Cvm Pres- Ii
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The Estrus Cycle:
the breeding pattern and behavior among the vertebrate are remarkably variable. this is dependent on whether or not the breeding cycle is seasonal or intraseasonal. Some mammals mate periodically throughout the year; others mate only during a restricted breeding season. except the higher primates, all female mammals permit mating only at definite times called “heat” or estrus.
Estrus = a period in the estrus cycle during which the female mammal is in a physiological and psychological readiness for mating, and hence sexually accepts the male.
Repetition of the estrus period at a set interval constitutes the estrus cycle.
Estrus cycle length; - varies remarkably in different vertebrate groups: Rat and Mouse - 4 to 5 day interval Dogs - every 6 months (twice yearly) Domestic cow and pig - 3 weeks (18 - 25 days) Deer family and wild sheep - mating only once a year.
Phases of the Estrus’s cycle
a) Estrus = a period of sexual desire and acceptance of mating b) Metestrus = a period of preparation for pregnancy - increasing P4 from developing CL c) Diestrus = period of uterine quiescence - peak P4 level *d) Anestrus = extended diestrus during which the ovary quiescent ( no functional structures); in seasonal breeders mostly d) Proestrus = period immediately preceding next estrus
The Estrus cycle cont’d
Uterine & Ovarian Changes during Estrus cycle: Ovulation Pituitary hormones releases ovum - FSH, LH, LTH Corpus hemorrhagicum Ovarian hormones Corpus luteum CL progesterone P4 Corpus albicantes Foll estrogen E2 non-functional
the production and the effects of estradiol (E2) are allocated precisely to the periods of proestrus and estrus – follicular phase
the production and effects of progesterone are allocated to the periods of metestrus and diestrus – luteal phase
considerable overlap in the presence and operation of these two hormones
The gonadotropic hormones from anterior pituitary: FSH; LH; LTH under the influence of GnHRH from the hypothalamus FSH, responsible for directing the maturation of the follicle and production of estrogen
LH, joins with FSH in promoting follicular maturation and estradiol production; also controls the development of the corpus luteum + P4 roduction
FSH and LH together are responsible for ovulation (major role by LH – cf. LH surge)
LTH, possibly identical to prolactin (PL) - regulates milk letdown; believes to stimulate progesterone secretion by the CL.
Regulation of the secretion and release of these hormones is via a negative feedback mechanism
CLEAVAGE - In Eutharian Mammals including human, monkey, mouse, etc
is holoblastic Microlecithal but the blastomeres tend to show size differences from the start of segmentation initial division occurs within 24 hr of ovulation and during uterine passage and within intact zona pellucida 24 hr post ovulation = first segmentation division within the zona pellucida, into a smaller and a larger blastomere 48 hr post ovulation = two successive divisions, first by the larger cell and then the lower cell, giving further smaller and larger cells/blastomeres.
Cleavage in eutharian mammals cont’d
Further cell divisions leads to attainment of 16-cell stage by 96 hr post ovulation; by which time the developing ovum has almost reached the uterus. This could be termed a blastocyst; and the cavity within called blastocyst cavity or primary yolk sac.
Day 5 post ovulation, the number of blastomeres has reached 100; uterine fluid is being absorbed thru the zona into the blastocyst, creating a single cavity; and also forcing rearrangement of the blastomeres into outer flattened cells and inner cells that are eccentrically located with the flattened outer cells.
At this stage, the embryo is referred to as a blastocyst; and the outer flattened cells called trophoblast, and the inner cells, called inner cell mass.
Gastrulation in mammals
Birds and mammals are both descendants of reptilian species. Therefore, it is not surprising that mammalian development parallels that of reptiles and birds.
What is surprising is that the gastrulation movements of reptilian and avian embryos, which evolved as an adaptation to yolky eggs, are retained even in the absence of large amounts of yolk in the mammalian embryo.
The mammalian inner cell mass can be envisioned as sitting atop an imaginary ball of yolk following instructions that seem more appropriate to its ancestors.
Modifications for Development Within Another Organism
Instead of developing in isolation within an egg, most mammals have evolved the remarkable strategy of developing within the mother herself. The mammalian embryo obtains nutrients directly from its mother and does not rely on stored yolk. This evolution has entailed a dramatic restructuring of the maternal anatomy (such as expansion of the oviduct to form the uterus) as well as the development of a fetal organ capable of absorbing maternal nutrients. This fetal organ—the placenta —is derived primarily from embryonic trophoblast cells, supplemented with mesodermal cells derived from the inner cell mass.
Placentation cont’d
first, the extraembryonic somatopleure elevates over all sides of the embryo to provide an amnion and chorion.
at same time the allantois grows out of the hindgut, expands into the coelom and fuses with chorion.
a large yolk is initially established but later declines rapidly as the allantois enlarges.
Placenta - a structure produced by appositioning of the extraembryonic membranes with the endometrium for the purpose of physiological exchange between the fetus and mother.
Origin : Formation - consists of two parts: - Fetal and Maternal sources
i) fetal placenta/component - furnished by the extraembryonic membranes amnion, chorion, allantois, yolk sac
chorion: most external and makes immediate contact with the uterine endometrium; vascular supply which is acquired from aIIantois.
Two sources of chorionic vascularization in mammals viz: i) the vitelline circulation provided by the yolk sac and ii) allantoic circulation provided by the allantois.
yolk sac; remains rudimentary in most mammals; allantois vascularizes the chorion.
The fetal circulation is therefore said to be chorioallantoic.
ii) maternal placenta/component - furnished by a single uterine endometrium. the established chorioallantoic membrane lies in apposition to the uterine endometrium.
as this membrane enlarges it establishes a close intimacy with the endometrium.
as the fetal placenta (chorioallantoic) has allantoic blood vessels running into and from the fetus, and maternal vessels run to and from the maternal placenta (endometrium), the two circulations are brought very close together. But there is no fusion between these two blood systems; fetal blood does not circulate in the mother & maternal blood does not circulate in the fetus. Materials are passed from one to the other through tissue barriers.
Placenta
(a structure produced by appositioning of the extraembryonic membranes with the endometrium for the purpose of physiological exchange between the fetus and mother).
CLASSIFICATION
via: i) histological (on basis of number of tissue barriers between the two circulations: ii) gross shape: fate of tissues)
i) histological
i) Epitheliochorial – 6 tissue layers
presenting apposition of fetal (chorionic epithelium) and maternal (endometrial epithelium) components and six tissue barriers between the two circulations. - in porcine, equine)
ii) Syndeschorial – 5 layers
intimate fusion of the maternal and fetal components results in destruction of the uterine epithelium; only 5 layers left. - in ruminants (sheep, cows, deer etc)
iii) Endotheliochorial – 4 layers
uterine mucosa is reduced and chorionic epithelium comes in contact with the walls of the maternal blood vessels: 4 layers - in carnivores – dogs, cats, etc
iv) Hemochorial- 3 layers
one in which the maternal endothelium also disappears and the chorionic epithelium is bathed directly in maternal blood; 3 layers. - in primates and rodents
ii) gross shape Placentas could also be classified on the basis of shape i) diffuse (Epitheliochorial) ii) cotyledonary (syndesmochorial) iii) zonary (endotheliochorial) iv) discoidal (Hemochorial) Note;
Apposition of fetal and maternal components and six tissue barriers between the two circulations, is the primitive type from which others have been derived. These derivations reflect a changeover from apposition to some degree of fusion of the two placental components which in turn involves some measure of destruction of maternal tissues.
In the ruminants (cattle, sheep) the fetal and maternal components are fused so intimately as to result in the destruction of the uterine epithelium, thus bringing the chorion into contact with the connective tissue of the uterine mucosa. Only five barriers therefore lie between the two bloodstreams. This situation is termed syndesmochorial (Figure 9.21b). Endotheliochorial placenta (Figure 9.21C) is one in which the uterine mucosa is reduced and the chorionic epithelium comes in contact with the walls of the maternal blood
* placentas exhibit thicker layer at earlier stages of development than later. For example, the primate placenta is initially Epitheliochorial and acquires its Hemochorial status later.
* Degree of union of the fetal and maternal components of the placenta varies:
i) Deciduous placenta = one in which the union of the components is so intimate that at birth a variable amount of maternal tissue is lost.
ii) Non-deciduous placenta = mere apposition of the two components of the placenta; and separation of the components is easily effected at birth and there is
.
no loss of maternal tissues
PLACENTA - FUNCTIONS:
i) Filtration - transmission of nutrients + oxygen into and nitrogenous wastes between fetus and mother
ii) Storage - stores materials such as fat, glycogen and iron
iii) Participates in the metabolism of protein
iv) Functions as endocrine gland (hCG, progesterone,
v) Protection of the fetus (physical and immunological)
The Human Placenta is characterized as:
i) Discoidal (reference to gross shape)
ii) Hemochorial (chorionic villi bathed in maternal blood)
iii) Deciduous (loss of endometrial tissues at birth)
iv) Chorioallantoic (by reason of allantoic blood supply to chorion even though the allantois proper is rudimentary
Human Development and Placentation Between 5 and 6 days postovulation, the blastocyst will be lying in the uterine cavity
the trophoblast differentiates into two portions viz: i) polar trophoblast = portion that overlies the inner cell mass;
this makes contact with the endometrium; multiplies rapidly and invades the endometrium deeply and establishes implantation
These invading trophoblasts differentiate into layers; - the original inner cellullar cytotrophoblast, - and outer syncytial syntrophoblast .
the syntrophoblast invades further and deeper into the endometrium; releases substances that block any rejection of the embryo by the mother: ; serves as nutritive and protective organ.
concurrently, the inner cell mass sort themselves and differentiate into primary endoderm and formative cells; and by further organization, form the forerunners of the anmion or amniogenic layer and amniotic cavity
by day 9 postovulation, blastocyst is totally beneath the uterine epithelium and fully embedded in the mucosa.
Events Between 13th and 20th clay post ovulation:
i) extraembryonic coelom formed, and villi formed from syncytrophoblastic strands;
the trophoblast then becomes chorion and chorionic villi through mesodermalization of the trophoblast and its villi.
ii) small secondary yolk sac is established
iii) formation of a definitive somatopleuric amnion; body stalk: rudimentary (a fingerlike projection of endoderm - a rudimentary allantois, ultimately grows into the matrix of the body stalk.
Note that, unlike the pig and many other mammals, the human allantois does not expand into the extraembryonic coelom, but remains as a rudiment within the body stalk)
iv) Conversion of the bilaminar embryonic disc to trilaminar disc. By 16-17 post ovulation, blastocyst is fully implanted in the endometrium and chorionic villi elaborated over entire surface. Once implantation is completed one could distinguish 3 topographical areas in the endometrium, viz: Placenta – Desidua basalis – maternal placenta D. capsularis D. parietalis EEM – Trophoblast placenta a) hindgut extension –to chorion or chorionic membrane b) Yolk sac Somatopleure c) chorion + d) amnion
i) desidua basalis - area immediately beneath the blastocyst
ii) desidua capsularis - that encapsulating the lumen-ward surface of the chorion
iii) desidua parietalis- area lining the remainder of the uterus
.
with advance gestation, the chorionic villi facing the decidua basalis enlarge and branch while the villi facing the decidau capsularis regress.
by 4th month gestation, decidau capsularis portion of the chorion is villusfree: and is called chorion leave
The decidua basalis chorion bearing villi is called chorion frondosum
Questions -review 1. Choose the correct answer from the following: The human placenta is described as: •Diffuse c) zonary b) Cotyledonary d) discoid 2. Which of the following is not a normal function of the human placenta: vii)Filtration iv) endocrine gland (hCG, progesterone, viii)Storage v) immuno-protective iii) metabolism of protein vi) fertilization 3. The trophoblast becomes chorion and chorionic villi through mesodermalization of the trophoblast and its villi. T/F 4. Select the correct word/answer from the following: The component(s) of the maternal placenta include: a) endometrium; b) chorion; c) amnion; d) allantois; e) chorioallantois 5. The human placenta is endocrine as it produces gonadotropins (LH/FSH) at pregnancy T/F
MORPHOGENESIS
GROWTH DIFFERENTIATION INDUCTTON TERAGOLOGY
Growth and Differentiation
two basic processes involved in the transformation of a single cell into a complex organism:
i) Growth = incr in size and/or no. of cells.
Ii) Differentiation = is the variation among cells descended from the original cell.
GROWTH:
3 basic mechanisms: i) increase in no. of cells ii) increase in size of cells iii) incr. in extracellular material
in early embryonic stages major growth mechanism is incr in size of cells by prolifera; embryo as a whole; also in the individual embr. structures.
DIFFERENTIATION in a population of daughter cells different from the parent cells
result in different cells; the different cells tend to form different structures.
The forming of these differentiated cells into their respective structures, is referred to as MORPHOGENESIS.
Morphogenic process = a process in which a mass of cells becomes a structure with a defined shape.
e.g. Single layer of epithelial cells transforms into a branched structure as a gland.
such
INDUCTION
developing organism made up of group of many organogenic fields which are interrelated.
Thus for normal devel of an organism as a whole, some mechanism must be present to ensure that an orderly sequence of events occurs
The phenomenon of ensuring that order prevails in the embryo as a whole, is that of induction.
INDUCTION = process whereby one population of cells, referred to as inductor, acts upon another population to change the behavior of the second population in some developmentally significant way, namely, to grow and/or undergo morphogenesis.
The result of induction is the emerg of highly complex structures and not merely the differentiation of a cell of tissue type.
INDUCTION initiates a preprogrammed series of events in the jnduced cell population.
In each system, inductors emerge in strict sequential manner usually a primary, secondary, tertiary etc inductors.
The induced cell population must be in a stage of differentiation, whereby they are COMPETENT to be affected by the inductors
The cells permanently induced are said to be DETERMINED.
Mechanism of Induction:
not exactly known.
assumed that some messengers are transferred from inductor to induced cell population;
Messengers such as macromolecules, collage; cell fractions; whole cell ions.
PRINCIPLES OF TERATOLOGY The Study of Abnormal Development
Timing of Teratogen:
development of the embryo consists of a group of cells that are growing--> differentiating --> and undergoing morphogenesis:
at different rates and at different times; But in strictly controlled sequence of events
Timing of these events --> necessary for normal development
Susceptibility to a teratogen varies with the developmental stage of the embryo at the time of exposure.
Teratogen -> embryo -->a) no effect b) severe effect
Critical Stages (CS) stage during development when the embryo will be most severely affected CS for each organ system and structure
CS of morphogenic field is time at which it is undergoing differentiation
The earlier in differentiation the attack, the more severe. e.g. Insult of time of NOTOCHORD induction malformation of entire HEAD ; at a later stage,--> only affects the eye.
Developmental Stages (DS)
The critical role of time is explained in terms of the effect produced at specific developmental stages.
Devel of embryo and its morphogenic field may be divided into 3 devel stages :
a) Stage of predifferentiation b) Stage of early differentiation & organogenesis c) stage of advance organogenesis = growth, histogenesis, functional maturation.
Pre-differentiation stage + teratogen a) lethal to embryo b)no effect at all Reason Cells at this stage have not undergone differentiation and so have same potential as the zygote. Therefore, insult ---> destroys all the cells and kill embryo or leaves cells which have full potential to form an entire embryo -> potential develop of normal embryo.
Early differentiation and organogenesis phase is the phase when the system is most susceptible to insults structural malformation.
Resistance to insults increases after the third stage; because this stage is after organs and structures’ form. Only functional and growth retardation rather than structural related may be
.
affected
It follows that the placenta is a product of the decidua basalis and chorion frondosum; the decidua basalis representing the maternal component; and the chorion frondosum representing the fetal component of the placenta
Umbilical cord - further expansion of the amnion presses the mesoderm of the body stalk into a cordlike structure within which the allantois and neck of the yolk sac are embedded.
This structure is the umbilical cord. The umbilical (allantoic) arteries and veins course through this cord. The right umbilical vein eventually disappears, and only rudiments of the yolk sac and allantois persist.
Umbilical cord = allantois + yolk sac + body stalk
ORGANOGENESIS
Formation of the organ system in the body: Skin Cardiovascular system Genitourinary system Digestive system CNS Others
THE SKIN
Embryogenesis Morpho-anatomy functions
Anatomy
keratin is the material from which hair, feathers, claws and etc., are made.
Stratum corneum = the outermost layer of the epidermis constitutes an impermeable membrane called stratum corneum.
The epidermis - stratum corneum (outer area) + stratum germinativum (inner portion) - has no blood vessels and no blood supply of its own; - depends on the vascularization of the dermis to which the structural
.
germinativum is adjacent
Functions - skin:
protective against: i) mechanical injuries ii) invasion by microorganisms iii) loss of moisture iv) predators - poison-producing glands; pigmentation; odor; external armour- scales etc
Thermoregulations physiologically - dissipation of heat via bloodstream (contraction and relaxation of blood vessels) physically by: evaporation of sweat by mammalian skin for cooling: conservation of heat via insulating materials as oil and fat,
.
mammalian hair, and feathers of birds
Excretory organ
supplements the kidney as a mechanism for excretion - most concentrated solute in sweat is NaCl; minerals such as K, Mg, and nitrogenous wastes also appear in sweat to some degree.
Organ of secretion:
(the integument is also an organ of secretion) a variety of cutaneous glands provides useful products: mucus-secreting glands in fishes and amphibians; oil-secreting glands in mammalian skin; milk-secreting glands of mammals
Skin cont’d Organ of respiration: in fishes and amphibians; in all other vertebrates including human, some degree of gaseous exchange takes (oxygen and carbon dioxide) takes place to some extent.
As a sense organ
skin serves as a vehicle for an array of cutaneous sense organs by which animals maintain contact with their external environment. these include: receptors for senses of: touch, temperature, pressure, and pain.
The Dermis
in contrast to the epidermis, dermis consists more of cell products than cells.
of
composed of network of connective tissue fibers derived from the original segmented dermatome
in the dermis are: lots of blood vessels, nerves, lymph vessels, smoothe muscle,
.
sense organs, deposits of fats, and array of epidermal and dermal derivatives
Origin of Skin:
all vertebrate skin originated from two embryonic sources: the ectoderm and mesoderm – somatopleure.
the entire ectoderm exclusive of the part dedicated to the neural tube, may be regarded as prospective epidermis of the skin.
The dermis originates from the dermatome subdivision of the epimere.
The dermatomal components of the somites resolve themselves into mesenchymal cells, which multiply and deploy to all regions of the embryo.
Function: keratin is the material from which hair, feathers, claws and etc., are made. Stratum corneum = the outermost layer of the epidermis constitutes an impermeable membrane called stratum corneum. The epidermis has no blood vessels and no blood supply of its own; depends on the vascularization of the dermis to which the stratura germinativum is adjacent.
Functions - skin:
protective against: i) mechanical injuries ii) invasion by microorganisms iii) loss of moisture iv) predators - poison-producing glands; pigmentation; odor; external armour- scales etc
Thermoregulations physiologically - dissipation of heat via bloodstream (contraction and relaxation of bvs) physically by: evaporation of sweat by mammalian skin for cooling: conservation of heat such insulating materials as oil and fat, mammalian hair, and feathers of birds.
- Excretory organ
supplements the kidney as a mechanism for excretion - most concentrated solute in sweat is NaCl; minerals such as K, Mg, and nitrogenous wastes also appear in sweat to some degree.
- Organ of secretion: (the integument is also an organ of secretion)
a variety of cutaneous glands provides useful products: mucus-secreting glands in fishes and amphibians; oil-secreting glands in mammalian skin; milk-secreting glands of mammals
Skin cont’d - organ of respiration: in fishes and amphibians; in all other vertebrates including human, some degree of gaseous exchange takes (oxygen and carbon dioxide) takes place to some extent.
- as a sense organ
skin serves as a vehicle for an array of cutaneous sense organs by which animals maintain contact with their external environment. these include: receptors for senses of: touch, temperature, pressure, and pain.
The Dermis
in contrast to the epidermis, dermis consists more of cell products than of cells. composed of network of connective tissue fibers derived from the original segmented dermatome in the dermis are: lots of blood vessels, nerves, lymph vessels, smoothe muscle, sense organs, deposits of fats, and array of epidermal and dermal derivatives.
Origin of Skin: all vertebrate skin originated from two embryonic sources: the ectoderm and mesoderm – somatopleure. the entire ectoderm exclusive of the part dedicated to the neural tube, may be regarded as prospective epidermis of the skin.
Origin of the Dermis
the dermis originates from the dermatome subdivision of the epimere. The dermatomal components of the somites resolve themselves into mesenchymal cells, which multiply and deploy to all regions of the embryo.
in mammals and birds, it appears that while the dermatome furnishes the dermis of the dorsal and dorsolateral portions of the body, the dermis of the flank and ventral surface of the body is a derivative of the mesoderm of the somatopleure.
Cutaneous Glands
glands in the skin are either unicellular or multicellular all glands originate in the epidermis unicellular glands: - are modified single cells scatteres among other cells in the epidermis multicellular glands: - formed as ingrowth from the stratum germinativum into the dermis; arc either tubular (straight, coiled, or branched) or alveolar (sacklike) (simple sacs, or moderately or elaborately branched - Fig 10-2)
1. Cyclostomes: unicellular type of gland: mostly mucus secreting giving thick, slimy protective skin coat as found in hagfishes
2. Fishes: both unicellular and alveolar glands
3. Amphibians: skin plays important role in respiration
alveolar mucous and glands predominate in amphibians there are also instances of specialized tubular glands as well unicellular glands also occur in amphibians - on snouts of larval frogs and salamander Functions of these glands include: READING assignment
4. Reptiles and Birds:
skin of birds and reptiles is virtually devoid of glands. existing glands secrete products related to breeding behavior and/or defense. in crocodiles and turtles, a gland beneath the jaw (crocodiles) secrete a substance called “musk’’ in the breeding season; no skin glands in lizard; some snakes have glands in the cloacaI region
In birds, the only gland is the uropygial gland (branched alveiolar) on dorsal side of bosy at base o the tail = supplies oil which the bird collects on its beak and uses in the preening of its plumage.
5. Mammals: have varying types of integumentary glands all derived from sebaceous and sweat glands sebaceous glands ; alveolar type; secrete oil; associated with hairs; their secretions serve to protect and lubricate the skin and hair; imparts an individual scent or odor to the animal; some lubricate the eyeball; some serve as scent glands (located near the anus)
Mammals cont’d
Sweat glands: - simple tubular glands; its watery secretions enables the skin’s function thermoregulation and excretion. Human, the horse and the bear, sweat glands are widespread over the body. Other mammals have no sweat glands at all. In others - rats, cats, mice, glands are confined to specific localities - underside of the paws; in deer - base of the tail; rabbits — on the lips; sheep, cattle, pigs — on the muzzle and snouts
Mammary or milk glands
are modified sweat glands found in all mammals
Embryogeny: first appearance of a pair of thickening of epidermal ectoderm along the ventrolateral body wall these thickening are called mammary ridge or milk line; further thickening of the ridge produces a roughly spherical mass projecting into the subjacent dermis cords of cells then ramify within the surrounding dermis which finally produce a branch-work.
Mammary or milk glands cont’d
In due course the cords hollow out to create a duct system converging at the surface, where an epidermal nipple is elevated,
At birth, no further development occurs in males, but elaboration of the mammae resumes in females with the advent of sexual maturity.
Nipples a) true nipples: rodents = single mammary duct; or carnivores and humans = several. ducts pass directly to the outside. True nipples: single duct in rodent multiple ducts in human & carnivores
Mammary glands cont’d
b) false nipple: or teat : found in horses and cattle: in which the ducts open into a chamber at the base of the projection, from where a secondary tube leads to the exterior and it is from the epithelium along the course of the milk line that the mammary glands arise.
_______________________ In humans, sea cows, elephants and bats = a single pair of pectoral mammae
In cattle, horses = one or two pairs of inguinal mammae appear
In rats, and mice = 3 pairs of thoracic and 3 pairs of abdominal mammaie
In pigs and dogs = five or six pairs are arranged in a continuous series from the chest to groin.
Scales
Epidermal scales:
are derivatives of the stratum corneum, the keratinized surface component of the dermis; well developed in reptiles; occurs in lizards, turtles, crocodiles snakes (devel from the stratum germinativum)
confined to the legs and feet in birds; when occurs in mammals, epidermal scales are found on the tail as in beavers, rats, mice, and opossum. are lacking in fishes: found rarely in amphibians
* Dermal scale: Read in text
FEATHERS
Functions include: For heat insulation; flight; protective coloration; sexual display
Types: i) hair feathers; ii) down feathers: iii) contour feathers
Hair feathers: smallest and simplest variety: scattered widely over the body under the contour feathers.
Down feathers: complex and composed of a short hollow basal quill, buried in the skin, from which they spray a number of filamentous barbs bearing tiny barbules along their margins. Underlying the contour feathers over much of the body, the down feathers provide effective insulation for the adult bird.
Scales cont’d
Contour feathers: the large feathers that ensheathe the body and provide its characteristic configuration. A typical contour feather consists of a lengthwise axis and a flat vane.
FORMATION: first there is aggregation of dermal cells immediately beneath the epidermis.
continued proliferation of the dermal aggregation results in a conical elevation, the dermal papilla, ensheathed by epidermis
thus formed the feather germ grows rapidly outward as a tapering epidermal cylinder filled with a vascular mesodermal core.
the epidermis at base of the papilla sinks in and the feather germ projects from a pit called follicle. From this point onward, down and contour feathers show developmental differences.
HAIR characteristic of mammals, hair is entirely epidermal in origin and composition structure of typical hair:
Compare the origins: Feather is d Dermal; Hair - epidermal
Two portions: projecting shaft and a root which lies in a pit called hair
follicle. At the base, the root expands into a hollow bulb into which a small dermal papilla projects, carrying blood vessels and nerves.
the living cells at the end of the root constantly multiply to lengthen the shaft; and in the process. the cells gradually die so that the hair proper becomes composed of dead, cornified cells.
Formation of hair:
formation of hair begins with epidermis alone unlike feathers whose development is initiated by a preliminary aggregation of dermal cells.
Hair cont’d
at the site of the presumptive hair, an epidermal nodule is formed by a local proliferation of the stratum germinativum (Fig. 10-17\)
the nodule extends down into the dermis as a tongue of tissue whose deeper end becomes enlarged to form a bulb (10-1 7B)
the bulb soon is inverted into a cup into which the vascular dermis and aggregates to form a dermal papilla (10- I 7C).
pushes
Concurrently, the deep-lying portion of the epidermal tongue splits to form a central strand and a surrounding epithelium.
the central strand becomes the root and shaft of the hair; the surrounding tissue provides the wall of the follicle. The follicular wall proliferates to form the sebaceous gland.
as the embryonic hair grows its shaft pushes towards the surface by making an opening through the center of the still solid epithelial cylinder.
Miscellaneous Beaks or bills, of birds: -- horny epidermal sheaths enclosing the bones of the upper and lower jaws constitute bills or beaks of birds. Similar cornification of epidermis on the edges of the digits serve as claws (reptile, birds, some mammals, some amphibians); hoofs (horses, cattle, deer, pigs, etc), and nails (humans and other primates).
The Skeleton
some form of skeleton in all living animals for protection and support all skeletons may be grouped into one category called CONNECTIVE TISSUE composed of varying quantities and combinations of mucopolysaccharide and proteins (collagen)
Connective tissue: - 2 functional groups binding c.t & supportive c.t i) binding c.t. = tendons, ligaments, and fasciae Tendons connect muscle to bones or cartilage Ligaments unit skeletal parts. Fasciae are sheets of c. t. serving to bind constituent cells together into a definitive organ; such as muscle cells into the mass of an individual muscle or nerve fibers into a nerve.
ii) supportive c. t.
= provide the skeleton around (endoskeleton) or within (exoskeleton) which an animal is built.
Skeletal Development
Notochord: present in all vertebrate embryos replaced by the vertebral column and base of the skull, in the adult vertebrates is still present in the lower Chordate adults (cyclostome and elasmobranchs) - as soft flexible rod beneath the neural tube composed of vesicular tissue, external sheath and a surrounding membrane run from the infundibulum of the brain back to the terminus of the fleshy part of the tail.
Skeletal Development cont’d
b) the Cartilage:
is a prominent skeletal material of the embryos of ALL vertebrate.
is a major skeletal material of adults of cyclostome, (chondrichthyes and a few degenerate groups of Osteichthymes. Bone is the major skeletal material in the higher vertebrates.
Is a c.t whose intracellular matrix is composed of complex protein within which there is spread a network of colagenous fibers.
the matrix is a product of cartilage cells (chondrocytes) which lie in small spaces (lacunae) within the matrix.
cartilage is surrounded by a dense c. tissue called perichondriurn, to which tendons and ligaments become attached.
Cartilage exists in several forms viz: a) hyaline cartilage: - homogeneous and translucent matrix with few fibers b) elastic cartilage - the matrix is fibrous and elastic c) calcified cartilage: - contains deposits of calcium salts hard but brittle textured
Skeletal Development cont’d
Formation cartilage is a derivative of embryonic mesenchymal cells. first, embryonic c.t. cells (mesenchyme) begin to aggregate. the branching mesenchymal cells gradually round up and definitive mesenchymal cells (chondrocytes) start secreting matrix. laying down of more and more matrix by chondrocytes forces the chondrocytes
;.
further apart
Skeletal Development cont’d
chondrocytes eventually become isolated in small spaces (lacunae) within the matrix. (fig. 11-2).
division and formation of more matrix by the internal chondrocytes adds to the growth of the matrix and cartilage
Deposition of more matrix from outer surface by the perichondrium supplements the growth of the cartilage.
This ability of cartilage to grow both by internal expansion and external deposition stands in strong contrast to the growth of bone.
c) Bone:
like cartilage, bone is a derivative of embryonic mesenchyme, consists of a matrix within which cells (osteocytes) are buried.
matrix is fibrillar; contains hard, complex mineral subst. composed mainly of crystals of calcium phosphate - Calcium hydroxide combination known as apatite.
Like cartilage, the bone cells (osteocytes) that secrete the matrix lie in lacunae.
Lacunae are linked with one another via canaliculi (a branching network of minute canals into which project the radiating processes of the osteocytes. In cartilage, the lacunae and hence, the chondrocytes. are isolated from each other.
Types:
* intramembraneous or dermal/membrane bone where the mesenchymal precursor is succeeded directly by bone.
intracartilaginous or cartilage/replacement bone = the original mesenchyme first provides a cartilaginous model, which is then destroyed and replaced by bone.
Note: These terms refer only to the sequence of events in bone development, for when fully formed, bones of both types are structurally alike.
Formation:
a) membrane bone:
first, mesenchymal cells congregate at the site of a future bone; cluster and arrange themselves in an interlacing network of strands.
each strand starts to secrete collagen resulting in a framework of collagenous fibers.
each fiber is invested by the c.t. cells that produced it; and deposition of calcium salts begin
the mesenchymal cells, now called OSTEOBLASTS extract the necessary raw materials from the blood supply in the area, and lay down bone salts around the fibers.
This then creates a scattering of bars and plates (trabeculae) of bone.
As more layers are added to the bone, some of the osteoblasts are caught in their own deposits and there, in their individual Iacunae.
In the Iacunae, the osteocytes cease to function as active bone formers; they assume maintenance of the already formed bone.
Osteocytes in lacunae are interconnected by canaliculi; and thus materials picked up by osteocytes adjacent to blood vessels, are distributed throughout the bone matrix.
Osteoblasts Osteocytes Osteoclast
formation of maintenance of breaking down of
the remaining osteoblasts continue to add to the trabeculae; and as they enlarge, come together and fuse into a continuous latticework.
bone in this condition is called cancellous or spongy bone. The area between the trabeculae are occupied by c.t., which is rapidly becomes vascularized; and so constitutes the bone marrow.
mesenchymal cell concentration appears around the primary cancellous bone to form the periosteum.
within the periosteum, certain cells assume the role of osteoblasts and resume deposition of bone.
Bone is laid down in dense, parallel sheets; resulting in an external layer of compact bone investing the cancellous bone within.
physiological maintenance of compact bone is facilitated by an array of canals constituting the Harvesian System.
The compact bone becomes perforated by long branching and anastomosing Harvesian canals.. which communicate externally with the periosteurn and internally with the marrow cavities.
Note that:
i) no fundamental difference between compact and cancellous bone, other than the degree of density.
ii) Both are a product of osteoblasts
iii) both are vital tissues maintained by entrapped but intercommunicating osteocytes.
iv) the Harvesian Systems are not of universal occurrence among vertebrates. Most amphibia, some reptiles, and many smaller mammals do not have them.
b) Cartilage bone
The intramembranous bone (described above) is largely confined with parts of the skull, and the shoulder girdle. Elsewhere, bone is preceded by cartilage
Events of formation are essentially the same as those in intramembranous bone development. Only difference is the added rise and decline of the cartilaginous precursor.
some bone is the product of the osteoblasts brought in by the invading mesensenchyme: some is a product of surviving cartilage cells that transform to osteoblast. ASSIGNED READING - for details.
BONE - AS A TISSUE:
Bone as a tissue is revealed in the formation of both types of bones from embryonic mesenchymal cells; and in the fact of the dynamism of bone - that is, bone is constantly being remodeled (formed by osteobIasts, and broken down by osteoclasts)
serves as the source of supply of calcium and phosphate required for the maintenance of physiological mechanims in the body,
is a reservoir of Ca and P that is drawn upon to satisfy structural requirements:
Bone is the reservoir for Ca and P, which are added to and substracted from bloodstream as required through the turnover mechanism in bone. e.g. In birds calcium for the egg shell is drawn from the bone.
BONE - AS AN ORGAN:
i) flat bones (of the skull) – composed of reduced cancellous area, containing masses of marrow, sandwiched between upper and lower layers of compact bone.
ii) long bones (of the appendages) derived from the cartilaginous model of bone formation.
Bone – structure and growth:
Consists of a long shaft, diaphysis, with epiphysis at each end. Ossification begins in the center of diaphysis and progresses toward each end.
Elongation is provided by steady creation of new cartilage at end of epiphyseal ends; and ossification follows until adult length is attained.
In lower vertebrates, the epiphyses remain cartilaginous throughout adult life.
In mammals and some reptiles, accessory ossifications later appear in the epiphyses, ossification that fuses with the diaphysis only after full adult size is reached (Fig 11-8E).
Irregular bone (of the vertebrae; limb girdles) formation commences with appearance of several primary endochondral centers;
which are supplemented by secondary centers.
The original cancellous bone is replaced in part and supplemented by periosteal deposition of compact bone.
Bone formation and development:
Influencing Factors
Bone formation and development: - Factors that influence: Bone size and proportions is determined by both intrinsic and extrinsic factors:
Experimental results indicate that early primordia possess inherent morphogenetic properties that can and do express themselves up to a point (regardless of environmental influences:
then sooner or later, extrinsic factors come into play in the molding of architectural details and in the attainment of appropriate size and proportions.
Mechanical factors: Muscle: localized growth of bone will cease in the absence of a proper muscle attachment; Size: the size and shape of the skull bones accommodate to the structures they invest: e.g.. Oversized eyes bring an enlargement of the orbital areas of the skull; Anomaly: genetically induced anomaly of a reduced or absent brain is accompanied by skull bone of greatly reduced size and abnormal shape; Abnormally large and thin skull bones accompany an anomalous brain of excessive size.
Nutritional factors vitamins C and D are essential for proper ossification vitamin A excess inhibits bone ossification.
Hormonal factors i) somatotropin hormone (STH) - commonly called growth hormone
Inadequate production of STH (from ant. pituitary) dwarfism of entire body, including the skeleton. Excess production of STH = disproportionate overgrowth of the skeleton.
ii) Gonadal hormones: governs the differences in form and
.
proportion of the skeletal parts in males and females
summary 1.
5.
Connective tissue i) Binding ligaments; tendons; fasciae ii) Supporting Cartilage i) hyaline ii) elastic iii) calcified iv) Features
chondrocytes perichondrium lacunae
3. Bone Features: - osteoblasts formation - osteocytes maintenance - osteoclasts remodeling - lacunae entrapping osteocytes and intercommunicating - canaliculi inter-communication Types: i) intramembranous/membrane/dermal ii) intracartilagenous/cartilage/replacement
The Skull
The ground plan of the vertebrate skull
embryologically comprises three parts viz:
i) the neurocranium (also called = endocranium) includes the capsules bounding the olfactory organs, the eyes, and internal ears, and the box enclosing the brain. Composed of cartilage, replacement bone or both endocranium is composed entirely of cartilage, it may be called chondronium.
ii) The splanchnocranium or visceral skeleton = which is the skeleton of the jaws and gill arches of their derivatives:
- mandibular arch - hyoid arch - gill arches
iii) the dermatocranium, also called surface skull = consists exclusively of dermal bone and are believed to have evolved from the external armour (exoskeleton) of primitive fishes.
The material source of the neurocranium is in the connective tissue membranes that initially envelop the embryonic brain and associated organs. Aggregation of mesenchyme appear and then chondrify into cartilage. This preliminary cartilage will be replaced by bone.
Additional aggregations of mesenchyme ossify directly to provide the overlying dermatocranium.
The spIanchnocranium- condensation of mesenchyme - from which the visceral arches arise are derived from the splanchnic mesoderm investing the pharyngeal gut.
Embryonic primordia of the skull
Speaking in terms of vertebrates and with reference to the neurocranium of the skull cartilage formation is initiated in six pairs of centers viz: i) pre-chordal cartilage ii) parachordal cartilage iii) occipital cartilage iv) orbital cartilage v) quadrate cartilage vi) Meckel’s cartilage
i) prechordal cartilages (trabeculae) ventral (formed beneath the anterior part of the brain) ii) parachordal cartilages - ventral to the brain and flank the notochord iii) occipital cartilage - at the level of transition from brain to spinal cord. The parachordals tend to fuse with occipital cartilages.
The next three centers of ossification are all associated with the developing sense organs: - orbital cartilage - quadrate cartilages - mandibular
iv) orbital cartilage: an auditory (otic) capsule - a cartilaginous; forms around primordial inner ear; medial to each eye and orbital cartilage provides a preliminary sidewall to the neurocranium: a nasal capsule forms around each olfactory sac.
Separate paired cartilages also appear within the mesodermal core of the embyonic viscera! arches, and collectively, constitute the splanchnocraniurn. These paired cartilages include:
v) quadrate cartilages - signifying the increase in the first /mandibular arch - is represented by the paired quadrate cartilages.
vi) mandibular (Meckel’s) cartilages. (a pair in the lower jaw).
Note: In tetrapods, especially in reptiles and mammals the bones of the upper jaw are affixed to the floor and sides of the neurocranium (obscuring the primary cranial bones
the floor of the braincase is formed by the arachnoids and the pre-sphenoid, which extends forward into the ethmoid region
sidewalls of the braincase are formed by the alisphenoid and orbitosphenoid.
the ethmoid complex is associated with the nasal passageways, including, in humans, the distinctive cribriform plate and the turbinate bones.
- the upper jaw is represented by the quadrate cartilage (or bone)
- the lower jaw is represented by the Merkel cartilage.
- Visceral arches I - VII are branchial or gill arches 7
- The dermatocraniurn = consists of longitudinal series of paired bones that provide a ROOF for the neurocranium
A. AMPHIBIAN SKULL:
- skull table is shortened and the snout is greatly lengthened. stabilization of the dermal bones - rostral as occurs in fishes have been eliminated prominent and elongated nasal and frontal bones endocranium is well ossified and is single structure (lacks movable parts)
the basioccipital bears a single condyle that articulates with the first vertebra. only manifestation of primary jaw is the small articular bone at the rear. much of the endocranium remains cartilaginous.
B. REPTILIA SKULL
skull is higher and narrower than that of amphibia some elements of the dermatocranium are eliminated (lost) notable deletions in the roof and sides of the neurocranium are the post-parietal and components of the temporal regions.
important modifications in the cheek bones: - whereas the regions in the stem reptiles and some modern amphibians were solidly roofed over,
in all the other reptiles, one or two openings make their appearance. The number and position of these openings provide the primary basis for the classification of the reptiles.
It was one of the single-opening groups, the Synapsida, that the mammals stemmed.
Unlike the modern amphibia in which there is an increasing tendency toward retention of embyonic cartilage, the reptilian braincase (endocraniurn) remains ossified
Articulation of the lower jaw is similar to that of the amphibian: i.e, the articular bone at the rear of the lower jaw abuts against the quadrate lying in the rear sidewall of the neuroeranium.
C BIRDS (AVES) Skull constriction essentially as those of their reptilian ancestors. - principal variation is the expansion of the cranial roof to accommodate the enlarged brain: - thinning of bones to provide lightness; - considerable variation of individual elements; - snakes and lizards; elongation of the pre-maxilla and nasal roof; - and loss of teeth and the maxilla that housed them.
MAMMALS
There is consistency of skull pattern within the mammals as a whole the nasal, frontal and parietal provide the cranial roof the skull.
In humans and primate, the frontal and parietal have become grossly expanded to accommodate the size of the brain; - the remaining dermal contributions to the roof and sides of the braincase are the squamosals in the cheek areas and the reduced lacrimals in the front of the orbits.
matching
A
1. Binds skeletal parts 2. Binds muscles and/or bones 3. Chondrocytes 4. Osteoblasts 5. Lacunae 6. canaliculi 7. Bone 8. Perichodrium 9. Osteoclasts 10. Tissue
b. c. d. e. f. g. h. i.
B
Cartilage Ligament Bone formation Epiphysis Periosteum Bone remodeling Blood Bone
The Estrus Cycle:
the breeding pattern and behavior among the vertebrate are remarkably variable. this is dependent on whether or not the breeding cycle is seasonal or intraseasonal. Some mammals mate periodically throughout the year; others mate only during a restricted breeding season. except the higher primates, all female mammals permit mating only at definite times called “heat” or estrus.
Estrus = a period in the estrus cycle during which the female mammal is in a physiological and psychological readiness for mating, and hence sexually accepts the male.
Repetition of the estrus period at a set interval constitutes the estrus cycle.
Estrus cycle length; - varies remarkably in different vertebrate groups: Rat and Mouse - 4 to 5 day interval Dogs - every 6 months (twice yearly) Domestic cow and pig - 3 weeks (18 - 25 days) Deer family and wild sheep - mating only once a year.
Phases of the Estrus’s cycle
a) Estrus = a period of sexual desire and acceptance of mating b) Metestrus = a period of preparation for pregnancy - increasing P4 from developing CL c) Diestrus = period of uterine quiescence - peak P4 level *d) Anestrus = extended diestrus during which the ovary quiescent ( no functional structures); in seasonal breeders mostly d) Proestrus = period immediately preceding next estrus
The Estrus cycle cont’d
Uterine & Ovarian Changes during Estrus cycle: Ovulation Pituitary hormones releases ovum - FSH, LH, LTH Corpus hemorrhagicum Ovarian hormones Corpus luteum CL progesterone P4 Corpus albicantes Foll estrogen E2 non-functional
the production and the effects of estradiol (E2) are allocated precisely to the periods of proestrus and estrus – follicular phase
the production and effects of progesterone are allocated to the periods of metestrus and diestrus – luteal phase
considerable overlap in the presence and operation of these two hormones
The gonadotropic hormones from anterior pituitary: FSH; LH; LTH under the influence of GnHRH from the hypothalamus FSH, responsible for directing the maturation of the follicle and production of estrogen
LH, joins with FSH in promoting follicular maturation and estradiol production; also controls the development of the corpus luteum + P4 roduction
FSH and LH together are responsible for ovulation (major role by LH – cf. LH surge)
LTH, possibly identical to prolactin (PL) - regulates milk letdown; believes to stimulate progesterone secretion by the CL.
Regulation of the secretion and release of these hormones is via a negative feedback mechanism
CLEAVAGE - In Eutharian Mammals including human, monkey, mouse, etc
is holoblastic Microlecithal but the blastomeres tend to show size differences from the start of segmentation initial division occurs within 24 hr of ovulation and during uterine passage and within intact zona pellucida 24 hr post ovulation = first segmentation division within the zona pellucida, into a smaller and a larger blastomere 48 hr post ovulation = two successive divisions, first by the larger cell and then the lower cell, giving further smaller and larger cells/blastomeres.
Cleavage in eutharian mammals cont’d
Further cell divisions leads to attainment of 16-cell stage by 96 hr post ovulation; by which time the developing ovum has almost reached the uterus. This could be termed a blastocyst; and the cavity within called blastocyst cavity or primary yolk sac.
Day 5 post ovulation, the number of blastomeres has reached 100; uterine fluid is being absorbed thru the zona into the blastocyst, creating a single cavity; and also forcing rearrangement of the blastomeres into outer flattened cells and inner cells that are eccentrically located with the flattened outer cells.
At this stage, the embryo is referred to as a blastocyst; and the outer flattened cells called trophoblast, and the inner cells, called inner cell mass.
Gastrulation in mammals
Birds and mammals are both descendants of reptilian species. Therefore, it is not surprising that mammalian development parallels that of reptiles and birds.
What is surprising is that the gastrulation movements of reptilian and avian embryos, which evolved as an adaptation to yolky eggs, are retained even in the absence of large amounts of yolk in the mammalian embryo.
The mammalian inner cell mass can be envisioned as sitting atop an imaginary ball of yolk following instructions that seem more appropriate to its ancestors.
Modifications for Development Within Another Organism
Instead of developing in isolation within an egg, most mammals have evolved the remarkable strategy of developing within the mother herself. The mammalian embryo obtains nutrients directly from its mother and does not rely on stored yolk. This evolution has entailed a dramatic restructuring of the maternal anatomy (such as expansion of the oviduct to form the uterus) as well as the development of a fetal organ capable of absorbing maternal nutrients. This fetal organ—the placenta —is derived primarily from embryonic trophoblast cells, supplemented with mesodermal cells derived from the inner cell mass.
Placentation cont’d
first, the extraembryonic somatopleure elevates over all sides of the embryo to provide an amnion and chorion.
at same time the allantois grows out of the hindgut, expands into the coelom and fuses with chorion.
a large yolk is initially established but later declines rapidly as the allantois enlarges.
Placenta - a structure produced by appositioning of the extraembryonic membranes with the endometrium for the purpose of physiological exchange between the fetus and mother.
Origin : Formation - consists of two parts: - Fetal and Maternal sources
i) fetal placenta/component - furnished by the extraembryonic membranes amnion, chorion, allantois, yolk sac
chorion: most external and makes immediate contact with the uterine endometrium; vascular supply which is acquired from aIIantois.
Two sources of chorionic vascularization in mammals viz: i) the vitelline circulation provided by the yolk sac and ii) allantoic circulation provided by the allantois.
yolk sac; remains rudimentary in most mammals; allantois vascularizes the chorion.
The fetal circulation is therefore said to be chorioallantoic.
ii) maternal placenta/component - furnished by a single uterine endometrium. the established chorioallantoic membrane lies in apposition to the uterine endometrium.
as this membrane enlarges it establishes a close intimacy with the endometrium.
as the fetal placenta (chorioallantoic) has allantoic blood vessels running into and from the fetus, and maternal vessels run to and from the maternal placenta (endometrium), the two circulations are brought very close together. But there is no fusion between these two blood systems; fetal blood does not circulate in the mother & maternal blood does not circulate in the fetus. Materials are passed from one to the other through tissue barriers.
Placenta
(a structure produced by appositioning of the extraembryonic membranes with the endometrium for the purpose of physiological exchange between the fetus and mother).
CLASSIFICATION
via: i) histological (on basis of number of tissue barriers between the two circulations: ii) gross shape: fate of tissues)
i) histological
i) Epitheliochorial – 6 tissue layers
presenting apposition of fetal (chorionic epithelium) and maternal (endometrial epithelium) components and six tissue barriers between the two circulations. - in porcine, equine)
ii) Syndeschorial – 5 layers
intimate fusion of the maternal and fetal components results in destruction of the uterine epithelium; only 5 layers left. - in ruminants (sheep, cows, deer etc)
iii) Endotheliochorial – 4 layers
uterine mucosa is reduced and chorionic epithelium comes in contact with the walls of the maternal blood vessels: 4 layers - in carnivores – dogs, cats, etc
iv) Hemochorial- 3 layers
one in which the maternal endothelium also disappears and the chorionic epithelium is bathed directly in maternal blood; 3 layers. - in primates and rodents
ii) gross shape Placentas could also be classified on the basis of shape i) diffuse (Epitheliochorial) ii) cotyledonary (syndesmochorial) iii) zonary (endotheliochorial) iv) discoidal (Hemochorial) Note;
Apposition of fetal and maternal components and six tissue barriers between the two circulations, is the primitive type from which others have been derived. These derivations reflect a changeover from apposition to some degree of fusion of the two placental components which in turn involves some measure of destruction of maternal tissues.
In the ruminants (cattle, sheep) the fetal and maternal components are fused so intimately as to result in the destruction of the uterine epithelium, thus bringing the chorion into contact with the connective tissue of the uterine mucosa. Only five barriers therefore lie between the two bloodstreams. This situation is termed syndesmochorial (Figure 9.21b). Endotheliochorial placenta (Figure 9.21C) is one in which the uterine mucosa is reduced and the chorionic epithelium comes in contact with the walls of the maternal blood
* placentas exhibit thicker layer at earlier stages of development than later. For example, the primate placenta is initially Epitheliochorial and acquires its Hemochorial status later.
* Degree of union of the fetal and maternal components of the placenta varies:
i) Deciduous placenta = one in which the union of the components is so intimate that at birth a variable amount of maternal tissue is lost.
ii) Non-deciduous placenta = mere apposition of the two components of the placenta; and separation of the components is easily effected at birth and there is
.
no loss of maternal tissues
PLACENTA - FUNCTIONS:
i) Filtration - transmission of nutrients + oxygen into and nitrogenous wastes between fetus and mother
ii) Storage - stores materials such as fat, glycogen and iron
iii) Participates in the metabolism of protein
iv) Functions as endocrine gland (hCG, progesterone,
v) Protection of the fetus (physical and immunological)
The Human Placenta is characterized as:
i) Discoidal (reference to gross shape)
ii) Hemochorial (chorionic villi bathed in maternal blood)
iii) Deciduous (loss of endometrial tissues at birth)
iv) Chorioallantoic (by reason of allantoic blood supply to chorion even though the allantois proper is rudimentary
Human Development and Placentation Between 5 and 6 days postovulation, the blastocyst will be lying in the uterine cavity
the trophoblast differentiates into two portions viz: i) polar trophoblast = portion that overlies the inner cell mass;
this makes contact with the endometrium; multiplies rapidly and invades the endometrium deeply and establishes implantation
These invading trophoblasts differentiate into layers; - the original inner cellullar cytotrophoblast, - and outer syncytial syntrophoblast .
the syntrophoblast invades further and deeper into the endometrium; releases substances that block any rejection of the embryo by the mother: ; serves as nutritive and protective organ.
concurrently, the inner cell mass sort themselves and differentiate into primary endoderm and formative cells; and by further organization, form the forerunners of the anmion or amniogenic layer and amniotic cavity
by day 9 postovulation, blastocyst is totally beneath the uterine epithelium and fully embedded in the mucosa.
Events Between 13th and 20th clay post ovulation:
i) extraembryonic coelom formed, and villi formed from syncytrophoblastic strands;
the trophoblast then becomes chorion and chorionic villi through mesodermalization of the trophoblast and its villi.
ii) small secondary yolk sac is established
iii) formation of a definitive somatopleuric amnion; body stalk: rudimentary (a fingerlike projection of endoderm - a rudimentary allantois, ultimately grows into the matrix of the body stalk.
Note that, unlike the pig and many other mammals, the human allantois does not expand into the extraembryonic coelom, but remains as a rudiment within the body stalk)
iv) Conversion of the bilaminar embryonic disc to trilaminar disc. By 16-17 post ovulation, blastocyst is fully implanted in the endometrium and chorionic villi elaborated over entire surface. Once implantation is completed one could distinguish 3 topographical areas in the endometrium, viz: Placenta – Desidua basalis – maternal placenta D. capsularis D. parietalis EEM – Trophoblast placenta a) hindgut extension –to chorion or chorionic membrane b) Yolk sac Somatopleure c) chorion + d) amnion
i) desidua basalis - area immediately beneath the blastocyst
ii) desidua capsularis - that encapsulating the lumen-ward surface of the chorion
iii) desidua parietalis- area lining the remainder of the uterus
.
with advance gestation, the chorionic villi facing the decidua basalis enlarge and branch while the villi facing the decidau capsularis regress.
by 4th month gestation, decidau capsularis portion of the chorion is villusfree: and is called chorion leave
The decidua basalis chorion bearing villi is called chorion frondosum
Questions -review 1. Choose the correct answer from the following: The human placenta is described as: •Diffuse c) zonary b) Cotyledonary d) discoid 2. Which of the following is not a normal function of the human placenta: vii)Filtration iv) endocrine gland (hCG, progesterone, viii)Storage v) immuno-protective iii) metabolism of protein vi) fertilization 3. The trophoblast becomes chorion and chorionic villi through mesodermalization of the trophoblast and its villi. T/F 4. Select the correct word/answer from the following: The component(s) of the maternal placenta include: a) endometrium; b) chorion; c) amnion; d) allantois; e) chorioallantois 5. The human placenta is endocrine as it produces gonadotropins (LH/FSH) at pregnancy T/F
MORPHOGENESIS
GROWTH DIFFERENTIATION INDUCTTON TERAGOLOGY
Growth and Differentiation
two basic processes involved in the transformation of a single cell into a complex organism:
i) Growth = incr in size and/or no. of cells.
Ii) Differentiation = is the variation among cells descended from the original cell.
GROWTH:
3 basic mechanisms: i) increase in no. of cells ii) increase in size of cells iii) incr. in extracellular material
in early embryonic stages major growth mechanism is incr in size of cells by prolifera; embryo as a whole; also in the individual embr. structures.
DIFFERENTIATION in a population of daughter cells different from the parent cells
result in different cells; the different cells tend to form different structures.
The forming of these differentiated cells into their respective structures, is referred to as MORPHOGENESIS.
Morphogenic process = a process in which a mass of cells becomes a structure with a defined shape.
e.g. Single layer of epithelial cells transforms into a branched structure as a gland.
such
INDUCTION
developing organism made up of group of many organogenic fields which are interrelated.
Thus for normal devel of an organism as a whole, some mechanism must be present to ensure that an orderly sequence of events occurs
The phenomenon of ensuring that order prevails in the embryo as a whole, is that of induction.
INDUCTION = process whereby one population of cells, referred to as inductor, acts upon another population to change the behavior of the second population in some developmentally significant way, namely, to grow and/or undergo morphogenesis.
The result of induction is the emerg of highly complex structures and not merely the differentiation of a cell of tissue type.
INDUCTION initiates a preprogrammed series of events in the jnduced cell population.
In each system, inductors emerge in strict sequential manner usually a primary, secondary, tertiary etc inductors.
The induced cell population must be in a stage of differentiation, whereby they are COMPETENT to be affected by the inductors
The cells permanently induced are said to be DETERMINED.
Mechanism of Induction:
not exactly known.
assumed that some messengers are transferred from inductor to induced cell population;
Messengers such as macromolecules, collage; cell fractions; whole cell ions.
PRINCIPLES OF TERATOLOGY The Study of Abnormal Development
Timing of Teratogen:
development of the embryo consists of a group of cells that are growing--> differentiating --> and undergoing morphogenesis:
at different rates and at different times; But in strictly controlled sequence of events
Timing of these events --> necessary for normal development
Susceptibility to a teratogen varies with the developmental stage of the embryo at the time of exposure.
Teratogen -> embryo -->a) no effect b) severe effect
Critical Stages (CS) stage during development when the embryo will be most severely affected CS for each organ system and structure
CS of morphogenic field is time at which it is undergoing differentiation
The earlier in differentiation the attack, the more severe. e.g. Insult of time of NOTOCHORD induction malformation of entire HEAD ; at a later stage,--> only affects the eye.
Developmental Stages (DS)
The critical role of time is explained in terms of the effect produced at specific developmental stages.
Devel of embryo and its morphogenic field may be divided into 3 devel stages :
a) Stage of predifferentiation b) Stage of early differentiation & organogenesis c) stage of advance organogenesis = growth, histogenesis, functional maturation.
Pre-differentiation stage + teratogen a) lethal to embryo b)no effect at all Reason Cells at this stage have not undergone differentiation and so have same potential as the zygote. Therefore, insult ---> destroys all the cells and kill embryo or leaves cells which have full potential to form an entire embryo -> potential develop of normal embryo.
Early differentiation and organogenesis phase is the phase when the system is most susceptible to insults structural malformation.
Resistance to insults increases after the third stage; because this stage is after organs and structures’ form. Only functional and growth retardation rather than structural related may be
.
affected
It follows that the placenta is a product of the decidua basalis and chorion frondosum; the decidua basalis representing the maternal component; and the chorion frondosum representing the fetal component of the placenta
Umbilical cord - further expansion of the amnion presses the mesoderm of the body stalk into a cordlike structure within which the allantois and neck of the yolk sac are embedded.
This structure is the umbilical cord. The umbilical (allantoic) arteries and veins course through this cord. The right umbilical vein eventually disappears, and only rudiments of the yolk sac and allantois persist.
Umbilical cord = allantois + yolk sac + body stalk
ORGANOGENESIS
Formation of the organ system in the body: Skin Cardiovascular system Genitourinary system Digestive system CNS Others
THE SKIN
Embryogenesis Morpho-anatomy functions
Anatomy
keratin is the material from which hair, feathers, claws and etc., are made.
Stratum corneum = the outermost layer of the epidermis constitutes an impermeable membrane called stratum corneum.
The epidermis - stratum corneum (outer area) + stratum germinativum (inner portion) - has no blood vessels and no blood supply of its own; - depends on the vascularization of the dermis to which the structural
.
germinativum is adjacent
Functions - skin:
protective against: i) mechanical injuries ii) invasion by microorganisms iii) loss of moisture iv) predators - poison-producing glands; pigmentation; odor; external armour- scales etc
Thermoregulations physiologically - dissipation of heat via bloodstream (contraction and relaxation of blood vessels) physically by: evaporation of sweat by mammalian skin for cooling: conservation of heat via insulating materials as oil and fat,
.
mammalian hair, and feathers of birds
Excretory organ
supplements the kidney as a mechanism for excretion - most concentrated solute in sweat is NaCl; minerals such as K, Mg, and nitrogenous wastes also appear in sweat to some degree.
Organ of secretion:
(the integument is also an organ of secretion) a variety of cutaneous glands provides useful products: mucus-secreting glands in fishes and amphibians; oil-secreting glands in mammalian skin; milk-secreting glands of mammals
Skin cont’d Organ of respiration: in fishes and amphibians; in all other vertebrates including human, some degree of gaseous exchange takes (oxygen and carbon dioxide) takes place to some extent.
As a sense organ
skin serves as a vehicle for an array of cutaneous sense organs by which animals maintain contact with their external environment. these include: receptors for senses of: touch, temperature, pressure, and pain.
The Dermis
in contrast to the epidermis, dermis consists more of cell products than cells.
of
composed of network of connective tissue fibers derived from the original segmented dermatome
in the dermis are: lots of blood vessels, nerves, lymph vessels, smoothe muscle,
.
sense organs, deposits of fats, and array of epidermal and dermal derivatives
Origin of Skin:
all vertebrate skin originated from two embryonic sources: the ectoderm and mesoderm – somatopleure.
the entire ectoderm exclusive of the part dedicated to the neural tube, may be regarded as prospective epidermis of the skin.
The dermis originates from the dermatome subdivision of the epimere.
The dermatomal components of the somites resolve themselves into mesenchymal cells, which multiply and deploy to all regions of the embryo.
Function: keratin is the material from which hair, feathers, claws and etc., are made. Stratum corneum = the outermost layer of the epidermis constitutes an impermeable membrane called stratum corneum. The epidermis has no blood vessels and no blood supply of its own; depends on the vascularization of the dermis to which the stratura germinativum is adjacent.
Functions - skin:
protective against: i) mechanical injuries ii) invasion by microorganisms iii) loss of moisture iv) predators - poison-producing glands; pigmentation; odor; external armour- scales etc
Thermoregulations physiologically - dissipation of heat via bloodstream (contraction and relaxation of bvs) physically by: evaporation of sweat by mammalian skin for cooling: conservation of heat such insulating materials as oil and fat, mammalian hair, and feathers of birds.
- Excretory organ
supplements the kidney as a mechanism for excretion - most concentrated solute in sweat is NaCl; minerals such as K, Mg, and nitrogenous wastes also appear in sweat to some degree.
- Organ of secretion: (the integument is also an organ of secretion)
a variety of cutaneous glands provides useful products: mucus-secreting glands in fishes and amphibians; oil-secreting glands in mammalian skin; milk-secreting glands of mammals
Skin cont’d - organ of respiration: in fishes and amphibians; in all other vertebrates including human, some degree of gaseous exchange takes (oxygen and carbon dioxide) takes place to some extent.
- as a sense organ
skin serves as a vehicle for an array of cutaneous sense organs by which animals maintain contact with their external environment. these include: receptors for senses of: touch, temperature, pressure, and pain.
The Dermis
in contrast to the epidermis, dermis consists more of cell products than of cells. composed of network of connective tissue fibers derived from the original segmented dermatome in the dermis are: lots of blood vessels, nerves, lymph vessels, smoothe muscle, sense organs, deposits of fats, and array of epidermal and dermal derivatives.
Origin of Skin: all vertebrate skin originated from two embryonic sources: the ectoderm and mesoderm – somatopleure. the entire ectoderm exclusive of the part dedicated to the neural tube, may be regarded as prospective epidermis of the skin.
Origin of the Dermis
the dermis originates from the dermatome subdivision of the epimere. The dermatomal components of the somites resolve themselves into mesenchymal cells, which multiply and deploy to all regions of the embryo.
in mammals and birds, it appears that while the dermatome furnishes the dermis of the dorsal and dorsolateral portions of the body, the dermis of the flank and ventral surface of the body is a derivative of the mesoderm of the somatopleure.
Cutaneous Glands
glands in the skin are either unicellular or multicellular all glands originate in the epidermis unicellular glands: - are modified single cells scatteres among other cells in the epidermis multicellular glands: - formed as ingrowth from the stratum germinativum into the dermis; arc either tubular (straight, coiled, or branched) or alveolar (sacklike) (simple sacs, or moderately or elaborately branched - Fig 10-2)
1. Cyclostomes: unicellular type of gland: mostly mucus secreting giving thick, slimy protective skin coat as found in hagfishes
2. Fishes: both unicellular and alveolar glands
3. Amphibians: skin plays important role in respiration
alveolar mucous and glands predominate in amphibians there are also instances of specialized tubular glands as well unicellular glands also occur in amphibians - on snouts of larval frogs and salamander Functions of these glands include: READING assignment
4. Reptiles and Birds:
skin of birds and reptiles is virtually devoid of glands. existing glands secrete products related to breeding behavior and/or defense. in crocodiles and turtles, a gland beneath the jaw (crocodiles) secrete a substance called “musk’’ in the breeding season; no skin glands in lizard; some snakes have glands in the cloacaI region
In birds, the only gland is the uropygial gland (branched alveiolar) on dorsal side of bosy at base o the tail = supplies oil which the bird collects on its beak and uses in the preening of its plumage.
5. Mammals: have varying types of integumentary glands all derived from sebaceous and sweat glands sebaceous glands ; alveolar type; secrete oil; associated with hairs; their secretions serve to protect and lubricate the skin and hair; imparts an individual scent or odor to the animal; some lubricate the eyeball; some serve as scent glands (located near the anus)
Mammals cont’d
Sweat glands: - simple tubular glands; its watery secretions enables the skin’s function thermoregulation and excretion. Human, the horse and the bear, sweat glands are widespread over the body. Other mammals have no sweat glands at all. In others - rats, cats, mice, glands are confined to specific localities - underside of the paws; in deer - base of the tail; rabbits — on the lips; sheep, cattle, pigs — on the muzzle and snouts
Mammary or milk glands
are modified sweat glands found in all mammals
Embryogeny: first appearance of a pair of thickening of epidermal ectoderm along the ventrolateral body wall these thickening are called mammary ridge or milk line; further thickening of the ridge produces a roughly spherical mass projecting into the subjacent dermis cords of cells then ramify within the surrounding dermis which finally produce a branch-work.
Mammary or milk glands cont’d
In due course the cords hollow out to create a duct system converging at the surface, where an epidermal nipple is elevated,
At birth, no further development occurs in males, but elaboration of the mammae resumes in females with the advent of sexual maturity.
Nipples a) true nipples: rodents = single mammary duct; or carnivores and humans = several. ducts pass directly to the outside. True nipples: single duct in rodent multiple ducts in human & carnivores
Mammary glands cont’d
b) false nipple: or teat : found in horses and cattle: in which the ducts open into a chamber at the base of the projection, from where a secondary tube leads to the exterior and it is from the epithelium along the course of the milk line that the mammary glands arise.
_______________________ In humans, sea cows, elephants and bats = a single pair of pectoral mammae
In cattle, horses = one or two pairs of inguinal mammae appear
In rats, and mice = 3 pairs of thoracic and 3 pairs of abdominal mammaie
In pigs and dogs = five or six pairs are arranged in a continuous series from the chest to groin.
Scales
Epidermal scales:
are derivatives of the stratum corneum, the keratinized surface component of the dermis; well developed in reptiles; occurs in lizards, turtles, crocodiles snakes (devel from the stratum germinativum)
confined to the legs and feet in birds; when occurs in mammals, epidermal scales are found on the tail as in beavers, rats, mice, and opossum. are lacking in fishes: found rarely in amphibians
* Dermal scale: Read in text
FEATHERS
Functions include: For heat insulation; flight; protective coloration; sexual display
Types: i) hair feathers; ii) down feathers: iii) contour feathers
Hair feathers: smallest and simplest variety: scattered widely over the body under the contour feathers.
Down feathers: complex and composed of a short hollow basal quill, buried in the skin, from which they spray a number of filamentous barbs bearing tiny barbules along their margins. Underlying the contour feathers over much of the body, the down feathers provide effective insulation for the adult bird.
Scales cont’d
Contour feathers: the large feathers that ensheathe the body and provide its characteristic configuration. A typical contour feather consists of a lengthwise axis and a flat vane.
FORMATION: first there is aggregation of dermal cells immediately beneath the epidermis.
continued proliferation of the dermal aggregation results in a conical elevation, the dermal papilla, ensheathed by epidermis
thus formed the feather germ grows rapidly outward as a tapering epidermal cylinder filled with a vascular mesodermal core.
the epidermis at base of the papilla sinks in and the feather germ projects from a pit called follicle. From this point onward, down and contour feathers show developmental differences.
HAIR characteristic of mammals, hair is entirely epidermal in origin and composition structure of typical hair:
Compare the origins: Feather is d Dermal; Hair - epidermal
Two portions: projecting shaft and a root which lies in a pit called hair
follicle. At the base, the root expands into a hollow bulb into which a small dermal papilla projects, carrying blood vessels and nerves.
the living cells at the end of the root constantly multiply to lengthen the shaft; and in the process. the cells gradually die so that the hair proper becomes composed of dead, cornified cells.
Formation of hair:
formation of hair begins with epidermis alone unlike feathers whose development is initiated by a preliminary aggregation of dermal cells.
Hair cont’d
at the site of the presumptive hair, an epidermal nodule is formed by a local proliferation of the stratum germinativum (Fig. 10-17\)
the nodule extends down into the dermis as a tongue of tissue whose deeper end becomes enlarged to form a bulb (10-1 7B)
the bulb soon is inverted into a cup into which the vascular dermis and aggregates to form a dermal papilla (10- I 7C).
pushes
Concurrently, the deep-lying portion of the epidermal tongue splits to form a central strand and a surrounding epithelium.
the central strand becomes the root and shaft of the hair; the surrounding tissue provides the wall of the follicle. The follicular wall proliferates to form the sebaceous gland.
as the embryonic hair grows its shaft pushes towards the surface by making an opening through the center of the still solid epithelial cylinder.
Miscellaneous Beaks or bills, of birds: -- horny epidermal sheaths enclosing the bones of the upper and lower jaws constitute bills or beaks of birds. Similar cornification of epidermis on the edges of the digits serve as claws (reptile, birds, some mammals, some amphibians); hoofs (horses, cattle, deer, pigs, etc), and nails (humans and other primates).
The Skeleton
some form of skeleton in all living animals for protection and support all skeletons may be grouped into one category called CONNECTIVE TISSUE composed of varying quantities and combinations of mucopolysaccharide and proteins (collagen)
Connective tissue: - 2 functional groups binding c.t & supportive c.t i) binding c.t. = tendons, ligaments, and fasciae Tendons connect muscle to bones or cartilage Ligaments unit skeletal parts. Fasciae are sheets of c. t. serving to bind constituent cells together into a definitive organ; such as muscle cells into the mass of an individual muscle or nerve fibers into a nerve.
ii) supportive c. t.
= provide the skeleton around (endoskeleton) or within (exoskeleton) which an animal is built.
Skeletal Development
Notochord: present in all vertebrate embryos replaced by the vertebral column and base of the skull, in the adult vertebrates is still present in the lower Chordate adults (cyclostome and elasmobranchs) - as soft flexible rod beneath the neural tube composed of vesicular tissue, external sheath and a surrounding membrane run from the infundibulum of the brain back to the terminus of the fleshy part of the tail.
Skeletal Development cont’d
b) the Cartilage:
is a prominent skeletal material of the embryos of ALL vertebrate.
is a major skeletal material of adults of cyclostome, (chondrichthyes and a few degenerate groups of Osteichthymes. Bone is the major skeletal material in the higher vertebrates.
Is a c.t whose intracellular matrix is composed of complex protein within which there is spread a network of colagenous fibers.
the matrix is a product of cartilage cells (chondrocytes) which lie in small spaces (lacunae) within the matrix.
cartilage is surrounded by a dense c. tissue called perichondriurn, to which tendons and ligaments become attached.
Cartilage exists in several forms viz: a) hyaline cartilage: - homogeneous and translucent matrix with few fibers b) elastic cartilage - the matrix is fibrous and elastic c) calcified cartilage: - contains deposits of calcium salts hard but brittle textured
Skeletal Development cont’d
Formation cartilage is a derivative of embryonic mesenchymal cells. first, embryonic c.t. cells (mesenchyme) begin to aggregate. the branching mesenchymal cells gradually round up and definitive mesenchymal cells (chondrocytes) start secreting matrix. laying down of more and more matrix by chondrocytes forces the chondrocytes
;.
further apart
Skeletal Development cont’d
chondrocytes eventually become isolated in small spaces (lacunae) within the matrix. (fig. 11-2).
division and formation of more matrix by the internal chondrocytes adds to the growth of the matrix and cartilage
Deposition of more matrix from outer surface by the perichondrium supplements the growth of the cartilage.
This ability of cartilage to grow both by internal expansion and external deposition stands in strong contrast to the growth of bone.
c) Bone:
like cartilage, bone is a derivative of embryonic mesenchyme, consists of a matrix within which cells (osteocytes) are buried.
matrix is fibrillar; contains hard, complex mineral subst. composed mainly of crystals of calcium phosphate - Calcium hydroxide combination known as apatite.
Like cartilage, the bone cells (osteocytes) that secrete the matrix lie in lacunae.
Lacunae are linked with one another via canaliculi (a branching network of minute canals into which project the radiating processes of the osteocytes. In cartilage, the lacunae and hence, the chondrocytes. are isolated from each other.
Types:
* intramembraneous or dermal/membrane bone where the mesenchymal precursor is succeeded directly by bone.
intracartilaginous or cartilage/replacement bone = the original mesenchyme first provides a cartilaginous model, which is then destroyed and replaced by bone.
Note: These terms refer only to the sequence of events in bone development, for when fully formed, bones of both types are structurally alike.
Formation:
a) membrane bone:
first, mesenchymal cells congregate at the site of a future bone; cluster and arrange themselves in an interlacing network of strands.
each strand starts to secrete collagen resulting in a framework of collagenous fibers.
each fiber is invested by the c.t. cells that produced it; and deposition of calcium salts begin
the mesenchymal cells, now called OSTEOBLASTS extract the necessary raw materials from the blood supply in the area, and lay down bone salts around the fibers.
This then creates a scattering of bars and plates (trabeculae) of bone.
As more layers are added to the bone, some of the osteoblasts are caught in their own deposits and there, in their individual Iacunae.
In the Iacunae, the osteocytes cease to function as active bone formers; they assume maintenance of the already formed bone.
Osteocytes in lacunae are interconnected by canaliculi; and thus materials picked up by osteocytes adjacent to blood vessels, are distributed throughout the bone matrix.
Osteoblasts Osteocytes Osteoclast
formation of maintenance of breaking down of
the remaining osteoblasts continue to add to the trabeculae; and as they enlarge, come together and fuse into a continuous latticework.
bone in this condition is called cancellous or spongy bone. The area between the trabeculae are occupied by c.t., which is rapidly becomes vascularized; and so constitutes the bone marrow.
mesenchymal cell concentration appears around the primary cancellous bone to form the periosteum.
within the periosteum, certain cells assume the role of osteoblasts and resume deposition of bone.
Bone is laid down in dense, parallel sheets; resulting in an external layer of compact bone investing the cancellous bone within.
physiological maintenance of compact bone is facilitated by an array of canals constituting the Harvesian System.
The compact bone becomes perforated by long branching and anastomosing Harvesian canals.. which communicate externally with the periosteurn and internally with the marrow cavities.
Note that:
i) no fundamental difference between compact and cancellous bone, other than the degree of density.
ii) Both are a product of osteoblasts
iii) both are vital tissues maintained by entrapped but intercommunicating osteocytes.
iv) the Harvesian Systems are not of universal occurrence among vertebrates. Most amphibia, some reptiles, and many smaller mammals do not have them.
b) Cartilage bone
The intramembranous bone (described above) is largely confined with parts of the skull, and the shoulder girdle. Elsewhere, bone is preceded by cartilage
Events of formation are essentially the same as those in intramembranous bone development. Only difference is the added rise and decline of the cartilaginous precursor.
some bone is the product of the osteoblasts brought in by the invading mesensenchyme: some is a product of surviving cartilage cells that transform to osteoblast. ASSIGNED READING - for details.
BONE - AS A TISSUE:
Bone as a tissue is revealed in the formation of both types of bones from embryonic mesenchymal cells; and in the fact of the dynamism of bone - that is, bone is constantly being remodeled (formed by osteobIasts, and broken down by osteoclasts)
serves as the source of supply of calcium and phosphate required for the maintenance of physiological mechanims in the body,
is a reservoir of Ca and P that is drawn upon to satisfy structural requirements:
Bone is the reservoir for Ca and P, which are added to and substracted from bloodstream as required through the turnover mechanism in bone. e.g. In birds calcium for the egg shell is drawn from the bone.
BONE - AS AN ORGAN:
i) flat bones (of the skull) – composed of reduced cancellous area, containing masses of marrow, sandwiched between upper and lower layers of compact bone.
ii) long bones (of the appendages) derived from the cartilaginous model of bone formation.
Bone – structure and growth:
Consists of a long shaft, diaphysis, with epiphysis at each end. Ossification begins in the center of diaphysis and progresses toward each end.
Elongation is provided by steady creation of new cartilage at end of epiphyseal ends; and ossification follows until adult length is attained.
In lower vertebrates, the epiphyses remain cartilaginous throughout adult life.
In mammals and some reptiles, accessory ossifications later appear in the epiphyses, ossification that fuses with the diaphysis only after full adult size is reached (Fig 11-8E).
Irregular bone (of the vertebrae; limb girdles) formation commences with appearance of several primary endochondral centers;
which are supplemented by secondary centers.
The original cancellous bone is replaced in part and supplemented by periosteal deposition of compact bone.
Bone formation and development:
Influencing Factors
Bone formation and development: - Factors that influence: Bone size and proportions is determined by both intrinsic and extrinsic factors:
Experimental results indicate that early primordia possess inherent morphogenetic properties that can and do express themselves up to a point (regardless of environmental influences:
then sooner or later, extrinsic factors come into play in the molding of architectural details and in the attainment of appropriate size and proportions.
Mechanical factors: Muscle: localized growth of bone will cease in the absence of a proper muscle attachment; Size: the size and shape of the skull bones accommodate to the structures they invest: e.g.. Oversized eyes bring an enlargement of the orbital areas of the skull; Anomaly: genetically induced anomaly of a reduced or absent brain is accompanied by skull bone of greatly reduced size and abnormal shape; Abnormally large and thin skull bones accompany an anomalous brain of excessive size.
Nutritional factors vitamins C and D are essential for proper ossification vitamin A excess inhibits bone ossification.
Hormonal factors i) somatotropin hormone (STH) - commonly called growth hormone
Inadequate production of STH (from ant. pituitary) dwarfism of entire body, including the skeleton. Excess production of STH = disproportionate overgrowth of the skeleton.
ii) Gonadal hormones: governs the differences in form and
.
proportion of the skeletal parts in males and females
summary 1.
5.
Connective tissue i) Binding ligaments; tendons; fasciae ii) Supporting Cartilage i) hyaline ii) elastic iii) calcified iv) Features
chondrocytes perichondrium lacunae
3. Bone Features: - osteoblasts formation - osteocytes maintenance - osteoclasts remodeling - lacunae entrapping osteocytes and intercommunicating - canaliculi inter-communication Types: i) intramembranous/membrane/dermal ii) intracartilagenous/cartilage/replacement
The Skull
The ground plan of the vertebrate skull
embryologically comprises three parts viz:
i) the neurocranium (also called = endocranium) includes the capsules bounding the olfactory organs, the eyes, and internal ears, and the box enclosing the brain. Composed of cartilage, replacement bone or both endocranium is composed entirely of cartilage, it may be called chondronium.
ii) The splanchnocranium or visceral skeleton = which is the skeleton of the jaws and gill arches of their derivatives:
- mandibular arch - hyoid arch - gill arches
iii) the dermatocranium, also called surface skull = consists exclusively of dermal bone and are believed to have evolved from the external armour (exoskeleton) of primitive fishes.
The material source of the neurocranium is in the connective tissue membranes that initially envelop the embryonic brain and associated organs. Aggregation of mesenchyme appear and then chondrify into cartilage. This preliminary cartilage will be replaced by bone.
Additional aggregations of mesenchyme ossify directly to provide the overlying dermatocranium.
The spIanchnocranium- condensation of mesenchyme - from which the visceral arches arise are derived from the splanchnic mesoderm investing the pharyngeal gut.
Embryonic primordia of the skull
Speaking in terms of vertebrates and with reference to the neurocranium of the skull cartilage formation is initiated in six pairs of centers viz: i) pre-chordal cartilage ii) parachordal cartilage iii) occipital cartilage iv) orbital cartilage v) quadrate cartilage vi) Meckel’s cartilage
i) prechordal cartilages (trabeculae) ventral (formed beneath the anterior part of the brain) ii) parachordal cartilages - ventral to the brain and flank the notochord iii) occipital cartilage - at the level of transition from brain to spinal cord. The parachordals tend to fuse with occipital cartilages.
The next three centers of ossification are all associated with the developing sense organs: - orbital cartilage - quadrate cartilages - mandibular
iv) orbital cartilage: an auditory (otic) capsule - a cartilaginous; forms around primordial inner ear; medial to each eye and orbital cartilage provides a preliminary sidewall to the neurocranium: a nasal capsule forms around each olfactory sac.
Separate paired cartilages also appear within the mesodermal core of the embyonic viscera! arches, and collectively, constitute the splanchnocraniurn. These paired cartilages include:
v) quadrate cartilages - signifying the increase in the first /mandibular arch - is represented by the paired quadrate cartilages.
vi) mandibular (Meckel’s) cartilages. (a pair in the lower jaw).
Note: In tetrapods, especially in reptiles and mammals the bones of the upper jaw are affixed to the floor and sides of the neurocranium (obscuring the primary cranial bones
the floor of the braincase is formed by the arachnoids and the pre-sphenoid, which extends forward into the ethmoid region
sidewalls of the braincase are formed by the alisphenoid and orbitosphenoid.
the ethmoid complex is associated with the nasal passageways, including, in humans, the distinctive cribriform plate and the turbinate bones.
- the upper jaw is represented by the quadrate cartilage (or bone)
- the lower jaw is represented by the Merkel cartilage.
- Visceral arches I - VII are branchial or gill arches 7
- The dermatocraniurn = consists of longitudinal series of paired bones that provide a ROOF for the neurocranium
A. AMPHIBIAN SKULL:
- skull table is shortened and the snout is greatly lengthened. stabilization of the dermal bones - rostral as occurs in fishes have been eliminated prominent and elongated nasal and frontal bones endocranium is well ossified and is single structure (lacks movable parts)
the basioccipital bears a single condyle that articulates with the first vertebra. only manifestation of primary jaw is the small articular bone at the rear. much of the endocranium remains cartilaginous.
B. REPTILIA SKULL
skull is higher and narrower than that of amphibia some elements of the dermatocranium are eliminated (lost) notable deletions in the roof and sides of the neurocranium are the post-parietal and components of the temporal regions.
important modifications in the cheek bones: - whereas the regions in the stem reptiles and some modern amphibians were solidly roofed over,
in all the other reptiles, one or two openings make their appearance. The number and position of these openings provide the primary basis for the classification of the reptiles.
It was one of the single-opening groups, the Synapsida, that the mammals stemmed.
Unlike the modern amphibia in which there is an increasing tendency toward retention of embyonic cartilage, the reptilian braincase (endocraniurn) remains ossified
Articulation of the lower jaw is similar to that of the amphibian: i.e, the articular bone at the rear of the lower jaw abuts against the quadrate lying in the rear sidewall of the neuroeranium.
C BIRDS (AVES) Skull constriction essentially as those of their reptilian ancestors. - principal variation is the expansion of the cranial roof to accommodate the enlarged brain: - thinning of bones to provide lightness; - considerable variation of individual elements; - snakes and lizards; elongation of the pre-maxilla and nasal roof; - and loss of teeth and the maxilla that housed them.
MAMMALS
There is consistency of skull pattern within the mammals as a whole the nasal, frontal and parietal provide the cranial roof the skull.
In humans and primate, the frontal and parietal have become grossly expanded to accommodate the size of the brain; - the remaining dermal contributions to the roof and sides of the braincase are the squamosals in the cheek areas and the reduced lacrimals in the front of the orbits.
matching
A
1. Binds skeletal parts 2. Binds muscles and/or bones 3. Chondrocytes 4. Osteoblasts 5. Lacunae 6. canaliculi 7. Bone 8. Perichodrium 9. Osteoclasts 10. Tissue
b. c. d. e. f. g. h. i.
B
Cartilage Ligament Bone formation Epiphysis Periosteum Bone remodeling Blood Bone
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