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COMPARATIVE VERTEBRATE MORPHOGENESIS ZOO 4690 FLORIDA ATLANTIC UNIVERSITY @ DAVIE BROWARD CAMPUS
Professor: James Kumi-Diaka (BSc. DVM, MSc, PhD. Vee-dip) Office: Science & Education Bldg. Room 278 Phone: (954)236-1135
Course Objective: 5. To expose students to the concept and principles of developmental biology (embryology and gross anatomy) with regard to structural and functional physiological development in vertebrates. Course Outline: 7. Introduction: historical perspective; phylum chordata; its characteristic features; concepts of homology and analogy. 8. Vertebrate phylogeny: classification. 9. Gametogenesis: definitions; nuclear and cytoplasmic; cell divisions; purpose of. 10. Fertilization: What is it?; purpose of; the process of; end-product of; parthenogenesis. 11. Early development: cleavage/segmentation holoblastic and meroblastic cleavages; formation of morula and blastula. 12. Gastrulation: the formation of gastrula; the process involved; formation of primary germ layers; extraembryonic membranes and their role in development.
Course Outline cont’d 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Induction: principle of; physiological significance; classification of. Placentation: amniotes and anamniotes; classification; functions of. The integument: components of: structure and derivatives of. Skeletal system I: components of; skull development; morphology. Skeletal system II: origin and developmental anatomy of vertebrae. Skeletal system III: appendicular skeleton, origin, development and morphology. Musculature I: muscle origin; fate of myotomes: classification; histoanatomy and topography. Muscular system II: development of brachiomeric & appendicular muscles. Development of coelomic cavities, and mesenteries. Respiratory system: evolution, morphogenesis. and derivatives of; classification. Digestive system: evolution and morphogenesis, derivatives of, classification of. Urogenital system I: kidney structure; morphogenesis; classification; primordial gonads. Urogenital system II: reproductive system; embryogenesis and morphoanatomy. Cardiovascular system I: circulatory system; morphogenesis of extraembryonic circulation. Cardiovascular system II: aortic arches and their derivatives. Cardiovascular system III: the heart - its embryonic origin and
Course Outline cont’d 1. 2. 3.
Nervous system I: morphogenesis and histoanatomy of neural tube (brain and spinal cord). Nervous system II: development of autonomic nervous system, spinal nerves, spinal cord. Nervous system llI: cranial nerves, special senses, olfactory, optic, otic, etc.
TEXT BOOK(S):
Any of the following text books will be OK
1. Analysis of Biological Development –Klaus Kalthoff, 2nd Ed. 2. Patten’s Foundations of Embryology 6th Edit, Bruce M Carlson 3. Principles of Development 2nd Edit, Lewis Wolpert
EXAMINATIONS; Exam I = 02/04/08 Exam II = 03/12/08 Exam III = 04/21/08
100% 100% 100%
EVALUATIONS: A >90%; B = 80-89%; C = 70-79%: D = 60 – 69%: F = < 60
PLEASE NOTE: i) All make-ups exams will draw 15% penalty ii) Exam results will NOT be given over the phone (do not). III) THERE ARE NO EXTRA-CREDITS
–
IV) NO CELL PHONES ALLOWED DURING EXAMS
PENALTY
25%
EMBRYOLOGY
Zygote - all higher animals start their lives from a single cell zygote - dual origin from two gametes – spermatozoon + ovum
Ontogeny - The time of fertilization represents the starting point in the life history - Ontogeny refers to the individual’s entire life span.
German biologist August Weismann (1834-1914) made the important distinction between the soma (body) and the germ-cell line (gametes). - thought that the germ-cell line was all-important for perpetuation of the species, and that the soma was primarily a vehicle for protecting and perpetuating the germ plasm. This viewpoint provides a convenient framework for looking at the perpetuation of life (Fig. 1-2).
Once an individual has passed the reproductive years, the remainder of its ontogeny does not provide direct physical input into the generative
Historical perspective
The origin of the embryo: Aristotle’ Theories i) the
Preformationist concept - everything in the embryo was preformed from very beginning and dimply got bigger during
– tiny
development - the sperm contained the embryo (homunculus human in the head of the sperm)
ii) Epigenesis (means ‘upon formation’) - new structures arose progressively during development
FUNDAMENTAL PROCESSES AND CONCEPTS IN DEVELOPMENT Cell Division and the Cell Cycle Is the fundamental properties of living systems, of vital importance
Cell division in vertebrates takes two forms—mitosis and meiosis.
Mitosis (for review, see Fig. 1-14) is the standard form of cell division in somatic cells and it results in two genetically equal daughter cells.
Meiosis (Fig. 3-7) is confined to certain stages of development of gametes Mitosis is one component of the cell cycle.
The life history of a cell can be divided into four periods (Fig. 1-15). Immediately after mitosis and the separation of the dividing cell into daughter cells, the G1 (gap I) period, often called the interphase commences. Its length is extremely variable. In rapidly cleaving embryos just after fertilization, the G1 phase is very short and sometimes may not even exist. At the other extreme, the G1 phase of mature neurons persists as the G0 phase throughout the remainder of the life of the cell because further cell division does no occur.
LIVE - THE ORGANISM
Protoplasm: The complex substance (organic and inorganic) of which all cells are composed. Protoplasm exists solely in the form of organisms and could only be spoken of in term of living things. The essence of the principle of evolution states that all organisms have arisen from common ancestry through a gradual process of change and diversification. However, it is generally held that the physical and chemical composition of the earth’s surface and atmosphere are no longer amenable to the de novo creation of life, it therefore follows that all present day organism have arisen from preexisting
Individuality: Vast differences between individuals in any given population. Differences are due mostly to genetic constitution: that is, among the individuals of a given population, two or more alleles occur at a large proportion of the gene loci i.e., although same numbers and kinds of genes in their chromosomes, the genes may occur in many alternate forms. Variations residing in the gene pool of a population provide the potential for evolutionary change.
The Gene:
Consists of DNA, an organic molecule composed of two long, twisted chains of structural units called nucleotide.
Each nucleotide unit is composed of the sugar deoxyribose, phosphoric acid, and a nitrogenous base of which there are four different kinds; viz: adenine, thymine, cytosine, and guanine (A,T,C,G).
Therefore there are four different kinds of nucleotide.
The nitrogenous bases in the nucleotide can fit only in four combinations: A-T; T-A; G-C; C-G.
The order of these base pairs varies, and the specificity of any part of the DNA molecule, depends upon this order.
Definitions
Chromatin- replicated DNA and assoc. proteins Chromatids- condensed chromatin Chromosomes- two sister chromatids attached by centromere Kinetochore- protein band along chromosome around centromere Mitotic spindles Kinetocore- fibers from centrosome attaching to kinetocore Polar- fibers extending centrosome to opposite centrosome Astral- fibers radiate outward from centrosome
Mutation:
Any changes in the code of nucleotide pairs in the DNA molecule will result in a corresponding change in the protein it codes and, by extension, in the cells and even the organisms of which the protein is a part.
Such alteration of the DNA molecule constitutes mutation. * The number of alleles at a gene locus is increased only through mutation.
Mutations are the sole source of new genetic variability and therefore lie at the root of evolutionary potential. It is genetic variability within the gene pool that is the raw material of evolution. For instance, fertilization, which is the union of reproductive cells, provides for a new and unique combination of genetic propensity.
Primary Terms in Embryogenesis:
REF: glossary – back of the text book:
*Embryology – the study of the embryo.
*Ontogeny – The developmental events or history of an individual that bring it into being: the totality of developmental operations.
*Embryogeny – origin of the embryo: the segment of ontogeny that precedes hatching or birth.
*Phylogeny – the succession of forms that culminates in any given structural entity is the evolutionary history or phylogeny of that entity. The term applies to the total body form or any part thereof.
*Homologous and Analogous – refer to structural and functional similarities:
Homologous – equivalent parts in the sense of having a common ancestry and regardless of structure and function are said to be homologous, or are homologues.
Analogous – parts having similar functions whether homologous or not, are said to be analogous, or are analogues.
Early embryonic stages of vertebrates reveals close likeness to fully formed fishes, amphibia and reptiles. However, as development proceeds the embryos of these different animals become more and more dissimilar
The Vertebrate Evolution
and Development
Modifications of development new features specific to classes/species
THE VERTEBRATE THE CHORDATE - features:
All vertebrates are chordates; but not all chordates are vertebrates.
Vertebrates share certain features with some nonvertebrates - features possessed by all chordates:
n
Notochord – lies along the dorsal midline; provides axis support for the body; present in embryos of all chordates: persists throughout life in some forms: In most chordates, notochord is transient & is replaced by the vertebral column.
n
Hollow neural tube – dorsal and above the notochord; the central nervous system of chordates.
n
Pharyngeal clefts (slits) – passageways from the pharyngeal portion of the digestive tract to the exterior: - respiratory organs (gills)carries over to adult life in aquatic chordates, and serves as - Only transient existence in non-terrestrial chordates
n
Sub-pharyngeal gland – not as salient a feature as (13); - Located ventrally in the pharynx; - binds iodine and related substance: - called endostyle in primitive chordates: - in vertebrates, its homologue is the thyroid gland. In addition to these exclusively diagnostic characteristics, most chordates also exhibit the following features:
n
Tail – rearward extension of the terminal opening of the digestive tract.
n n n
Liver – lies ventral to the digestive tract, and kidneys (distinct tubular unit) rest dorsal to the digestive tract Heart – ventral to the digestive tract; pumping blood via a closed system of vessels. Internal skeleton (over and beyond the notochord) – provides support and protection.
Other features shared with other animal phyla and are thus not exclusively diagnostic:
n
Bilaterally symmetrical body – right and left are mirror images of each other.
n
Metamerism or segmentation – serial repetition of such structures as nerves, blood vessels, muscles and certain other parts.
n
Coelom – all chordates have a true body cavity, or coelom.
Fig 2-2 reveals the diagnostic characteristics of a true vertebrate (FIG-2).
* All of the non-vertebrate chordates are commonly referred to as PROTOCHORDATES.
CLASSIFICATION OF VERTEBRATES A. Mammalian -muscular diaphragm for breathing; -a four-chambered heart; -a single aorta; -two occipital condyles; -nourish young by lactation -has functional placenta (except in the egg-laying it.
mammals) to nourish the fetus and remove waste products from
A.
Aves
-
- 4-chambered heart - endotherm (warm-blooded) - attainment of high and constant metabolic rate - internal thermogenesis heat is derived from within the body, - characterized by feathers and adaptations for flight - uses yolk in eggs to nourish the young/embryo
C) Amphibia - for the most part semi-aquatic; -
are ectothermic. mostly egg-laying Includes: frogs, toads, salamanders.
B.
Pisces
– various classes of fishes - all built around a common blueprint as a superclass Pisces. 4 major classes of the Pisces: 1. Agnatha - jaw1ss ; extinct except for the lampreys and hagfhes 2. Placodermi – armored , jawed fishes; 3. Chondricthyes – modern fishes, the sharks, skates and rays; 4. Osteichthyes – advanced bony fishes which constitute half the living species of vertebrates.
overview THE
VERTEBRATE:
Pisces amphibia reptilian aves mammalia Are chordates –> identical anatomic features
Same/common developmental process
EMBRYOGENY
Since all animals are related in some degree, it is possible to sketch an outline of the early stages of development that applies to all classes of vertebrates.
The basic events involved in development are:
n n n n n
Gamete maturation – gametogenesis Fertilization Cleavage Gastrulation Organogenesis organ system
overview n
Gamete maturation : - gametogenesis spermatozoa + oocytes (both haploid) - a period of chromosomal reduction from diploid to haploid in both spermatozoa and ova.
n
Fertilization - gametes zygote (unicellular/diploid) fertilization initiates two events viz: – Activation of the ovum to start dividing i.e. initiates development, and – Establishment of diploidy - union of the two haploid gametes.
overview c) Cleavage - a period of segmentation of the zygote - series of mitotic divisions blastula (multi-cellular) d) Gastrulation - further mitotic divisions of blastula gastrula establish primary germs layers: ectoderm (outer); endoderm (inner); and mesoderm (middle). e) Organogenesis organ system formation
EMBRYONIC ORGANIZATION: The Ectoderm - the outer strata of the embryo represents the embryonic skin. The Endoderm - inner stratum is the future lining of the
GIT; the space it encompasses is the cavity of the GIT, the archenteron.
The
Mesoderm - between skin and embryonic gut; Neural Tube -ectodermal in origin; along dorsal axis of the embryo;
formed by invagination of the neural plate and joining of the neural folds in the midline; ; anterior portion will form the brain; the remainder will form the spinal cord of the adult.
overview
The neural crest - arise from cells within the neural folds; - and occupies either side of the neural tube; - derivatives include - ganglia of the cranial and spinal nerves: - some of the cells migrate to parts of the body to form different tissues including the sympathetic nervous system (SNS), sensory ganglia of cranial nerves V, VII, IX and X; adrenal medulla, pigment cells; teeth parts of the skull.
The neural crest
Is Ectodermal in origin Adult derivatives:
- ganglia of the spinal nerves - sympathetic nervous system (SNS), - sensory ganglia of cranial nerves V, VII, IX and X - adrenal medulla - teeth - parts of the skull. - pigment cells
The Notochord and Mesoderm - notochord is derived from mesodermal cells that segregate from median dorsal mesoderm; lies between the gut floor and the developing neural tube above. The lateral mesoderm spreads ventrolaterally and the spreading sheets on either side meet in the ventral midline. At same time these mesodermal sheets split into two layers: – splanchnic mesoderm and – somatic mesoderm
overview
The cavity between them is the coelom - a true cavity bounded by an epithelium of mesodermal origin.
The splanchnic mesoderm unites with adjacent endoderm to form the splanchnopleure.
The somatic mesoderm unites with ectoderm to form the
.
somatopleure
Dorsoventral division of the mesoderm into:
n
Epimere - most dorsal and flanks the neural tube and notochord; gives the following derivatives – sclerotome, dermatome and myotome;
n
Mesomere - narrow; between the epimere and hypomere;
n
Hypomere - most ventral and flanks the gut.
Note:
When we speak of an organ or part of it being derived from a given germ laver, we have in mind only the intrinsic functional portion of that organ; for there is no structure in the animal body that is the product of a single germ layer exclusively.
For illustration: e) The intestine is considered to be endodermal in origin, yet only its secreting and absorbing interior lining is so derived. b)
The muscles, peritoneum, connective tissues, blood vessels, and nerves, which constitute the bulk of the GIT, are derived
.
from mesoderm and ectoderm
overview a)
In normal development there are instances in which the cells of one germ layer give rise to structures customarily farmed by those of other germ layers.
c)
The developing tail of vertebrates is illustrative.
e)
The muscles of the tail are a product of ectoderm, whereas muscles in general are mesodermal in character.
g)
Further, the skeleton is ordinarily derived from mesodermal connective tissue,
i)
yet the gill skeleton is produced by neural crest cells originally associated with the nervous system.
* Yet the organ-germ layer concept is still useful as a descriptive scheme of classification and identification. Furthermore, there is a high degree of homology between these layers and the products provided by them. What are the Components of Development?
Development - a series of changes culminating in the formation of a complex organism from relatively small and simple, one cell organism. Includes: i) Growth; ii) morphogenesis; iii) differentiation
overview n
Growth - developmental increase in mass (synthesis of new protoplasm - cytoplasm, nucleic and cytoplasmic products) accompanied by cell division which is characteristic of growth.
n
Morphogenesis - generation of new form; the
organized movements of multiplying cells and the attendant physical reshaping of areas. A sheet of cells may undergo delamination (splitting) into two or more layers; a cord may hollow out (undergo cavitation); there may be folding, invagination (inpocketing), evagination (outpocketing).
n
Differentiation - encompasses a host of
operations culminating in increasing diversification of form and function. Events by which cells and other parts become different from one another and also different from what they were originally.
* Morphological differentiation - as they multiply, cells become structurally different from other cells. E.g. from a general ectoderm --> nerve cells and epidermal cells acquire distinguishing features of size, shape, and internal architecture.
*
Behavioral differentiation -
*
Chemical differentiation - as an
acquisition of special functional capabilities: e.g. nerve cells come to transport electrical disturbances; muscles to contract; gland cells to secrete special products. etc. illustration, consider the egg. An egg has complex chemical composition. As it cleaves and progressively become a blastula, gastrula and embryo, individual cells and areas of cells become biochemically different from one another. The process by which this fixity of developments prospect is acquired is called DETERMINATION.
overview Differentiation
- could therefore be redefined
as the series of events by which materials become chemically determined and then assume distinctive form and function.
THE GAMETES:
In all vertebrates reproduction is effected by sexual means with the use of specialized germ cells called gametes - spermatozoa in the in
.
male and ova in the female
The Primordial Germ cells: There are different concepts about the origin of the germ cells found in the adult gonads.
n
One doctrine states that the PGCs are merely special endodermal wandering cells, having nothing to do with reproduction; and that gamete formations is a matter of differentiation of somatic cells within the gonads.
i.
Another doctrine: - PGCs are extragonadal cells gonads gametes.
In vertebrates, identification of the PGCs comes later in development ; are external to prospective gonads and external to embryo when first recognized
Most recent notion: - PGCs are the sole progenitors of the gametes.
iv.
ii.
How do the PGCs reach the presumptive gonads? Extra-vascular/mesenteric route: The PGCs may migrate, following cell surface signals such as fibronectin in the dorsal mesentery and peritoneum into the region of the developing gonads (genital ridge). The vascular route: PGCs may go through the vascular system to the genital ridge. - birds and reptiles
Developmental Period Stages Embryogenesis Stages of embryogenesis: fertilization cleavage gastrulation organogenesis fertilization histogenesis (differentiated tissues) A.
overview n
Fertilization - union of egg and sperm. Classification of egg: micro-, meso-, and macrolecithal. One polarity axis - animal-vegetal axis.
Fertilization establishes: Stimulation of egg Diploidy (2n) Fig 1.6
9.
Cleavage Definition Classification Blastomeres End result of cleavage – Fig. 1.7
n
Gastrulation - involves movement of cells. - Morphogenetic movements - cells rearrange; migrate; spread; bend and fold forming a gastrula. - End-product of gastruIation 3 germ layers. ectoderm mesoderm endoderm
READING
– text book: Chap 12 - migration of the germ cells - fertilization - gametogenesis spermatogenesis +
oogenesis
Gametogenesis
The totality of preparatory events leading to the formation of the haploid gametes from diploid gonials through the process of mitotic and meiotic (reduction) divisions.
Spermatogenesis
Occurs in the seminiferous tubules in the testis
A.
Spermatogenesis: i Spermatocytogenesis - involving mitotic and meiotic divisions of the spermatogonia through formation of spermatocytes and spermatids. ii Spermiogenesis (spermateliosis) morphogenesis of the spermatid to yield the characteristic spermatozoa ; no cell division involved. iii Spermiation - release of spermatozoa into the lumen of the seminiferous tubules iv Emission
spermatogenesis Spermatogonia Spermatocytes –primary - secondary Spermatids - round -elongated Spermatozoa
The Spermatozoa Sperm - great morphological diversity between kinds of vertebrates.
Head and tail (neck-midpiece-principal piece/tail-endpiece).
Head - spheroidal in teleosts; rod or lance-shaped in amphibia; spirally twisted in passerine birds; spoon-shaped in humans and many other mammals; hooked in mouse and rats.
Serves two functions: genetic (in the DNA), and activating
.
Mitosis and Meiosis
Spermatogenesis
Spermatocytogene sis Spermiogenesis Spermiation Emission Ejaculation
Spermatogenesis
Spermatogenesis starts in testes in seminiferous tubules Spermatogenesis starts in outermost layer of tube and daughter cells move towards lumen.
Male Reproductive Tract Sperm pass through
Seminiferous tubules Epididymis Vas Deferens Ejaculatory Duct Urethra Penis (within)
Seminiferous Tubules
In testes Very convoluted: if stretched, 1.5 miles! (in one testicle only) Sertoli cells= Sustentacular cells: sustain sperm Leydig cells= Interstitial cells: make androgens
Functions of Sustentacular Cells
Secretion of Müllerian-inhibiting factor Support of mitosis and meiosis Support of spermiogenesis Secretion of androgen-binding protein Secretion of inhibin Maintenance of blood-testis barrier
Seminiferous Tubules
Spermatozoon
Head Neck Tail Lacking many intracellular structures At this step, can’t fertilize or move coordinately
A.
Oogenesis:
The totality of the processes involving mitotic and meiotic divisions of the diploid oogonia, culminating in the formation haploid ovum, and nonfunctional polar bodies.
The Vertebrate Egg: The nutritive substance in all eggs is the yolk. Yolk is primarily produced in the liver, and is transported in a soluble form via the bloodstream to the ovary. In the ovary, the yolk is transferred by ovarian cells that surround the oocyte and is deposited as platelets or granules. The vertebrate is classified according to amount of yolk in the cytoplasm.
Oogenesis
Step 1 Oogonia are small cells that increase in number by mitosis When they enlarge slightly, they are considered primary oocytes
Primary Follicle Oocyte enclosed by Follicle cells Primary oocytes become enclosed by a single layer of follicle cells Follicle cells from the surface layer of the ovary Primary follicle is the structure including a primary oocyte enclosed in follicle cells
Oogenesis At puberty, the primary follicles enlarge and accumulate yolk caused by
rapidly the
follicle stimulating hormone (FSH) from the gland
pituitary pushed to
which is called the
As the yolk increases, the oocyte is one side as a small disc The disc is called the blastodisc The blastodisc contains the enlarged nucleus of the primary oocyte germinal vesicle YOLK
BLASTODISC
Oogenesis
First meiotic division of the primary oocyte occurs just before ovulation The result is the first polar body pinching off and the secondary oocyte The secondary oocyte contains most of the yolk and the blastodisc
Fertilization The second meiotic division begins but stops at metaphase The secondary oocyte (sometimes called the ovum at this stage) is mature enough to be fertilized and Ovulation occurs
Oogenesis/Fertilization
The secondary oocyte (ovum) is released from the ovary as the follicle ruptures It enters the infundibulum of the oviduct where it will encounter sperm cells As sperm penetrates the blastodisc, meiosis begins again and the result is the second polar body and the mature ovum
READING
– text book Chapter 8 - fertilization - cleavage amphioxus xenopus chick sea urchin - gastrulation
Cleavage and Blastulation Cleavage-
Occurs in eggs activated by fertilization. Cleavage is the subdivision of eggs into cells called blastomeres. The blastomeres divide quantitatively into smaller units until they are of such a size that they can readily undergo the subsequent events of blastulation, gastrulation, and interaction that are involved in formation of tissues and organs
Cleavage & Blastulation
The blastodisc initiates cleavage
First cleavage occurs in the zygote by way of mitosis and the result is two cells called blastomeres Cleavage occurs in the cytoplasm, not the yolk Second cleavage = 4 blastomeres Cleavage (mitosis) continues until the blastula forms
Blastula Blastulation •The formation of a segmentation cavity or blastocoele within a mass of cleaving blastomeres. Rearrangement of blastomeres around this cavity forms the type of definitive blastula, characteristic of each species •The definitive blastula is thought to terminate cleavage stages •The wall of the blastula is a mosaic of cellular areas, each of which will normally produce a certain structure during subsequent development. In other words, each area of cells in the wall of the blastula has a certain prospective fate which will be realized in normal development.
The Vertebrate Egg: Nutritive substance in all eggs is the yolk. Yolk is produced in the liver transported in a soluble form via the bloodstream to the ovary.
In the ovary, the yolk is transferred by ovarian cells that surround the oocyte is deposited as platelets or granules.
Classification of egg: The vertebrate is classified according to amount of yolk in the cytoplasm: microlecithal: mesolecithal: macrolecithal
Eggs:
n
Microlecithal - small amount of yolk, scattered evenly in cytoplasm (Amphioxus; tunicates; eutherian mammals) Mesolecithal - relatively larger amount of yolk in cytoplasm; not quite evenly scattered (Amphihia; dipnoi; Petromyzontia). Macrolecithal - enormous amount of yolk as food reserve (Myxinoidea; Chondrichtyes; Osteichthyes;reptiles; and birds).
n
n
*
Telolecithal eggs (mesolecithal and
macrolecithal) - where yolk is concentrated in one hemisphere the vegetal pole of the egg (greatest concentration of yolk), than the animal pole (smallest amount of yolk).
* The amount and distribution of yolk influences the pattern of cleavage and movement of cells and tissues during gastrulation.
Fertilization
Union of the ovum and spermatozoon to form the zygote.
Occurs in fluid medium
- All the events involved in
fertilization (internal and external) take place in a fluid medium.
Barriers encountered by the spermatozoa include: cumulus oophorus; corona radiata (encasing the ova); zona pellucida; the vitelline membrane; mucus in the genitalia (internal fertilization).
In some vertebrates (mouse) the zona pellucida contains glycoprotein (zona glycoprotein) which acts as receptor for the binding protein of the sperm membrane.
The acrosome produces hyaluronidase (lytic enzyme) which dissolves the egg barriers.
The acrosome of many different kinds of sperm have been shown to produce a specific protein called bindin, which reacts with bindin receptors (glycoprotein) on the ovum’s vitelline membrane on contact, thus attaching the tip of the acrosome to the membrane.
Acrosomal Reaction:
Morphological changes and biochemical events undertaken by the spermatozoa in order to acquire fertilizing capability.
Identified in several vertebrates and invertebrates.
Capacitation: Sperm of several species require a period of exposure to compounds - glycosaminoglycans (GAGs) in the female genitalia before the bindin reaction can occur. This preparation is part of what is known as sperm
.
capacitation
Steps in AcR
n
Disappearance of the Acrosomal membrane at the apical region. The Acrosomal and sperm membranes become continuous (Fig 5-2A). At the same time, disintegration of Acrosomal granule, as does the portion of the egg membrane exposed to the granule (signifying release of lytic enzymes).
n n
d) Indentation of that region of the acrosomal membrane abutting the nucleus (arrow in 5-2A). e) Elongation and deepening of the indentation, thus creating a steady elongating Acrosomal filament that traverses the egg envelope and finally approaches the plasma membrane of the ovum (5-3C). f) Zygote Formation: Establishes definitive contact between the two gametes (with the meeting of the sperm and egg plasma membranes - one continuous membrane. The contents are now essentially components of one cell - the zygote.
b)
Activation of : Contact of the acrosome with the plasma membrane of the egg activates the otherwise quiescent egg into action:
Polyspermy: Definition of: Significance of : Blockage of polyspermy: Block polyspermy: n
–
a) Fast block to polyspermy: - contact between acrosome and ovum membrane results in electrical charges change in membrane potential across the surface of the egg, immediately preventing further sperm penetration of the egg. b) Slow block to polyspermy: - cortical reaction; I cortical vesicles discharge cortical granules to fuse with egg plasma membrane chemical alteration in the membrane I inactivating its bindin receptors releasing any other sperm that had attached to it thus preventing further fertilization. This constitutes the slow block to polyspermy.
i.
Consequences of sperm penetration: a) Establishment of the plane of bilateral symmetry sperm penetration of ovum rearrangement of its internal constituents leading to the establishment of the plane of bilateral symmetry of the embryo to come. b) Alterations in the physiological properties of the egg; changes in permeability ; and alterations in the rate of metabolism; ovum becomes a truly dynamic entity and embarks on physicochemical transformations that lead into the period of cleavage.
Establishment of Diploidy - union of the haploid nuclei: Fertilization establishes/restores the diploid number of chromosomes for the new organism. Fertilization restores diploidy which represents a new combination of hereditary prospects. In mammals and most other vertebrates, each pronucleus (male and female) loses its membrane, and concomitantly its chromatin resolves into a haploid set of chromosomes. The two set of chromosomes arrange themselves across the spindle. This arrangement, marking the readiness for the first cleavage division, represents completion of the process of fertilization.
* Fertilization is completed with formation of the zygote and
.
initiation of cleavage
Stages in the Events of Fertilization
Fertilization-sequelae - Summary
1. Prevention of polyspermy via: - fast: changes in electrical membrane potential - slow: cortical reaction (granules fusing with egg membrane 2. Establishment of diploidy 3. Establishment of plane of bilateral symmetry in presumptive embryo 4. Establishment of dynamic entity of the ovum physiological transformation initiation of cleavage
PARTHENOGENESIS
Fertilization of the ovum to the zygote without spermatozoa.
This indicates that activation and nuclear phases of fertilization are essentially distinct; that development does not absolutely require a union of egg and sperm.
The eggs of every animal group, including mammals, can be started on a course of development, often to the completion of a new individual, by a host of chemical and physical agents.
This strongly suggests that the ovum possesses in itself all the capacities to form an embryo, needing only some agents to trigger the action.
In normal development, the spindle for the first cleavage division is assembled around the centriole brought by the sperm.
What is the source of the centriole in parthenogenesis? This may reside in the egg. In some cases, spindle appear to arise spontaneously.
The easiest way to induce parthenogenicity in the frog is to stab the egg with a very fine needle. A method that is effective only if the needle carries
some foreign protein into the egg. The protein provides a center around which the cleavage spindle arises
Although parthenogenesis occurs naturally in some vertebrates (some birds and amphibians), it rarely occurs in others such as the whip-tailed lizards (Cnemidophorus uniparens), all the adults of which are females.
Recent experiments have shown that elements of both male and female genomes are necessary for full development in mammals.
Expectedly, the chromosome numbers in parthenogenetic individuals are haploid. In some cases, there is tendency for the restoration of diploidy via: fusion of haploid cleavage nuclei prior to cell division; fusion of egg nucleus with a polar body nucleus before cleavage begins.
Advanced or complete development occurs only if regulation to diploidy is accomplished: haploid individuals generally succumb in early embyonic stages.
Cleavage Eggs – Types: Isolecithal - small amount of yolk evenly distributed; mammals. Mesolecithal - moderate yolk in vegetal pole; amphibians. Macrolecithal – large amount of yolk in the egg or Telolecithal - large yolk filling almost entire egg; fish, reptiles, birds. Centrolecithal - yolk concentrated in the central cytoplasm; insects, arthropods.
Cleavage Pattern
Holoblastic - entire egg cleaves during cytokinesis. – Complete Holoblastic blastomere. – Partial Holoblastic micromeres +
macromeres Meroblastic - characterized by partial cytokinesis; two types:
Discoidal cleavage - restricted to animal
pole, yolk-free area. *Vegetal pole remains uncleaved in telolecithal eggs.
Amphibian Cleavage From 128 cell stage = blastula Several layers thick; Larger blastomeres in vegetal pole – macromeres. Smaller blastomeres at animal pole – micromeres.
Mammalian Cleavage
Cleavage Cellular Mechanism
Cleavage Cellular Mechanism Experimental evidence:
In isolecithal eggs - mitotic spindle is centered cleavage furrows form all around the circumference.
In mesolecithal eggs - mitotic spindle is displaced to the animal pole - the furrow first appears in animal pole then cuts towards the vegetal pole.
In telolecithal eggs - mitotic spindle is eccentric the cleavage furrow forms only near the mitotic apparatus.
Cleavage & Gastrulation Cleavage - general consideration:
Series of a regular rhythm of mitotic divisions of the egg following activation either by the spermatozoon or by parthenogenetic agent, and ending with formation of a blastula.
The individual cells during the process of segmentation are called blastomeres.
During the period of segmentation there is no increase in cytoplasmic mass (embryo does not grow); segmentation leads to increase in the number of blastomeres but not the size; ; there is no growth during these mitotic divisions as the G1 and G2 phases of the cell cycle immediately before and after DNA synthesis are eliminated.
With each cell division there is additional DNA synthesis and protein, to provide for the increased nuclei, at the expense of cytoplasmic reserve nuclear gain is balanced by cytoplasmic loss: leading to numerous small blastomeres each with its nucleus and cytoplasm.
Since the area of constriction that divides the cytoplasm (cytokinesis) is organized by the centriole of the cell, the plane of cell division is always at right angle to the long axis of the spindle
Variations in Cleavage Pattern: b.
Amount of yolk in egg cytoplasm Influences the rate and pattern of cleavage division; the greater the amount of yolk and the more unevenly distributed the yolk, the more the mitotic spindle in the egg and later in the blastomeres are displaced from the center of the cell toward the less yolky ends; and upon division, blastomeres of unequal size result. In an egg with most heavily laden yolk (birds), segmentation is confined to a small region (blastodisc) at the animal pole.
a.
Cytoplasmic organization Exemplified by the spiral cleavage in annelids and mollusks - where the rotational movement of blastomeres resulting from alternately tilted obliquely to the right and left, so that successive generations of blastomeres are oriented in a twisted fashion.
Summary – cleavage pattern n Holoblastic (total) Cleavage: The whole egg divides as do the blastomeres: – Equal holobalstic: In microlecithal eggs (mammals) - blastomeres of equal sizes – Unequal holoblastic: In mesolecithal eggs (amphibians) – blastomeres of unequal sizes
n
Partial Cleavage: (Teleost fish) part of egg remains undivided; in moderately macrolecithal eggs
n
Meroblastic Cleavage: (reptiles; bird & primitive mammals) - extremely macrolecithal eggs –segmentation is confined to small area at the animal pole.
GASTRULATION – commences at the conclusion of segmentation & formation of the blastula
Gastrulation -
GASTRULATION
is by a process of rapid cell multiplication by which the blastula is molded into a stratified structure - gastrula involves flattening and invagination (inward folding) of the
prospective endodermal plate into the blastocoel
the new cavity is called gastrocoel (archenteron) which opens to the exterior by the blastopore
further migration of cells and elongation of the gastrula establishes two layers: a) an outer ectoderm, neural and epidermal, and b) inner cell mass encompassing prospective notochord, mesoderm and endoderm
the organs of the body are all derivatives of these three primary germ layers.
Note:
In amphoxius, the mesodermal pouches are hollow from the beginning which is to say that they contain a coelum.
Thus the Amphioxus features an enterocoel, (the coelomic spaces trace their origin to the cavity of the primitive gut).
The vertebrates feature a schizophoel - a coelom formed by the splitting of an originally solid layer of mesoderm.
In each case the end result is the same: somatic mesoderm associated with epidermal ectoderm to form somatopleure; and splanchnic mesoderm associated with endoderm (endodermal gut) to form splanchnopleure.
AMPHIBIAN
- FROG
eggs are mesolecithal - moderate amount of yolk present in form of oval granules scattered throughout the cytoplasm, but show gradual concentration to one pole; telolecithal - greater concentration of yolk located at the vegetal pole. Cleavage: follows the pattern seen in the amphioxus with minor differences:
Gastrulation
(cont’d)
involves invagination, involution and stretching and spreading of cells of the blastula to form a spherical gastrula surrounded externally by ectoderm and containing endodermal and mesodermal components internally, comparable to the situation in amphioxus.
throughout gastrulation the embryo retains its spherical shape and a uniform size.
endodermal ridges meet in the dorsal midline and unite to create a tubular endodermal gut.
mesoderm in the dorsal midline is set off as a definitive notochord, separate from the presumptive epimere, mesomere and hypomere the forerunners of other organs.
the overlying ectoderm would give rise to neural tube.
Avian
first cleavage division is an irregular shallow furrow in the central portion of the blastodisc.
this furrow does not cut completely through the depth of the disc; and this results in two incompletely separated blastomeres.
the second division occurs at right angle to the first but does not cut entirely through the depth of the disc.
further incomplete divisions result in irregular pattern of cleavage, creating two groups of blastomeres: a) completely bounded central blastomeres and b) incompletely bounded marginal blastomeres.
further divisions of the blastomeres create a stratification of cells. This results in a compact plate several cells thick; which is called the blastoderm.
the bulk of the yolk is external to the blastoderm (which still contains some yolk cells).
The bilaminar blastoderm represents the blastula.; the epiblast represents the animal half; and hypoblast (endoderm) represents the vegetal half; and the intervening space, represent the blastocoel.
Gastrulation the hypoblast (endoderm) is already present in part at beginning of gastrulation.
Major differences in the fate of embryonic cells of avian and amphibian?
i) formation of extraembryonic organs from the blastoderm to support embryo prior to hatching
The epiblast of the entire area opaca is destined to provide extraembryonic ectoderm.
The epiblast (upper layer) is the presumptive ectoderm, mesoderm, and intraembryonic endoderm.
The anterior two-thirds of the area pellucida is prospective ectoderm.
The greater part of the ectodermal area is prospective dermis and the bulk of this is extraembryonic.
ii) Invagination in the frog involves folding and retraction of a sheet of cells into the blastocoel, formation of archenteron and a blastopore: in the avian, the separation of these layers from the ectoderm involves the process of convergence and ingression.
the process of convergence and ingression initiate the arrangement of mesodermal and endodermal tissues between the epiblast and hypoblast
the final stages of gastrulation in the avian embryo involve a continuation of convergence and ingression of cells culminating in the establishment of the germ layers and other presumptive organs- somite and epimere, notochord etc.
Placenta
formation in amniotes
*The four extraembryonic membranes are: i) amnion - protective ii) chorion - respiratory and nutritive Iii) allantois - respiratory and nutritive iv) yolk sac - nutrition (source of albumin)
Salient points:
a) Amniota: - amnion-possessing vertebrate such as:
.
mammals: birds and some reptiles
b) ANAMNIOTA: - cyclostome, fishes, and amphibian only EEM is the yolk sac (lacking other membranes including the amnion.
c) OVIPAROUS:- external deposition of eggs and young developing independent of parents. all birds; most fishes, amphibians and reptiles
d) VIVIPAROUS. - retention of egg by a parent (in female reprod. tract); or specially designed pouch in male or female,) ; and young are “born alive”.
THE
REPRODUCTIVE CYCLE
The ovarian cycles in mammals shedding of blood and cellular debris from the uterus at montly intervals. The menstrual cycle represents the integration of three very different activities: (1) the ovarian cycle, the function of which is to mature and
release an oocyce, (2) the uterine cycle, the function of which is to provide the appropriate environment to the developing blastocyst to implant (3) the cervical cycle, the function of which is to allow sperm to enter the female reproductive tract only at the appropriate time. These three functions are integrated through the hormones of the pituitary, hypothalamus, and ovary.
The majority of the oocytes within the adult human ovary are mainly in the prolonged diplotene stage of the first meiotic prophase (often referred to as the dictyate state. Each oocyte is envelopped by a primordial follicle consisting of a single layer of epithelial granulosa cells and a less-organized layer of mesenchymal thecal cells
Periodically, a group of primordial follicles enters a stage of follicular growth. During this time, the oocyte undergoes a 500fold increase in volume (corresponding to an increase in oocyte diameter from 10 micro-meter in a primordial follicle to 80 micrometer to a fully developed oocyte.
the secondary stage of follicle development is marked by the appearance of the zona between the granulosa cells and the ovum.
antrum: a follicular cavity is formed by creation of a cleft within the granulosa cells; this is called antrum
tertiary follicle; a follicle with an antrum is called tertiary follicle further increase in size -, antrum enlarges, oocyte still attaches to granulosa on one size. cumulus - the granulosa investing the ovum is now called cumulus.
theca cells - as the follicle grow, a framework of connective tissue is laid around the ovum, outside the granulosa cells: this is the theca layer outside the parietal granulosa.
oocyte, still encased in the zona pellucida and remnants of the cumulus, stretches and floats in the antral fluid; the release of the cumulus from the theca-granulosa causes them to extend out into the follicular fluid in a mass of projections called corona radiata.
Ovulation:
the mature follicle, called the Graafian follicle, finally bursts open to release the oocyte. This process is called ovulation.
in most mammals ovulation is followed by completion of the first meiotic division and extrusion of the first polar body.
in uniparous mammals, such as the human female, only one follicle develops into maturity and only one follicle ovulate, generally.
in litter-bearing mammals (pluriparous) several follicles attain maturity, and several ova are eleased at ovulation.
* Considerable variability in timing of ovulation in mammals may occur hours before or after heat: in human female ovulation may occasionally occur almost a any time in the estrus cycle.
Ovulation is not spontaneous in all mammals. In rabbit, cat, mink ovulation takes place only after nervous stimulation derived from the act of copulation. - these are called induced-ovulators
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 intra-seasonal. 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 anestrus proestrus estrus etc
Professor: James Kumi-Diaka (BSc. DVM, MSc, PhD. Vee-dip) Office: Science & Education Bldg. Room 278 Phone: (954)236-1135
Course Objective: 5. To expose students to the concept and principles of developmental biology (embryology and gross anatomy) with regard to structural and functional physiological development in vertebrates. Course Outline: 7. Introduction: historical perspective; phylum chordata; its characteristic features; concepts of homology and analogy. 8. Vertebrate phylogeny: classification. 9. Gametogenesis: definitions; nuclear and cytoplasmic; cell divisions; purpose of. 10. Fertilization: What is it?; purpose of; the process of; end-product of; parthenogenesis. 11. Early development: cleavage/segmentation holoblastic and meroblastic cleavages; formation of morula and blastula. 12. Gastrulation: the formation of gastrula; the process involved; formation of primary germ layers; extraembryonic membranes and their role in development.
Course Outline cont’d 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Induction: principle of; physiological significance; classification of. Placentation: amniotes and anamniotes; classification; functions of. The integument: components of: structure and derivatives of. Skeletal system I: components of; skull development; morphology. Skeletal system II: origin and developmental anatomy of vertebrae. Skeletal system III: appendicular skeleton, origin, development and morphology. Musculature I: muscle origin; fate of myotomes: classification; histoanatomy and topography. Muscular system II: development of brachiomeric & appendicular muscles. Development of coelomic cavities, and mesenteries. Respiratory system: evolution, morphogenesis. and derivatives of; classification. Digestive system: evolution and morphogenesis, derivatives of, classification of. Urogenital system I: kidney structure; morphogenesis; classification; primordial gonads. Urogenital system II: reproductive system; embryogenesis and morphoanatomy. Cardiovascular system I: circulatory system; morphogenesis of extraembryonic circulation. Cardiovascular system II: aortic arches and their derivatives. Cardiovascular system III: the heart - its embryonic origin and
Course Outline cont’d 1. 2. 3.
Nervous system I: morphogenesis and histoanatomy of neural tube (brain and spinal cord). Nervous system II: development of autonomic nervous system, spinal nerves, spinal cord. Nervous system llI: cranial nerves, special senses, olfactory, optic, otic, etc.
TEXT BOOK(S):
Any of the following text books will be OK
1. Analysis of Biological Development –Klaus Kalthoff, 2nd Ed. 2. Patten’s Foundations of Embryology 6th Edit, Bruce M Carlson 3. Principles of Development 2nd Edit, Lewis Wolpert
EXAMINATIONS; Exam I = 02/04/08 Exam II = 03/12/08 Exam III = 04/21/08
100% 100% 100%
EVALUATIONS: A >90%; B = 80-89%; C = 70-79%: D = 60 – 69%: F = < 60
PLEASE NOTE: i) All make-ups exams will draw 15% penalty ii) Exam results will NOT be given over the phone (do not). III) THERE ARE NO EXTRA-CREDITS
–
IV) NO CELL PHONES ALLOWED DURING EXAMS
PENALTY
25%
EMBRYOLOGY
Zygote - all higher animals start their lives from a single cell zygote - dual origin from two gametes – spermatozoon + ovum
Ontogeny - The time of fertilization represents the starting point in the life history - Ontogeny refers to the individual’s entire life span.
German biologist August Weismann (1834-1914) made the important distinction between the soma (body) and the germ-cell line (gametes). - thought that the germ-cell line was all-important for perpetuation of the species, and that the soma was primarily a vehicle for protecting and perpetuating the germ plasm. This viewpoint provides a convenient framework for looking at the perpetuation of life (Fig. 1-2).
Once an individual has passed the reproductive years, the remainder of its ontogeny does not provide direct physical input into the generative
Historical perspective
The origin of the embryo: Aristotle’ Theories i) the
Preformationist concept - everything in the embryo was preformed from very beginning and dimply got bigger during
– tiny
development - the sperm contained the embryo (homunculus human in the head of the sperm)
ii) Epigenesis (means ‘upon formation’) - new structures arose progressively during development
FUNDAMENTAL PROCESSES AND CONCEPTS IN DEVELOPMENT Cell Division and the Cell Cycle Is the fundamental properties of living systems, of vital importance
Cell division in vertebrates takes two forms—mitosis and meiosis.
Mitosis (for review, see Fig. 1-14) is the standard form of cell division in somatic cells and it results in two genetically equal daughter cells.
Meiosis (Fig. 3-7) is confined to certain stages of development of gametes Mitosis is one component of the cell cycle.
The life history of a cell can be divided into four periods (Fig. 1-15). Immediately after mitosis and the separation of the dividing cell into daughter cells, the G1 (gap I) period, often called the interphase commences. Its length is extremely variable. In rapidly cleaving embryos just after fertilization, the G1 phase is very short and sometimes may not even exist. At the other extreme, the G1 phase of mature neurons persists as the G0 phase throughout the remainder of the life of the cell because further cell division does no occur.
LIVE - THE ORGANISM
Protoplasm: The complex substance (organic and inorganic) of which all cells are composed. Protoplasm exists solely in the form of organisms and could only be spoken of in term of living things. The essence of the principle of evolution states that all organisms have arisen from common ancestry through a gradual process of change and diversification. However, it is generally held that the physical and chemical composition of the earth’s surface and atmosphere are no longer amenable to the de novo creation of life, it therefore follows that all present day organism have arisen from preexisting
Individuality: Vast differences between individuals in any given population. Differences are due mostly to genetic constitution: that is, among the individuals of a given population, two or more alleles occur at a large proportion of the gene loci i.e., although same numbers and kinds of genes in their chromosomes, the genes may occur in many alternate forms. Variations residing in the gene pool of a population provide the potential for evolutionary change.
The Gene:
Consists of DNA, an organic molecule composed of two long, twisted chains of structural units called nucleotide.
Each nucleotide unit is composed of the sugar deoxyribose, phosphoric acid, and a nitrogenous base of which there are four different kinds; viz: adenine, thymine, cytosine, and guanine (A,T,C,G).
Therefore there are four different kinds of nucleotide.
The nitrogenous bases in the nucleotide can fit only in four combinations: A-T; T-A; G-C; C-G.
The order of these base pairs varies, and the specificity of any part of the DNA molecule, depends upon this order.
Definitions
Chromatin- replicated DNA and assoc. proteins Chromatids- condensed chromatin Chromosomes- two sister chromatids attached by centromere Kinetochore- protein band along chromosome around centromere Mitotic spindles Kinetocore- fibers from centrosome attaching to kinetocore Polar- fibers extending centrosome to opposite centrosome Astral- fibers radiate outward from centrosome
Mutation:
Any changes in the code of nucleotide pairs in the DNA molecule will result in a corresponding change in the protein it codes and, by extension, in the cells and even the organisms of which the protein is a part.
Such alteration of the DNA molecule constitutes mutation. * The number of alleles at a gene locus is increased only through mutation.
Mutations are the sole source of new genetic variability and therefore lie at the root of evolutionary potential. It is genetic variability within the gene pool that is the raw material of evolution. For instance, fertilization, which is the union of reproductive cells, provides for a new and unique combination of genetic propensity.
Primary Terms in Embryogenesis:
REF: glossary – back of the text book:
*Embryology – the study of the embryo.
*Ontogeny – The developmental events or history of an individual that bring it into being: the totality of developmental operations.
*Embryogeny – origin of the embryo: the segment of ontogeny that precedes hatching or birth.
*Phylogeny – the succession of forms that culminates in any given structural entity is the evolutionary history or phylogeny of that entity. The term applies to the total body form or any part thereof.
*Homologous and Analogous – refer to structural and functional similarities:
Homologous – equivalent parts in the sense of having a common ancestry and regardless of structure and function are said to be homologous, or are homologues.
Analogous – parts having similar functions whether homologous or not, are said to be analogous, or are analogues.
Early embryonic stages of vertebrates reveals close likeness to fully formed fishes, amphibia and reptiles. However, as development proceeds the embryos of these different animals become more and more dissimilar
The Vertebrate Evolution
and Development
Modifications of development new features specific to classes/species
THE VERTEBRATE THE CHORDATE - features:
All vertebrates are chordates; but not all chordates are vertebrates.
Vertebrates share certain features with some nonvertebrates - features possessed by all chordates:
n
Notochord – lies along the dorsal midline; provides axis support for the body; present in embryos of all chordates: persists throughout life in some forms: In most chordates, notochord is transient & is replaced by the vertebral column.
n
Hollow neural tube – dorsal and above the notochord; the central nervous system of chordates.
n
Pharyngeal clefts (slits) – passageways from the pharyngeal portion of the digestive tract to the exterior: - respiratory organs (gills)carries over to adult life in aquatic chordates, and serves as - Only transient existence in non-terrestrial chordates
n
Sub-pharyngeal gland – not as salient a feature as (13); - Located ventrally in the pharynx; - binds iodine and related substance: - called endostyle in primitive chordates: - in vertebrates, its homologue is the thyroid gland. In addition to these exclusively diagnostic characteristics, most chordates also exhibit the following features:
n
Tail – rearward extension of the terminal opening of the digestive tract.
n n n
Liver – lies ventral to the digestive tract, and kidneys (distinct tubular unit) rest dorsal to the digestive tract Heart – ventral to the digestive tract; pumping blood via a closed system of vessels. Internal skeleton (over and beyond the notochord) – provides support and protection.
Other features shared with other animal phyla and are thus not exclusively diagnostic:
n
Bilaterally symmetrical body – right and left are mirror images of each other.
n
Metamerism or segmentation – serial repetition of such structures as nerves, blood vessels, muscles and certain other parts.
n
Coelom – all chordates have a true body cavity, or coelom.
Fig 2-2 reveals the diagnostic characteristics of a true vertebrate (FIG-2).
* All of the non-vertebrate chordates are commonly referred to as PROTOCHORDATES.
CLASSIFICATION OF VERTEBRATES A. Mammalian -muscular diaphragm for breathing; -a four-chambered heart; -a single aorta; -two occipital condyles; -nourish young by lactation -has functional placenta (except in the egg-laying it.
mammals) to nourish the fetus and remove waste products from
A.
Aves
-
- 4-chambered heart - endotherm (warm-blooded) - attainment of high and constant metabolic rate - internal thermogenesis heat is derived from within the body, - characterized by feathers and adaptations for flight - uses yolk in eggs to nourish the young/embryo
C) Amphibia - for the most part semi-aquatic; -
are ectothermic. mostly egg-laying Includes: frogs, toads, salamanders.
B.
Pisces
– various classes of fishes - all built around a common blueprint as a superclass Pisces. 4 major classes of the Pisces: 1. Agnatha - jaw1ss ; extinct except for the lampreys and hagfhes 2. Placodermi – armored , jawed fishes; 3. Chondricthyes – modern fishes, the sharks, skates and rays; 4. Osteichthyes – advanced bony fishes which constitute half the living species of vertebrates.
overview THE
VERTEBRATE:
Pisces amphibia reptilian aves mammalia Are chordates –> identical anatomic features
Same/common developmental process
EMBRYOGENY
Since all animals are related in some degree, it is possible to sketch an outline of the early stages of development that applies to all classes of vertebrates.
The basic events involved in development are:
n n n n n
Gamete maturation – gametogenesis Fertilization Cleavage Gastrulation Organogenesis organ system
overview n
Gamete maturation : - gametogenesis spermatozoa + oocytes (both haploid) - a period of chromosomal reduction from diploid to haploid in both spermatozoa and ova.
n
Fertilization - gametes zygote (unicellular/diploid) fertilization initiates two events viz: – Activation of the ovum to start dividing i.e. initiates development, and – Establishment of diploidy - union of the two haploid gametes.
overview c) Cleavage - a period of segmentation of the zygote - series of mitotic divisions blastula (multi-cellular) d) Gastrulation - further mitotic divisions of blastula gastrula establish primary germs layers: ectoderm (outer); endoderm (inner); and mesoderm (middle). e) Organogenesis organ system formation
EMBRYONIC ORGANIZATION: The Ectoderm - the outer strata of the embryo represents the embryonic skin. The Endoderm - inner stratum is the future lining of the
GIT; the space it encompasses is the cavity of the GIT, the archenteron.
The
Mesoderm - between skin and embryonic gut; Neural Tube -ectodermal in origin; along dorsal axis of the embryo;
formed by invagination of the neural plate and joining of the neural folds in the midline; ; anterior portion will form the brain; the remainder will form the spinal cord of the adult.
overview
The neural crest - arise from cells within the neural folds; - and occupies either side of the neural tube; - derivatives include - ganglia of the cranial and spinal nerves: - some of the cells migrate to parts of the body to form different tissues including the sympathetic nervous system (SNS), sensory ganglia of cranial nerves V, VII, IX and X; adrenal medulla, pigment cells; teeth parts of the skull.
The neural crest
Is Ectodermal in origin Adult derivatives:
- ganglia of the spinal nerves - sympathetic nervous system (SNS), - sensory ganglia of cranial nerves V, VII, IX and X - adrenal medulla - teeth - parts of the skull. - pigment cells
The Notochord and Mesoderm - notochord is derived from mesodermal cells that segregate from median dorsal mesoderm; lies between the gut floor and the developing neural tube above. The lateral mesoderm spreads ventrolaterally and the spreading sheets on either side meet in the ventral midline. At same time these mesodermal sheets split into two layers: – splanchnic mesoderm and – somatic mesoderm
overview
The cavity between them is the coelom - a true cavity bounded by an epithelium of mesodermal origin.
The splanchnic mesoderm unites with adjacent endoderm to form the splanchnopleure.
The somatic mesoderm unites with ectoderm to form the
.
somatopleure
Dorsoventral division of the mesoderm into:
n
Epimere - most dorsal and flanks the neural tube and notochord; gives the following derivatives – sclerotome, dermatome and myotome;
n
Mesomere - narrow; between the epimere and hypomere;
n
Hypomere - most ventral and flanks the gut.
Note:
When we speak of an organ or part of it being derived from a given germ laver, we have in mind only the intrinsic functional portion of that organ; for there is no structure in the animal body that is the product of a single germ layer exclusively.
For illustration: e) The intestine is considered to be endodermal in origin, yet only its secreting and absorbing interior lining is so derived. b)
The muscles, peritoneum, connective tissues, blood vessels, and nerves, which constitute the bulk of the GIT, are derived
.
from mesoderm and ectoderm
overview a)
In normal development there are instances in which the cells of one germ layer give rise to structures customarily farmed by those of other germ layers.
c)
The developing tail of vertebrates is illustrative.
e)
The muscles of the tail are a product of ectoderm, whereas muscles in general are mesodermal in character.
g)
Further, the skeleton is ordinarily derived from mesodermal connective tissue,
i)
yet the gill skeleton is produced by neural crest cells originally associated with the nervous system.
* Yet the organ-germ layer concept is still useful as a descriptive scheme of classification and identification. Furthermore, there is a high degree of homology between these layers and the products provided by them. What are the Components of Development?
Development - a series of changes culminating in the formation of a complex organism from relatively small and simple, one cell organism. Includes: i) Growth; ii) morphogenesis; iii) differentiation
overview n
Growth - developmental increase in mass (synthesis of new protoplasm - cytoplasm, nucleic and cytoplasmic products) accompanied by cell division which is characteristic of growth.
n
Morphogenesis - generation of new form; the
organized movements of multiplying cells and the attendant physical reshaping of areas. A sheet of cells may undergo delamination (splitting) into two or more layers; a cord may hollow out (undergo cavitation); there may be folding, invagination (inpocketing), evagination (outpocketing).
n
Differentiation - encompasses a host of
operations culminating in increasing diversification of form and function. Events by which cells and other parts become different from one another and also different from what they were originally.
* Morphological differentiation - as they multiply, cells become structurally different from other cells. E.g. from a general ectoderm --> nerve cells and epidermal cells acquire distinguishing features of size, shape, and internal architecture.
*
Behavioral differentiation -
*
Chemical differentiation - as an
acquisition of special functional capabilities: e.g. nerve cells come to transport electrical disturbances; muscles to contract; gland cells to secrete special products. etc. illustration, consider the egg. An egg has complex chemical composition. As it cleaves and progressively become a blastula, gastrula and embryo, individual cells and areas of cells become biochemically different from one another. The process by which this fixity of developments prospect is acquired is called DETERMINATION.
overview Differentiation
- could therefore be redefined
as the series of events by which materials become chemically determined and then assume distinctive form and function.
THE GAMETES:
In all vertebrates reproduction is effected by sexual means with the use of specialized germ cells called gametes - spermatozoa in the in
.
male and ova in the female
The Primordial Germ cells: There are different concepts about the origin of the germ cells found in the adult gonads.
n
One doctrine states that the PGCs are merely special endodermal wandering cells, having nothing to do with reproduction; and that gamete formations is a matter of differentiation of somatic cells within the gonads.
i.
Another doctrine: - PGCs are extragonadal cells gonads gametes.
In vertebrates, identification of the PGCs comes later in development ; are external to prospective gonads and external to embryo when first recognized
Most recent notion: - PGCs are the sole progenitors of the gametes.
iv.
ii.
How do the PGCs reach the presumptive gonads? Extra-vascular/mesenteric route: The PGCs may migrate, following cell surface signals such as fibronectin in the dorsal mesentery and peritoneum into the region of the developing gonads (genital ridge). The vascular route: PGCs may go through the vascular system to the genital ridge. - birds and reptiles
Developmental Period Stages Embryogenesis Stages of embryogenesis: fertilization cleavage gastrulation organogenesis fertilization histogenesis (differentiated tissues) A.
overview n
Fertilization - union of egg and sperm. Classification of egg: micro-, meso-, and macrolecithal. One polarity axis - animal-vegetal axis.
Fertilization establishes: Stimulation of egg Diploidy (2n) Fig 1.6
9.
Cleavage Definition Classification Blastomeres End result of cleavage – Fig. 1.7
n
Gastrulation - involves movement of cells. - Morphogenetic movements - cells rearrange; migrate; spread; bend and fold forming a gastrula. - End-product of gastruIation 3 germ layers. ectoderm mesoderm endoderm
READING
– text book: Chap 12 - migration of the germ cells - fertilization - gametogenesis spermatogenesis +
oogenesis
Gametogenesis
The totality of preparatory events leading to the formation of the haploid gametes from diploid gonials through the process of mitotic and meiotic (reduction) divisions.
Spermatogenesis
Occurs in the seminiferous tubules in the testis
A.
Spermatogenesis: i Spermatocytogenesis - involving mitotic and meiotic divisions of the spermatogonia through formation of spermatocytes and spermatids. ii Spermiogenesis (spermateliosis) morphogenesis of the spermatid to yield the characteristic spermatozoa ; no cell division involved. iii Spermiation - release of spermatozoa into the lumen of the seminiferous tubules iv Emission
spermatogenesis Spermatogonia Spermatocytes –primary - secondary Spermatids - round -elongated Spermatozoa
The Spermatozoa Sperm - great morphological diversity between kinds of vertebrates.
Head and tail (neck-midpiece-principal piece/tail-endpiece).
Head - spheroidal in teleosts; rod or lance-shaped in amphibia; spirally twisted in passerine birds; spoon-shaped in humans and many other mammals; hooked in mouse and rats.
Serves two functions: genetic (in the DNA), and activating
.
Mitosis and Meiosis
Spermatogenesis
Spermatocytogene sis Spermiogenesis Spermiation Emission Ejaculation
Spermatogenesis
Spermatogenesis starts in testes in seminiferous tubules Spermatogenesis starts in outermost layer of tube and daughter cells move towards lumen.
Male Reproductive Tract Sperm pass through
Seminiferous tubules Epididymis Vas Deferens Ejaculatory Duct Urethra Penis (within)
Seminiferous Tubules
In testes Very convoluted: if stretched, 1.5 miles! (in one testicle only) Sertoli cells= Sustentacular cells: sustain sperm Leydig cells= Interstitial cells: make androgens
Functions of Sustentacular Cells
Secretion of Müllerian-inhibiting factor Support of mitosis and meiosis Support of spermiogenesis Secretion of androgen-binding protein Secretion of inhibin Maintenance of blood-testis barrier
Seminiferous Tubules
Spermatozoon
Head Neck Tail Lacking many intracellular structures At this step, can’t fertilize or move coordinately
A.
Oogenesis:
The totality of the processes involving mitotic and meiotic divisions of the diploid oogonia, culminating in the formation haploid ovum, and nonfunctional polar bodies.
The Vertebrate Egg: The nutritive substance in all eggs is the yolk. Yolk is primarily produced in the liver, and is transported in a soluble form via the bloodstream to the ovary. In the ovary, the yolk is transferred by ovarian cells that surround the oocyte and is deposited as platelets or granules. The vertebrate is classified according to amount of yolk in the cytoplasm.
Oogenesis
Step 1 Oogonia are small cells that increase in number by mitosis When they enlarge slightly, they are considered primary oocytes
Primary Follicle Oocyte enclosed by Follicle cells Primary oocytes become enclosed by a single layer of follicle cells Follicle cells from the surface layer of the ovary Primary follicle is the structure including a primary oocyte enclosed in follicle cells
Oogenesis At puberty, the primary follicles enlarge and accumulate yolk caused by
rapidly the
follicle stimulating hormone (FSH) from the gland
pituitary pushed to
which is called the
As the yolk increases, the oocyte is one side as a small disc The disc is called the blastodisc The blastodisc contains the enlarged nucleus of the primary oocyte germinal vesicle YOLK
BLASTODISC
Oogenesis
First meiotic division of the primary oocyte occurs just before ovulation The result is the first polar body pinching off and the secondary oocyte The secondary oocyte contains most of the yolk and the blastodisc
Fertilization The second meiotic division begins but stops at metaphase The secondary oocyte (sometimes called the ovum at this stage) is mature enough to be fertilized and Ovulation occurs
Oogenesis/Fertilization
The secondary oocyte (ovum) is released from the ovary as the follicle ruptures It enters the infundibulum of the oviduct where it will encounter sperm cells As sperm penetrates the blastodisc, meiosis begins again and the result is the second polar body and the mature ovum
READING
– text book Chapter 8 - fertilization - cleavage amphioxus xenopus chick sea urchin - gastrulation
Cleavage and Blastulation Cleavage-
Occurs in eggs activated by fertilization. Cleavage is the subdivision of eggs into cells called blastomeres. The blastomeres divide quantitatively into smaller units until they are of such a size that they can readily undergo the subsequent events of blastulation, gastrulation, and interaction that are involved in formation of tissues and organs
Cleavage & Blastulation
The blastodisc initiates cleavage
First cleavage occurs in the zygote by way of mitosis and the result is two cells called blastomeres Cleavage occurs in the cytoplasm, not the yolk Second cleavage = 4 blastomeres Cleavage (mitosis) continues until the blastula forms
Blastula Blastulation •The formation of a segmentation cavity or blastocoele within a mass of cleaving blastomeres. Rearrangement of blastomeres around this cavity forms the type of definitive blastula, characteristic of each species •The definitive blastula is thought to terminate cleavage stages •The wall of the blastula is a mosaic of cellular areas, each of which will normally produce a certain structure during subsequent development. In other words, each area of cells in the wall of the blastula has a certain prospective fate which will be realized in normal development.
The Vertebrate Egg: Nutritive substance in all eggs is the yolk. Yolk is produced in the liver transported in a soluble form via the bloodstream to the ovary.
In the ovary, the yolk is transferred by ovarian cells that surround the oocyte is deposited as platelets or granules.
Classification of egg: The vertebrate is classified according to amount of yolk in the cytoplasm: microlecithal: mesolecithal: macrolecithal
Eggs:
n
Microlecithal - small amount of yolk, scattered evenly in cytoplasm (Amphioxus; tunicates; eutherian mammals) Mesolecithal - relatively larger amount of yolk in cytoplasm; not quite evenly scattered (Amphihia; dipnoi; Petromyzontia). Macrolecithal - enormous amount of yolk as food reserve (Myxinoidea; Chondrichtyes; Osteichthyes;reptiles; and birds).
n
n
*
Telolecithal eggs (mesolecithal and
macrolecithal) - where yolk is concentrated in one hemisphere the vegetal pole of the egg (greatest concentration of yolk), than the animal pole (smallest amount of yolk).
* The amount and distribution of yolk influences the pattern of cleavage and movement of cells and tissues during gastrulation.
Fertilization
Union of the ovum and spermatozoon to form the zygote.
Occurs in fluid medium
- All the events involved in
fertilization (internal and external) take place in a fluid medium.
Barriers encountered by the spermatozoa include: cumulus oophorus; corona radiata (encasing the ova); zona pellucida; the vitelline membrane; mucus in the genitalia (internal fertilization).
In some vertebrates (mouse) the zona pellucida contains glycoprotein (zona glycoprotein) which acts as receptor for the binding protein of the sperm membrane.
The acrosome produces hyaluronidase (lytic enzyme) which dissolves the egg barriers.
The acrosome of many different kinds of sperm have been shown to produce a specific protein called bindin, which reacts with bindin receptors (glycoprotein) on the ovum’s vitelline membrane on contact, thus attaching the tip of the acrosome to the membrane.
Acrosomal Reaction:
Morphological changes and biochemical events undertaken by the spermatozoa in order to acquire fertilizing capability.
Identified in several vertebrates and invertebrates.
Capacitation: Sperm of several species require a period of exposure to compounds - glycosaminoglycans (GAGs) in the female genitalia before the bindin reaction can occur. This preparation is part of what is known as sperm
.
capacitation
Steps in AcR
n
Disappearance of the Acrosomal membrane at the apical region. The Acrosomal and sperm membranes become continuous (Fig 5-2A). At the same time, disintegration of Acrosomal granule, as does the portion of the egg membrane exposed to the granule (signifying release of lytic enzymes).
n n
d) Indentation of that region of the acrosomal membrane abutting the nucleus (arrow in 5-2A). e) Elongation and deepening of the indentation, thus creating a steady elongating Acrosomal filament that traverses the egg envelope and finally approaches the plasma membrane of the ovum (5-3C). f) Zygote Formation: Establishes definitive contact between the two gametes (with the meeting of the sperm and egg plasma membranes - one continuous membrane. The contents are now essentially components of one cell - the zygote.
b)
Activation of : Contact of the acrosome with the plasma membrane of the egg activates the otherwise quiescent egg into action:
Polyspermy: Definition of: Significance of : Blockage of polyspermy: Block polyspermy: n
–
a) Fast block to polyspermy: - contact between acrosome and ovum membrane results in electrical charges change in membrane potential across the surface of the egg, immediately preventing further sperm penetration of the egg. b) Slow block to polyspermy: - cortical reaction; I cortical vesicles discharge cortical granules to fuse with egg plasma membrane chemical alteration in the membrane I inactivating its bindin receptors releasing any other sperm that had attached to it thus preventing further fertilization. This constitutes the slow block to polyspermy.
i.
Consequences of sperm penetration: a) Establishment of the plane of bilateral symmetry sperm penetration of ovum rearrangement of its internal constituents leading to the establishment of the plane of bilateral symmetry of the embryo to come. b) Alterations in the physiological properties of the egg; changes in permeability ; and alterations in the rate of metabolism; ovum becomes a truly dynamic entity and embarks on physicochemical transformations that lead into the period of cleavage.
Establishment of Diploidy - union of the haploid nuclei: Fertilization establishes/restores the diploid number of chromosomes for the new organism. Fertilization restores diploidy which represents a new combination of hereditary prospects. In mammals and most other vertebrates, each pronucleus (male and female) loses its membrane, and concomitantly its chromatin resolves into a haploid set of chromosomes. The two set of chromosomes arrange themselves across the spindle. This arrangement, marking the readiness for the first cleavage division, represents completion of the process of fertilization.
* Fertilization is completed with formation of the zygote and
.
initiation of cleavage
Stages in the Events of Fertilization
Fertilization-sequelae - Summary
1. Prevention of polyspermy via: - fast: changes in electrical membrane potential - slow: cortical reaction (granules fusing with egg membrane 2. Establishment of diploidy 3. Establishment of plane of bilateral symmetry in presumptive embryo 4. Establishment of dynamic entity of the ovum physiological transformation initiation of cleavage
PARTHENOGENESIS
Fertilization of the ovum to the zygote without spermatozoa.
This indicates that activation and nuclear phases of fertilization are essentially distinct; that development does not absolutely require a union of egg and sperm.
The eggs of every animal group, including mammals, can be started on a course of development, often to the completion of a new individual, by a host of chemical and physical agents.
This strongly suggests that the ovum possesses in itself all the capacities to form an embryo, needing only some agents to trigger the action.
In normal development, the spindle for the first cleavage division is assembled around the centriole brought by the sperm.
What is the source of the centriole in parthenogenesis? This may reside in the egg. In some cases, spindle appear to arise spontaneously.
The easiest way to induce parthenogenicity in the frog is to stab the egg with a very fine needle. A method that is effective only if the needle carries
some foreign protein into the egg. The protein provides a center around which the cleavage spindle arises
Although parthenogenesis occurs naturally in some vertebrates (some birds and amphibians), it rarely occurs in others such as the whip-tailed lizards (Cnemidophorus uniparens), all the adults of which are females.
Recent experiments have shown that elements of both male and female genomes are necessary for full development in mammals.
Expectedly, the chromosome numbers in parthenogenetic individuals are haploid. In some cases, there is tendency for the restoration of diploidy via: fusion of haploid cleavage nuclei prior to cell division; fusion of egg nucleus with a polar body nucleus before cleavage begins.
Advanced or complete development occurs only if regulation to diploidy is accomplished: haploid individuals generally succumb in early embyonic stages.
Cleavage Eggs – Types: Isolecithal - small amount of yolk evenly distributed; mammals. Mesolecithal - moderate yolk in vegetal pole; amphibians. Macrolecithal – large amount of yolk in the egg or Telolecithal - large yolk filling almost entire egg; fish, reptiles, birds. Centrolecithal - yolk concentrated in the central cytoplasm; insects, arthropods.
Cleavage Pattern
Holoblastic - entire egg cleaves during cytokinesis. – Complete Holoblastic blastomere. – Partial Holoblastic micromeres +
macromeres Meroblastic - characterized by partial cytokinesis; two types:
Discoidal cleavage - restricted to animal
pole, yolk-free area. *Vegetal pole remains uncleaved in telolecithal eggs.
Amphibian Cleavage From 128 cell stage = blastula Several layers thick; Larger blastomeres in vegetal pole – macromeres. Smaller blastomeres at animal pole – micromeres.
Mammalian Cleavage
Cleavage Cellular Mechanism
Cleavage Cellular Mechanism Experimental evidence:
In isolecithal eggs - mitotic spindle is centered cleavage furrows form all around the circumference.
In mesolecithal eggs - mitotic spindle is displaced to the animal pole - the furrow first appears in animal pole then cuts towards the vegetal pole.
In telolecithal eggs - mitotic spindle is eccentric the cleavage furrow forms only near the mitotic apparatus.
Cleavage & Gastrulation Cleavage - general consideration:
Series of a regular rhythm of mitotic divisions of the egg following activation either by the spermatozoon or by parthenogenetic agent, and ending with formation of a blastula.
The individual cells during the process of segmentation are called blastomeres.
During the period of segmentation there is no increase in cytoplasmic mass (embryo does not grow); segmentation leads to increase in the number of blastomeres but not the size; ; there is no growth during these mitotic divisions as the G1 and G2 phases of the cell cycle immediately before and after DNA synthesis are eliminated.
With each cell division there is additional DNA synthesis and protein, to provide for the increased nuclei, at the expense of cytoplasmic reserve nuclear gain is balanced by cytoplasmic loss: leading to numerous small blastomeres each with its nucleus and cytoplasm.
Since the area of constriction that divides the cytoplasm (cytokinesis) is organized by the centriole of the cell, the plane of cell division is always at right angle to the long axis of the spindle
Variations in Cleavage Pattern: b.
Amount of yolk in egg cytoplasm Influences the rate and pattern of cleavage division; the greater the amount of yolk and the more unevenly distributed the yolk, the more the mitotic spindle in the egg and later in the blastomeres are displaced from the center of the cell toward the less yolky ends; and upon division, blastomeres of unequal size result. In an egg with most heavily laden yolk (birds), segmentation is confined to a small region (blastodisc) at the animal pole.
a.
Cytoplasmic organization Exemplified by the spiral cleavage in annelids and mollusks - where the rotational movement of blastomeres resulting from alternately tilted obliquely to the right and left, so that successive generations of blastomeres are oriented in a twisted fashion.
Summary – cleavage pattern n Holoblastic (total) Cleavage: The whole egg divides as do the blastomeres: – Equal holobalstic: In microlecithal eggs (mammals) - blastomeres of equal sizes – Unequal holoblastic: In mesolecithal eggs (amphibians) – blastomeres of unequal sizes
n
Partial Cleavage: (Teleost fish) part of egg remains undivided; in moderately macrolecithal eggs
n
Meroblastic Cleavage: (reptiles; bird & primitive mammals) - extremely macrolecithal eggs –segmentation is confined to small area at the animal pole.
GASTRULATION – commences at the conclusion of segmentation & formation of the blastula
Gastrulation -
GASTRULATION
is by a process of rapid cell multiplication by which the blastula is molded into a stratified structure - gastrula involves flattening and invagination (inward folding) of the
prospective endodermal plate into the blastocoel
the new cavity is called gastrocoel (archenteron) which opens to the exterior by the blastopore
further migration of cells and elongation of the gastrula establishes two layers: a) an outer ectoderm, neural and epidermal, and b) inner cell mass encompassing prospective notochord, mesoderm and endoderm
the organs of the body are all derivatives of these three primary germ layers.
Note:
In amphoxius, the mesodermal pouches are hollow from the beginning which is to say that they contain a coelum.
Thus the Amphioxus features an enterocoel, (the coelomic spaces trace their origin to the cavity of the primitive gut).
The vertebrates feature a schizophoel - a coelom formed by the splitting of an originally solid layer of mesoderm.
In each case the end result is the same: somatic mesoderm associated with epidermal ectoderm to form somatopleure; and splanchnic mesoderm associated with endoderm (endodermal gut) to form splanchnopleure.
AMPHIBIAN
- FROG
eggs are mesolecithal - moderate amount of yolk present in form of oval granules scattered throughout the cytoplasm, but show gradual concentration to one pole; telolecithal - greater concentration of yolk located at the vegetal pole. Cleavage: follows the pattern seen in the amphioxus with minor differences:
Gastrulation
(cont’d)
involves invagination, involution and stretching and spreading of cells of the blastula to form a spherical gastrula surrounded externally by ectoderm and containing endodermal and mesodermal components internally, comparable to the situation in amphioxus.
throughout gastrulation the embryo retains its spherical shape and a uniform size.
endodermal ridges meet in the dorsal midline and unite to create a tubular endodermal gut.
mesoderm in the dorsal midline is set off as a definitive notochord, separate from the presumptive epimere, mesomere and hypomere the forerunners of other organs.
the overlying ectoderm would give rise to neural tube.
Avian
first cleavage division is an irregular shallow furrow in the central portion of the blastodisc.
this furrow does not cut completely through the depth of the disc; and this results in two incompletely separated blastomeres.
the second division occurs at right angle to the first but does not cut entirely through the depth of the disc.
further incomplete divisions result in irregular pattern of cleavage, creating two groups of blastomeres: a) completely bounded central blastomeres and b) incompletely bounded marginal blastomeres.
further divisions of the blastomeres create a stratification of cells. This results in a compact plate several cells thick; which is called the blastoderm.
the bulk of the yolk is external to the blastoderm (which still contains some yolk cells).
The bilaminar blastoderm represents the blastula.; the epiblast represents the animal half; and hypoblast (endoderm) represents the vegetal half; and the intervening space, represent the blastocoel.
Gastrulation the hypoblast (endoderm) is already present in part at beginning of gastrulation.
Major differences in the fate of embryonic cells of avian and amphibian?
i) formation of extraembryonic organs from the blastoderm to support embryo prior to hatching
The epiblast of the entire area opaca is destined to provide extraembryonic ectoderm.
The epiblast (upper layer) is the presumptive ectoderm, mesoderm, and intraembryonic endoderm.
The anterior two-thirds of the area pellucida is prospective ectoderm.
The greater part of the ectodermal area is prospective dermis and the bulk of this is extraembryonic.
ii) Invagination in the frog involves folding and retraction of a sheet of cells into the blastocoel, formation of archenteron and a blastopore: in the avian, the separation of these layers from the ectoderm involves the process of convergence and ingression.
the process of convergence and ingression initiate the arrangement of mesodermal and endodermal tissues between the epiblast and hypoblast
the final stages of gastrulation in the avian embryo involve a continuation of convergence and ingression of cells culminating in the establishment of the germ layers and other presumptive organs- somite and epimere, notochord etc.
Placenta
formation in amniotes
*The four extraembryonic membranes are: i) amnion - protective ii) chorion - respiratory and nutritive Iii) allantois - respiratory and nutritive iv) yolk sac - nutrition (source of albumin)
Salient points:
a) Amniota: - amnion-possessing vertebrate such as:
.
mammals: birds and some reptiles
b) ANAMNIOTA: - cyclostome, fishes, and amphibian only EEM is the yolk sac (lacking other membranes including the amnion.
c) OVIPAROUS:- external deposition of eggs and young developing independent of parents. all birds; most fishes, amphibians and reptiles
d) VIVIPAROUS. - retention of egg by a parent (in female reprod. tract); or specially designed pouch in male or female,) ; and young are “born alive”.
THE
REPRODUCTIVE CYCLE
The ovarian cycles in mammals shedding of blood and cellular debris from the uterus at montly intervals. The menstrual cycle represents the integration of three very different activities: (1) the ovarian cycle, the function of which is to mature and
release an oocyce, (2) the uterine cycle, the function of which is to provide the appropriate environment to the developing blastocyst to implant (3) the cervical cycle, the function of which is to allow sperm to enter the female reproductive tract only at the appropriate time. These three functions are integrated through the hormones of the pituitary, hypothalamus, and ovary.
The majority of the oocytes within the adult human ovary are mainly in the prolonged diplotene stage of the first meiotic prophase (often referred to as the dictyate state. Each oocyte is envelopped by a primordial follicle consisting of a single layer of epithelial granulosa cells and a less-organized layer of mesenchymal thecal cells
Periodically, a group of primordial follicles enters a stage of follicular growth. During this time, the oocyte undergoes a 500fold increase in volume (corresponding to an increase in oocyte diameter from 10 micro-meter in a primordial follicle to 80 micrometer to a fully developed oocyte.
the secondary stage of follicle development is marked by the appearance of the zona between the granulosa cells and the ovum.
antrum: a follicular cavity is formed by creation of a cleft within the granulosa cells; this is called antrum
tertiary follicle; a follicle with an antrum is called tertiary follicle further increase in size -, antrum enlarges, oocyte still attaches to granulosa on one size. cumulus - the granulosa investing the ovum is now called cumulus.
theca cells - as the follicle grow, a framework of connective tissue is laid around the ovum, outside the granulosa cells: this is the theca layer outside the parietal granulosa.
oocyte, still encased in the zona pellucida and remnants of the cumulus, stretches and floats in the antral fluid; the release of the cumulus from the theca-granulosa causes them to extend out into the follicular fluid in a mass of projections called corona radiata.
Ovulation:
the mature follicle, called the Graafian follicle, finally bursts open to release the oocyte. This process is called ovulation.
in most mammals ovulation is followed by completion of the first meiotic division and extrusion of the first polar body.
in uniparous mammals, such as the human female, only one follicle develops into maturity and only one follicle ovulate, generally.
in litter-bearing mammals (pluriparous) several follicles attain maturity, and several ova are eleased at ovulation.
* Considerable variability in timing of ovulation in mammals may occur hours before or after heat: in human female ovulation may occasionally occur almost a any time in the estrus cycle.
Ovulation is not spontaneous in all mammals. In rabbit, cat, mink ovulation takes place only after nervous stimulation derived from the act of copulation. - these are called induced-ovulators
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 intra-seasonal. 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 anestrus proestrus estrus etc
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