Lecture 6: Osseous Tissue and Bone Structure
Topics:
Skeletal cartilage Structure and function of bone tissues Types of bone cells Structures of the two main bone tissues Bone membranes Bone formation Minerals, recycling, and remodeling Hormones and nutrition Fracture repair The effects of aging
The Skeletal System
Skeletal system includes: bones
of the skeleton cartilages, ligaments, and connective tissues
Skeletal Cartilage Contains no blood vessels or nerves Surrounded by the perichondrium (dense irregular connective tissue) that resists outward expansion Three types – hyaline, elastic, and fibrocartilage
Hyaline Cartilage Provides support, flexibility, and resilience Is the most abundant skeletal cartilage Is present in these cartilages:
Articular
– covers the ends of long bones Costal – connects the ribs to the sternum Respiratory – makes up larynx, reinforces air passages Nasal – supports the nose
Elastic Cartilage Similar to hyaline cartilage, but contains elastic fibers Found in the external ear and the epiglottis
Fibrocartilage Highly compressed with great tensile strength Contains collagen fibers Found in menisci of the knee and in intervertebral discs
Growth of Cartilage Appositional – cells in the perichondrium secrete matrix against the external face of existing cartilage Interstitial – lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from within Calcification of cartilage occurs
During
normal bone growth During old age
Bones and Cartilages of the Human Body
Figure 6.1
Functions of the Skeletal System 1. 2. 3. 4. 5. 6.
Support Storage of minerals (calcium) Storage of lipids (yellow marrow) Blood cell production (red marrow) Protection Leverage (force of motion)
Bone (Osseous) Tissue Supportive connective tissue Very dense Contains specialized cells Produces solid matrix of calcium salt deposits and collagen fibers
Characteristics of Bone Tissue
Dense matrix, containing: deposits
of calcium salts osteocytes within lacunae organized around blood vessels
Canaliculi: form
pathways for blood vessels exchange nutrients and wastes
Osteocyte and canaliculi
Characteristics of Bone Tissue
Periosteum: covers
outer surfaces of bones consist of outer fibrous and inner cellular layers Contains osteblasts responsible for bone growth in thickness
Endosteum Covers
inner surfaces of bones
Bone Matrix Solid ground is made of mineral crystals 2/3 of bone matrix is calcium phosphate, Ca3(PO4)2:
reacts
with calcium hydroxide, Ca(OH)2 to form crystals of hydroxyapatite, Ca10(PO4)6(OH)2 which incorporates other calcium salts and ions
Bone Matrix
Matrix Proteins: 1/3
of bone matrix is protein fibers (collagen)
Question: why aren’t bones made of ALL collagen if it’s so strong?
Bone Matrix Mineral salts make bone rigid and compression resistant but would be prone to shattering Collagen fibers add extra tensile strength but mostly add tortional flexibility to resist shattering
Chemical Composition of Bone: Organic
Cells: Osteoblasts
– bone-forming cells Osteocytes – mature bone cells Osteoprogenitor cells – grandfather cells Osteoclasts – large cells that resorb or break down bone matrix
Osteoid – unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen; becomes calcified later
There are four major types of cells
in matrix only
periosteum + endo
endosteum only
1. Osteoblasts
Immature bone cells that secrete matrix compounds (osteogenesis)
Eventually become surrounded by calcified bone and then they become osteocytes Figure 6–3 (2 of 4)
2.Osteocytes
Mature bone cells that maintain the bone matrix
Figure 6–3 (1 of 4)
Osteocytes
Live in lacunae Found between layers (lamellae) of matrix Connected by cytoplasmic extensions through canaliculi in lamellae (gap junctions) Do not divide (remember G0?) Maintain protein and mineral content of matrix Help repair damaged bone
3. Osteoprogenitor Cells Mesenchyme stem cells that divide to produce osteoblasts Are located in inner, cellular layer of periosteum Assist in fracture repair
4. Osteoclasts
Secrete acids and protein-digesting enzymes
Figure 6–3 (4 of 4)
Osteoclasts Giant, mutlinucleate cells Dissolve bone matrix and release stored minerals (osteolysis) Often found lining in endosteum lining the marrow cavity Are derived from stem cells that produce macrophages
Homeostasis
Bone building (by osteocytes and -blasts) and bone recycling (by osteoclasts) must balance: more
breakdown than building, bones become weak exercise causes osteocytes to build bone
Bone cell lineage summary
Osteoprogenitor cells osteoblasts osteocytes
Osteoclasts are related to macrophages (blood cell derived)
Gross Anatomy of Bones: Bone Textures Compact bone – dense outer layer Spongy bone – honeycomb of trabeculae filled with yellow bone marrow
Compact Bone
Figure 6–5
Osteon The basic structural unit of mature compact bone Osteon = Osteocytes arranged in concentric lamellae around a central canal containing blood vessels
Lamella
– weight-bearing, column-like matrix tubes composed mainly of collagen
Three Lamellae Types
Concentric Lamellae Circumferential Lamellae Lamellae
wrapped around the long bone line tree
rings Binds inner osteons together
Interstitial Lamellae Found
between the osteons made up of concentric lamella They are remnants of old osteons that have been partially digested and remodeled by osteoclast/osteoblast activity
Compact Bone
Figure 6–5
Microscopic Structure of Bone: Compact Bone
Figure 6.6a, b
Microscopic Structure of Bone: Compact Bone
Figure 6.6a
Microscopic Structure of Bone: Compact Bone
Figure 6.6b
Microscopic Structure of Bone: Compact Bone
Figure 6.6c
Spongy Bone
Figure 6–6
Spongy Bone Tissue
Makes up most of the bone tissue in short, flat, and irregularly shaped bones, and the head (epiphysis) of long bones; also found in the narrow rim around the marrow cavity of the diaphysis of long bone
Spongy Bone Does not have osteons The matrix forms an open network of trabeculae Trabeculae have no blood vessels
Bone Marrow
The space between trabeculae is filled with marrow which is highly vascular Red bone marrow supplies nutrients to osteocytes in trabeculae forms red and white blood cells Yellow bone marrow yellow because it stores fat
Question: Newborns have only red marrow. Red changes into yellow marrow in some bones as we age. Why?
Location of Hematopoietic Tissue (Red Marrow)
In infants Found
in the medullary cavity and all areas of spongy bone
In adults Found
in the diploë of flat bones, and the head of the femur and humerus
Bone Membranes
Periosteum – double-layered protective membrane Covers
all bones, except parts enclosed in joint capsules (continuois w/ synovium) Made up of:
outer, fibrous layer (tissue?) inner, cellular layer (osteogenic layer) is composed of osteoblasts and osteoclasts
Secured
to underlying bone by Sharpey’s fibers
Endosteum – delicate membrane covering internal surfaces of bone
Sharpy’s (Perforating) Fibers Collagen fibers of the outer fibrous layer of periosteum, connect with collagen fibers in bone Also connect with fibers of joint capsules, attached tendons, and ligaments Tendons are “sewn” into bone via periosteum
Periosteum
Figure 6–8a
Functions of Periosteum 1. 2.
3.
Isolate bone from surrounding tissues Provide a route for circulatory and nervous supply Participate in bone growth and repair
Endosteum
Figure 6–8b
Endosteum
An incomplete cellular layer: lines
the marrow cavity covers trabeculae of spongy bone lines central canals
Contains osteoblasts, osteoprogenitor cells, and osteoclasts Is active in bone growth and repair
Bone Development
Human bones grow until about age 25 Osteogenesis:
bone formation
Ossification: the
process of replacing other tissues with bone
Osteogenesis and ossification lead to: The
formation of the bony skeleton in embryos Bone growth until early adulthood Bone thickness, remodeling, and repair through life
Calcification The process of depositing calcium salts Occurs during bone ossification and in other tissues
Formation of the Bony Skeleton Begins at week 8 of embryo development Ossification
Intramembranous
ossification – bone develops from a fibrous membrane Endochondral ossification – bone forms by replacing hyaline cartilage
Intramembranous Ossification Note: you don’t have to know the steps of this process in detail
Also called dermal ossification (because it occurs in the dermis) produces
dermal bones such as mandible and
clavicle
Formation of most of the flat bones of the skull and the clavicles Fibrous connective tissue membranes are formed by mesenchymal cells
The Birth of Bone
When new bone is born, either during development or regeneration, it often starts out as spongy bone (even if it will later be remodeled into compact bone)
Endochondral Ossification Note: you DO have to know this one
Begins in the second month of development Uses hyaline cartilage “bones” as models for bone construction then ossifies cartilage into bone Common, as most bones originate as hyaline cartilage This is like a “trick” the body uses to allow long bones to grow in length when bones can only grow by appositional growth
Bone formation in a chick embryo
Stained to represent hardened bone (red) and cartilage (blue)
: This image is the cover illustration from The Atlas of Chick Development by Ruth Bellairs and Mark Osmond, published by Academic Press (New York) in 1998
Fetal Primary Ossification Centers
Figure 6.15
Stages of Endochondral Ossification
Bone models form out of hyaline cartilage Formation of bone collar Cavitation of the hyaline cartilage Invasion of internal cavities by the periosteal bud, and spongy bone formation Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates
Stages of Endochondral Ossification Secondary ossificaton center
Deteriorating cartilage matrix
Hyaline cartilage Primary ossification center
Spongy bone formation
Epiphyseal blood vessel
Medullary cavity
Articular cartilage Spongy bone
Epiphyseal plate cartilage
Bone collar
1 Formation of bone collar around hyaline cartilage model.
Blood vessel of periosteal bud 2 Cavitation of the hyaline cartilage within the cartilage model.
3 Invasion of internal cavities by the periosteal bud and spongy bone formation.
4 Formation of the medullary cavity as ossification continues; appearance of secondary ossification centers in the epiphyses in preparation for stage 5.
5 Ossification of the epiphyses; when completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages.
Figure 6.8
Endochondral Ossification: Step 1 (Bone Collar)
Blood vessels grow around the edges of the cartilage Cells in the perichondrium change to osteoblasts: producing
a layer of superficial bone (bone collar) around the shaft which will continue to grow and become compact bone (appositional growth)
Figure 6–9 (Step 2)
Endochondral Ossification: Step 2 (Cavitation)
Chondrocytes in the center of the hyaline cartilage of each bone model: enlarge form
struts and calcify die, leaving cavities in cartilage
Figure 6–9 (Step 1)
Endochondral Ossification: Step 3 (Invasion)
Periosteal bud brings blood vessels into the cartilage: bringing
osteoblasts and osteoclasts spongy bone develops at the primary ossification center
Figure 6–9 (Step 3)
Endochondral Ossification: Step 4a (Remodelling)
Remodeling creates a marrow (medullary) cavity: bone
replaces cartilage at the metaphyses Diaphysis elongates
Figure 6–9 (Step 4)
Endochondral Ossification: Step 4b (2° Ossification)
Capillaries and osteoblasts enter the epiphyses: creating
secondary ossification centers (perinatal)
Figure 6–9 (Step 5)
Endochondral Ossification: Step 5 (Elongation)
Epiphyses fill with spongy bone but cartilage remains at two sites: ends
of bones within the joint cavity = articular cartilage cartilage at the metaphysis = epiphyseal cartilage (plate) Figure 6–9 (Step 6)
Postnatal Bone Growth
Growth in length of long bones Cartilage
on the side of the epiphyseal plate closest to the epiphysis is relatively inactive Cartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth Cells of the epiphyseal plate proximal to the resting cartilage form three functionally different zones: growth, transformation, and osteogenic
Functional Zones in Long Bone Growth Growth zone – cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysis Transformation zone – older cells enlarge, the matrix becomes calcified, cartilage cells die, and the matrix begins to deteriorate Osteogenic zone – new bone formation occurs
Growth in Length of Long Bone
Figure 6.9
Postnatal bone growth
Remember that bone growth can only occur from the outside (appositional growth). So this type of endochondral growth is a way for bones to grow from the inside and lengthen because it is the cartilage that is growing, not the bone
Key Concept As epiphyseal cartilage grows through the division of chondrocytes it pushes the ends of the bone outward in length. At the “inner” (shaft) side of the epiphyseal plate, recently born cartilage gets turned into bone, but as long as the cartilage divides and extends as fast or faster than it gets turned into bone, the bone will grow longer
Long Bone Growth and Remodeling Growth in length – cartilage continually grows and is replaced by bone as shown Remodeling – bone is resorbed and added by appositional growth as shown compact bone thickens and strengthens long bones with layers of circumferential lamellae
Long Bone Growth and Remodeling
Figure 6.10
Appositional Growth
Epiphyseal Lines
When long bone stops growing, between the ages of 18 – 25: epiphyseal
cartilage disappears epiphyseal plate closes visible on X-rays as an epiphyseal line
At this point, bone has replaced all the cartilage and the bone can no longer grow in length
Epiphyseal Lines
Figure 6–10
Hormonal Regulation of Bone Growth During Youth During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone During puberty, testosterone and estrogens:
Initially
promote adolescent growth spurts Cause masculinization and feminization of specific parts of the skeleton Later induce epiphyseal plate closure, ending long bone growth
Remodeling
Remodeling continually recycles and renews bone matrix Turnover rate varies within and between bones If deposition is greater than removal, bones get stronger If removal is faster than replacement, bones get weaker Remodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces
Bone Deposition
Occurs where bone is injured or added strength is needed Requires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and manganese Alkaline phosphatase is essential for mineralization of bone Sites of new matrix deposition are revealed by the: Osteoid
seam – unmineralized band of bone matrix Calcification front – abrupt transition zone between the osteoid seam and the older mineralized bone
Effects of Exercise on Bone Mineral recycling allows bones to adapt to stress Heavily stressed bones become thicker and stronger
Response to Mechanical Stress
Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it Observations supporting Wolff’s law include Long
bones are thickest midway along the shaft (where bending stress is greatest) Curved bones are thickest where they are most likely to buckle
Trabeculae form along lines of stress Large, bony projections occur where heavy, active muscles attach
Response to Mechanical Stress
Figure 6.12
Bone Resorption
Accomplished by osteoclasts Resorption bays – grooves formed by osteoclasts as they break down bone matrix Resorption involves osteoclast secretion of: Lysosomal
enzymes that digest organic matrix Acids that convert calcium salts into soluble forms
Dissolved matrix is transcytosed across the osteoclast cell where it is secreted into the interstitial fluid and then into the blood
Bone Degeneration Bone degenerates quickly Up to 1/3 of bone mass can be lost in a few weeks of inactivity
Minerals, vitamins, and nutrients Rewired for bone growth A dietary source of calcium and phosphate salts: plus
small amounts of magnesium, fluoride, iron, and manganese
Protein, vitamins C, D, and A
Hormones for Bone Growth and Maintenance
Table 6–2
Calcitriol
The hormone calcitriol: synthesis
requires vitamin D3 (cholecalciferol) made in the kidneys (with help from the liver) helps absorb calcium and phosphorus from digestive tract
The Skeleton as Calcium Reserve
Bones store calcium and other minerals Calcium is the most abundant mineral in the body Calcium ions in body fluids must be closely regulated because: Calcium ions are vital to: membranes neurons muscle cells, blood
clotting
especially heart cells
Calcium Regulation: Hormonal Control
Homeostasis is maintained by calcitonin and parathyroid hormone which control storage, absorption, and excretion Rising blood Ca2+ levels trigger the thyroid to release calcitonin Calcitonin stimulates calcium salt deposit in bone Falling blood Ca2+ levels signal the parathyroid glands to release PTH PTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood
Hormonal Control of Blood Ca
PTH; calcitonin secreted
Calcitonin stimulates calcium salt deposit in bone
Thyroid gland
Rising blood Ca2+ levels Calcium homeostasis of blood: 9–11 mg/100 ml Falling blood Ca2+ levels
Thyroid gland Osteoclasts degrade bone matrix and release Ca2+ into blood
Parathyroid glands
PTH
Parathyroid glands release parathyroid hormone (PTH)
Figure 6.11
Calcitonin and Parathyroid Hormone Control
Bones: where
Digestive tract: where
calcium is stored calcium is absorbed
Kidneys: where
calcium is excreted
Parathyroid Hormone (PTH)
Produced by parathyroid glands in neck Increases calcium ion levels by: stimulating osteoclasts increasing intestinal absorption of calcium decreases calcium excretion at kidneys
Calcitonin
Secreted by cells in the thyroid gland Decreases calcium ion levels by: inhibiting
osteoclast
activity increasing calcium excretion at kidneys
Actually plays very small role in adults
Fractures
Fractures: cracks
or breaks in bones caused by physical stress
Fractures are repaired in 4 steps
Fracture Repair Step 1: Hematoma
Hematoma formation Torn
blood vessels hemorrhage A mass of clotted blood (hematoma) forms at the fracture site Site becomes swollen, painful, and inflamed
Bone cells in the area die Figure 6.13.1
Fracture Repair Step 2: Soft Callus
Cells of the endosteum and periosteum divide and migrate into fracture zone Granulation tissue (soft callus) forms a few days after the fracture from fibroblasts and endothelium Fibrocartilaginous callus forms to stabilize fracture external callus of hyaline cartilage surrounds break internal callus of cartilage and collagen develops in marrow cavity
Capillaries grow into the tissue and phagocytic cells begin cleaning debris Figure 6.13.2
Stages in the Healing of a Bone Fracture
The fibrocartilaginous callus forms when: Osteoblasts
and fibroblasts migrate to the fracture and begin reconstructing the bone Fibroblasts secrete collagen fibers that connect broken bone ends Osteoblasts begin forming spongy bone Osteoblasts furthest from capillaries secrete an externally bulging cartilaginous matrix that later calcifies
Fracture Repair Step 3: Bony Callus
Bony callus formation New
spongy bone trabeculae appear in the fibrocartilaginous callus Fibrocartilaginous callus converts into a bony (hard) callus Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later Figure 6.13.3
Fracture Repair Step 4: Remodeling
Bone remodeling Excess
material on the bone shaft exterior and in the medullary canal is removed Compact bone is laid down to reconstruct shaft walls Remodeling for up to a year
reduces bone callus may never go away completely
Usually
heals stronger than surrounding bone Figure 6.13.4
Clinical advances in bone repair
Electrical stimulation of fracture site.
Ultrasound.
Daily treatment results in decreased healing time of fracture by about 25% to 35% in broken arms and shinbones. Stimulates cartilage cells to make bony callus.
Free vascular fibular graft technique.
results in increased rapidity and completeness of bone healing electrical field may prevent parathyroid hormone from activating osteoclasts at the fracture site thereby increasing formation of bone and minimizing breakdown of bone,
Uses pieces of fibula to replace bone or splint two broken ends of a bone. Fibula is a non-essential bone, meaning it does not play a role in bearing weight; however, it does help stabilize the ankle.
Bone substitutes.
synthetic material or crushed bones from cadavers serve as bone fillers (Can also use sea coral).
Aging and Bones Bones become thinner and weaker with age Osteopenia begins between ages 30 and 40 Women lose 8% of bone mass per decade, men 3%
Osteoporosis
Severe bone loss which affects normal function Group of diseases in which bone reabsorption outpaces bone deposit The epiphyses, vertebrae, and jaws are most affected, resulting in fragile limbs, reduction in height, tooth loss Occurs most often in postmenopausal women Bones become so fragile that sneezing or stepping off a curb can cause fractures Over age 45, occurs in: 29%
of women 18% of men
Notice what happens in osteoporosis
Osteoporosis: Treatment Calcium and vitamin D supplements Increased weight-bearing exercise Hormone (estrogen) replacement therapy (HRT) slows bone loss Natural progesterone cream prompts new bone growth Statins increase bone mineral density PPIs may decrease density
Hormones and Bone Loss Estrogens and androgens help maintain bone mass Bone loss in women accelerates after menopause
Cancer and Bone Loss
Cancerous tissues release osteoclastactivating factor: stimulates produces
osteoclasts
severe osteoporosis
Paget’s Disease Characterized by excessive bone formation and breakdown An excessively high ratio of spongy to compact bone is formed Reduced mineralization causes spotty weakening of bone Osteoclast activity wanes, but osteoblast activity continues to work
Developmental Aspects of Bones Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes and cartilages that form the embryonic skeleton The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms At birth, most long bones are well ossified (except for their epiphyses)
Developmental Aspects of Bones By age 25, nearly all bones are completely ossified In old age, bone resorption predominates A single gene that codes for vitamin D docking determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life
SUMMARY
Skeletal cartilage Structure and function of bone tissues Types of bone cells Structures of compact bone and spongy bone Bone membranes, peri- and endosteum Ossification: intramembranous and endochondral Bone minerals, recycling, and remodeling Hormones and nutrition Fracture repair The effects of aging
The Major Types of Fractures
Simple (closed): bone end does not break the skin Compound (open): bone end breaks through the skin Nondisplaced – bone ends retain their normal position Displaced – bone ends are out of normal alignment Complete – bone is broken all the way through Incomplete – bone is not broken all the way through Linear – the fracture is parallel to the long axis of the bone Transverse – the fracture is perpendicular to the long axis of the bone Comminuted – bone fragments into three or more pieces; common in the elderly Figure 6–16 (1 of 9)
Types of fractures (just FYI)
More fractures