bone physiology
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Bone Physiology. Chris van ZylKHC. Physical Structure:. Composed of cells and predominantly collagenous extracellular matrix (type I collagen) called osteoid which become mineralized giving bone rigidity and strength Compact (cortical) bone Dense rigid outer shell - PowerPoint PPT PresentationTRANSCRIPT
BONE PHYSIOLOGYChris van ZylKHC
Physical Structure:Composed of cells and predominantly collagenous extracellular matrix (type I collagen) called osteoid which become mineralized giving bone rigidity and strength
Compact (cortical) bone Dense rigid outer shell Minimal gaps and spaces Accounts for 80% of the total bone mass of an
adult skeleton
Physical Structure: Trabecular (cancellous)
bone
Central zone of interconnecting trabeculae
Network of rod- and plate-like elements
Make the overall organ lighter
Allow room for blood vessels and marrow
Microscopic Structure:
Haphazard organization of collagen fibers
Mechanically weak Produced when
osteoblasts produce osteoid rapidly
E.g. Fetal bones, fractures, Paget’s
Woven:
Microscopic Structure:
Regular parallel alignment of collagen into sheets
Mechanically strong Fibers run in opposite
directions in alternating layers
Replaces woven bone after fracture
Lamellar:
Cellular Structure:
Derived from osteoprogenitor cells The bone-forming cells Synthesize osteoid, mediates its
mineralization Found lined up along bone surfaces
Osteoblasts
Osteoblast Stimulation
Stimulated to increase bone mass through increased secretion of osteoid
Stimulated by the secretion of: Growth Hormone Thyroid Hormone Sex Hormones (oestrogens + androgens)
These hormones also promote increased secretion of osteoprotegerin Inhibits osteoclast stimulation
Osteoblast Stimulation Vit D + PTH + Osteocytes stimulates
osteoblasts to secrete cytokines: Stimulate bone resorption via osteoclasts Differentiation of progenitor cells to
osteoclasts Decrease Osteoprotegerin
Cellular Structure:
Derived from macrophage monocyte cell-line Phagocytic cells Responsible for bone resorption Important along with osteoblasts in the constant
turnover and refashioning of bone
Osteoclasts
Osteoclast Inhibition Rate of bone resorption inhibited by:
Calcitionin (C cells of thyroid) Osteoprotegerin (osteoblasts)
Cellular Structure:
Mature bone cells Inactive osteoblasts, trapped and
surrounded by bone matrix Function:
Formation of bone Matrix maintenance Calcium homeostasis
Osteocytes
Bone matrix
Type I Collagen Ground substance
proteoglycans Non-collagen molecules
involved in mineralization regulation
70% inorganic salts, 30% organic matrix
Organic matrix:
Bone matrix
Polymer of numerous elongated overlapping tropocollagen subunits
Hole zones initial site of mineral deposition
Controls water content in bone
Regulating formation of collagen fibers in a form appropriate for mineralization
Type I collagen: Ground substance proteoglycans:
Calcium and phosphate in form of hydroxyapatite
Bone matrix
Non-collagen molecules:
Inorganic component:
Osteocalcin: Binds calcium
Osteonectin: bridging function
between collagen and mineral component
How is bone formed?
Collagen synthesized by osteoblasts Secreted as osteoid
After maturation phase Amorphous calcium phosphate precipitates
in hole zones Mineralization foci expand + coalesce into
hydroxyapatite crystals 20% remains amorphous for readily
available calcium buffer
How is bone formed? Concentration of calcium + phosphate in
extracellular fluid greater than required for spontaneous calcium deposition
Inhibited by pyrophosphate
Deposition of calcium controlled by osteoblasts which secretes alkaline phosphatase vesicles
Neutralizes pyrophosphate
Bone development and growth Develops in 2 ways (2 types of
ossification) Both involve replacement of primitive
collagenous supporting tissue by bone Resultant woven bone is then
extensively remodelled by resorption and appositional growth to form mature adult lamellar bone
Thereafter the process occurs at much reduced rate to accommodate functional stresses and to effect calcium homeostatis
Two types of occification: Endochondral ossification Intramembranous ossification
Bone development and growth
Endochondral ossification E.g. long bones, vertebra, pelvis, base of skull Hyaline cartilage is first formed in a shape
corresponding closely to future bone Cartilage model is covered - perichondrium Bone matrix deposition - replacing the existing
cartilage
Intramembranous ossification E.g. vault of skull,
maxilla, mandible Deposition of bone in
primitive mesenchymal tissue
Direct replacement of mesenchyme by bone Cell differentiation into
osteogenic tissue These become
impregnated with calcium salts
Remodeling/Bone turnover
Process of resorption followed by replacement of bone, with little change in shape
Occurs throughout a person's life Purpose:
To regulate calcium homeostasis Repair micro-damaged bones To shape and sculpture the skeleton during
growth
The role of bone in calcium homeostasis
Bone contains 99% of total body calcium Bone resorption releases calcium into
systemic circulation Bone formation actively binds calcium,
removing it from blood stream Ca2+ homeostasis controlled by:
Parathyroid hormone (parathyroid glands) Calcitonin (Thyroid) Calcitriol (Vit D3)
Increases serum Ca2+
Increases bone resorption by osteoclasts indirectly Mediated by paracrines e.g. osteoprotegerin
Enhances renal reabsorption of calcium Increases intestinal absorption of calciam
Via effects on Vit D
The role of bone in calcium homeostasis
Parathyroid hormone:
Released when plasma Ca2+ increases Decreases bone resorption Increases renal calcium excretion
Enhances intestinal absorption of calcium Facilitates renal reabsorption Helps mobilize Ca2+ out of bone
The role of bone in calcium homeostasis
Calcitonin
Calcitriol
References: Human Physiology an Integrated
Approach Dee Unglaub Silverthorn
Wheater’s Functional Histology B. Young, J.W. Heath
en.wikipedia.org/wiki/Bone July 2012