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