Histology, Development, & Growth of Bone Histology of bone Bone development Bone growth Role of bone in calcium homeostasis

Review of bone composition Bone: 35% organic, 65% inorganic - organic component is collagen: gives flexible strength - inorganic component is calcium and phosphate - calcium phosphate interacts with calcium hydroxide to form hydroxyapatite crystals - gives compression strength Result: strong, yet flexible structure

Bone Cells Bone cells: osteoblasts, osteocytes, osteoclasts - osteocytes are located in lacunae - osteocytes are connected to each other by thin cytoplasmic projections with gap junctions - the cell processes are enclosed in canaliculi

Types of bone: spongy Cancellous (spongy) bone - Cancellous bone consists of network of trabeculae (lighter than compact bone) - Trabeculae do not have direct blood supply, osteocytes receive nutrition through diffusion via canaliculi - Trabeculae are lined by osteoblasts - The orientation of trabeculae is influenced by stress on the bone - Found in areas without much bone stress, or where stress comes from several directions - Site of blood cell formation (red marrow)

Effects of stress on spongy bone

Types of bone: compact Compact bone - More dense than spongy bone - Has a direct vascular supply in matrix - osteons - circumferential lamellae veincapillary osteons interstitial lamellae spongy bone perforating canal central canal

Bone development (ossification) n n Ossification: the formation of bone by osteoblasts n n Two types: intramembranous and endochondral n n Intramembranous: - type of bone formation in bones of skull and clavicle - start with connective tissue membrane containing osteoprogenitor cells - osteoblasts form, produce trabecullae (spicules) of woven bone at centers of ossification

Bone formation n n Intramembranous (cont.): - cancellous bone forms as trabecullae radiate out of centers of ossification - periosteum forms around bone - outer layer of compact bone is formed - as process continues, woven bone is removed and replaced by mature bone

Bone formation Endochondral Ossification - bone produced from a cartilage model

Bone formation Formation of Secondary Ossification Center

Growth of Bones n n There are two types of bone growth: endochondral and appositional n n Endochondral growth: growth of cartilage, which is replaced by bone.

Endochondral Bone Growth n n The epiphyseal plate is organized into four zones: a) zone of resting cartilage (epiphyseal surface): chondrocytes don't divide rapidly b) zone of proliferation: densely packed columns of chondrocytes, rapidly dividing c) zone of hypertrophy: chondrocytes mature and enlarge d) zone of calcification: thin layer of calcified matrix, chondrocytes die, vessels grow into the area bringing in osteoblasts, which form bone.

Endochondral Bone Growth zone of resting cartilage zone of proliferation zone of hypertrophy zone of calcification

Growth of Bones Appositional bone growth - responsible for increased diameter (not length) of bones - osteoblasts add matrix to surface of bone: rate varies - osteoclasts remove bone at fixed rate

Growth of Bones Bone growth is influenced by many factors - nutrition: calcium availability & collagen synthesis - calcium intake - role of vitamin D in calcium absorption - vitamin C required for collagen synthesis (scurvy) - vitamin A stimulates osteoblast activity - stimulated by hormones - growth hormone - thyroid hormone - estrogen and testosterone - stimulated by weight-bearing stress/exercise

Role of Bone in Calcium Homeostasis Calcium plays important roles in: - bone formation - controlling permeability of cell membranes - contraction of muscle cells - hormone action (second messenger signaling) Sources of calcium: - diet (main source) - bone reservoir

Regulation of bone calcium Parathyroid hormone (PTH) - from parathyroid glands - reduces loss of calcium from the kidneys, small intestine - causes kidney to activate vitamin D - increases osteoclast activity - PTH levels increase in response to decreased blood Ca ++ Calcitonin - from the thyroid gland - decreases osteoclast activity (calcium returned to bone) -increasing the rate of excretion of calcium ions at the kidneys - calcitonin levels increase in response to increased blood calcium levels

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