Chapter 8: Bone

Bone SUMMARY OF KEY POINTS

• Bone is a type of connective tissue with a calcified extracellular matrix (ECM), specialized to support the body, protect many internal organs, and act as the body’s Ca²⁺ reservoir.

• Osteoblasts differentiate from (stem) osteoprogenitor cells and secrete components of the initial matrix, called osteoid, that allow matrix mineralization to occur.

• Important components of osteoid include type I collagen, the protein osteocalcin, which binds Ca²⁺ and matrix vesicles with enzymes generating PO₄⁻.

• High concentrations of Ca²⁺ and PO₄⁻ ions cause formation of hydroxyapatite crystals, whose growth gradually calcifies the entire matrix.

• Osteocytes differentiate further from osteoblasts when they become enclosed within matrix lacunae and act to maintain the matrix and detect mechanical stresses on bone.

• Osteocytes maintain communication with adjacent cells via a network of long dendritic processes that extend through the matrix via narrow canaliculi radiating from each lacuna.

• Osteoclasts are very large cells, formed by fusion of several blood monocytes, which locally erode bone matrix during osteogenesis and bone remodeling.

• Periosteum is a layer of dense connective tissue on the outer surface of bone, bound to bone matrix by bundles of type I collagen called perforating (or Sharpey) fibers.

• Regions of periosteum adjacent to bone are rich in osteoprogenitor cells and osteoblasts that mediate much bone growth and remodeling.

• The endosteum is a thin layer of active and inactive osteoblasts, which lines all the internal surfaces within bone; osteoblasts here are also required for bone growth.

• Dense bone immediately beneath the periosteum is called compact bone; deep to the compact bone are small bony trabeculae or spicules of cancellous (or spongy) bone (Table 8–1).

• In long bones of the limbs, these two types of mature bone tissue occur in both the knobby, bulbous ends, called epiphyses, and in the intervening shaft or diaphysis.

• Immature bone, called woven bone, is formed during osteogenesis or repair and has a calcified matrix with randomly arranged collagen fibers.

• By the action of osteoclasts and osteoblasts, woven bone undergoes rapid turnover and is remodeled into lamellar bone with new matrix deposited in distinct layers with parallel collagen bundles; both compact and cancellous bone is lamellar bone.

• Most lamellar bone consists of lamellae organized concentrically around small central canals containing blood vessels and nerves; this organization is called an osteon or Haversian system.

• Within each osteon, osteocytic lacunae occur between the lamellae, with canaliculi radiating through the lamellae, which allow all cells to communicate with the central canal.

• Bones of the skull and jaws form initially by intramembranous ossification, with osteoblasts differentiating directly from progenitor cells in condensed “membranes” of mesenchyme.

• All other bones form by endochondral ossification, in which osteoprogenitor cells surround and then invade hyaline cartilage models...