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  • 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.

Major Cells & Matrix Components of Bone
  • 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 & Endosteum
  • 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.

Types & Organization of Bone
  • 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...

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