Bone is composed of osteogenic cells, organic matrix, and mineral.

Osteogenic cells: osteoblasts, osteocytes, and osteoclasts, derived from mesenchymal cells

Matrix (⅓ mass): collagen and proteoglycans

Mineral (⅔ mass): calcium phosphate crystals deposited as hydroxyapatite.


Osteoblast lining cells form a polarized layer and depoit osteoid. Osteocytes live within canaliculi of the mineralized bone matrix. Together osteoblasts and osteocytes regulate the flow of mineral ions from extracellular fluid to the mineralized bone matrix. Osteoclasts are found in sites of bone remodeling.


There are two types of bone matrix: hard compact cortical bone, which surrounds marrow cavities within the shafts of long bone, and spongy cancellous bone forming trabeculae cavities that enclose hematopoietic marrow and fatty marrow.

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