Osteoblast

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Histological appearance of bone tissue formation.

Osteoblasts are bone-forming cells derived from osteoprogenitor stem cells which arise from mesenchymal tissue. They are mostly located in the periosteum and the endosteum but may also occur within compact bone, in regions of remodeling.

Histologically, active osteoblasts, which are engaged in bone matrix synthesis, appear as large, cuboidal to columnar mononuclear cells with abundant basophilic cytoplasm. This appearance reflects the increased synthesis of type I collagen, proteoglycan and various glycoproteins which are the organic components of bone matrix. Inactive osteoblasts, on the other hand, usually have a flattened appearance.

Active osteoblasts initially synthesize and secrete osteoid, which is a semisolid, organic form of bone matrix, between the osteoblast layer and the existing bone surface. The osteoid subsequently hardens into bone with the deposition of calcium salts which bind to glycoproteins such as osteocalcin. Additionally, osteoblasts release membrane-bound matrix vesicles containing alkaline phosphate and other enzymes that increase local phosphate ion concentrations, ultimately facilitating bone mineralization.

Osteoblasts entrapped within small hollow spaces (lacunae) of the bone matrix they produce differentiate into mature bone cells called osteocytes. These osteocytes typically lie side by side, forming a layer at the surfaces of bone matrix.