Osteoblast

bone-forming cells derived from mesenchyme that deposit an eosinophilic osteoid matrix within collagenous tissue, which subsequently becomes mineralized. In the process of intramembranous ossification, some of the osteoblasts on the surface of the osteoid and woven-fibered bone become entrapped in spaces in the matrix called lacunae and then become known as osteocytes. In endochondral ossification, a similar process is followed, and the osteoblasts produce a subperiosteal collar of bone enclosing cartilaginous tissue in the centre of the developing bone. Vascular channels then perforate the shell of bone, disrupting the lacunae and creating spaces that fill with embryonic bone marrow. Osteoblasts appear and lay down osteoid tissue in the cartilage matrix. As in intramembranous bone formation, osteoblasts become trapped within the developing bone as osteocytes. Osteoblasts also are active in collagen and mucopolysaccharide production in bone.

Osteoblasts decrease in number and size as the skeleton reaches maturity, but dormant cells remain capable of responding and producing new bone, as in the healing of a fracture. In later life, the activity of osteoblasts is influenced directly by circulating levels of parathyroid hormone.