Osteoblasts

Bone-forming cells are called osteoblasts. They are characterized by high levels of alkaline phosphatase, an enzyme required for matrix mineralization, and display structural features reflecting their intense secretory activity (e.g., prominent endoplasmic reticulum).

Osteoblasts are arranged as a closely packed layer of cells on growing bone surfaces, with each cell producing around three times its own volume of bone in about 3 days. Newly synthesized bone matrix is produced in an unmineralized form (termed osteoid) and consists of highly crosslinked collagen I fibers (which give the tissue its tensile strength) and a number of noncollagenous proteins such as osteocalcin. Osteoid is also rich in osteo-blast-derived growth factors - insulin-like growth factor II (IGF-II) and transforming growth factor fi -and these may regulate local bone turnover. Once formed, osteoid is mineralized. In cortical bone, crystal growth begins at sites along the collagen fibrils and is regulated by inhibitory molecules released by the osteoblasts.

Approximately 10-20% of osteoblasts become entombed in the matrix that they have produced and are termed osteocytes. They remain linked to the bone surface via long cell processes and appear to respond to mechanical loading. Osteocytes may therefore be responsible for coupling of mechanical stimulation to bone growth.

Osteoblasts contribute to the control of bone resorption, responding to bone-resorbing signals by producing degradative enzymes and by releasing molecules that increase osteoclast activity and recruitment. Osteoblasts may therefore coordinate bone turnover by switching from bone formation to the control of bone resorption.

Origin of the osteoblast Osteoblasts are derived from stem cells located near bone surfaces. These stem cells can give rise to cartilage, fat and fibrous tissues in experimental systems, suggesting that osteoblasts form part of a superfamily of connective-tissue cells.

The stem cells divide and mature into preosteo-blasts, an intermediate cell type, which displays some osteoblast-like features, e.g., type I collagen production, alkaline phosphatase and osteonectin mRNA, but which lacks the intense alkaline phos-phatase activity and highly developed endoplasmic reticulum of the mature cells.

Terminal differentiation into mature osteoblasts is associated with the cessation of cell division, the production of osteopontin and osteocalcin, and changes in both oncogene expression and nuclear protein-DNA interactions. Further changes in regulatory nuclear proteins accompany the onset of mineralization.

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