Osteoblasts are bone-forming cells. Although this is a useful definition for the purpose of discussion, osteoblasts do not actually form complete bone. Rather, they synthesize the organic (ie, nonmineral) component of bone, the primary element of which is osteoid (a protein matrix of type I collagen).
Osteocalcin and bone sialoproteins are other organic products of osteoblasts. Through these products, the osteoblasts regulate the mineralization of osteoid. Bone, therefore, is a composite of organic products of cellular synthesis and mineral deposited by extracellular matrix calcification.3,4 Because osteoblasts are metabolically active, they typically contain a large Golgi apparatus (Fig. 4).
This organelle is responsible for secreting type I collagen, the prime component of the bone matrix. Osteoblasts also contain a large nucleus that is usually oriented away from the osteoid-producing side of the cell. In addition to collagen, osteoblasts also produce an enzyme called alkaline phosphatase. The precise function of alkaline phosphatase is still unknown; however, based on the observation that patients who are producing bone (ie, those with fractures or bone-forming tumors) have increased serum levels of alkaline phosphatase, researchers reasonably conclude that it plays some role in the mineralization of osteoid. Alkaline phosphatase is therefore a useful clinical marker of bone formation activity. Osteoblasts are derived from the mesenchymal cell line. Once a mesenchymal cell becomes committed to the bone cell lineage, it is known as a preosteoblast. These preosteoblasts become osteoblasts when properly stimulated. The precise signals responsible are not known, but substances shown to have a role in the regulation of the differentiation of this cell line include bone morphogenetic proteins, growth factors, interleukins, insulin-derived growth factor, and platelet-derived growth factor. The organic matrix secreted by osteoblasts is primarily type I collagen. Approximately 10% of the mass of the osteoid is composed of other matrix proteins and growth factors. Mineralization of the matrix occurs via a controlled process that has not been completely identified. It is known, however, that calcium must be locally concentrated (to form precipitates) and that energy is required to initiate the process. Some of the noncollagenous matrix proteins apparently promote or inhibit mineralization and modulate crystal size.