Normal Bone

Osteonectin is a 32,000 dalton bone-specific protein that binds selectively to both hydroxyapatite and collagen. When osteonectin is bound to insolubilized type I collagen, the resultant complex binds synthetic apatite crystals and free calcium ions. The osteonectin-collagen complexes also nucleate mineral phase deposition from metastable balanced salt solutions, Antibodies to osteonectin cross-react with bone and, to a lesser extent, dentin, but not with other tissues. The protein is localized to mineralized bone trabeculae and occurs at higher levels in the matrix than in the cells of bone. These studies suggest that osteonectin is a tissue-specific protein, linking the bone mineral and collagen phases, perhaps initiating active mineralization in normal skeletal tissue.

Vesicles have been identified within the cartilage matrix of the upper tibial epiphyseal plate of normal mice. They were seen at all levels within the plate and usually did not appear to be in contact with cartilage cells. Vesicles were concentrated within the matrix of the longitudinal septa from the proliferative zone downward. They varied considerably in size (∼300 A to ∼1 µ) and in shape. They were bounded by unit membranes, and contained materials of varying density including, rarely, ribosomes. A close association was demonstrated between matrix vesicles and calcification: in the lower hypertrophic and calcifying zones of the epiphysis, vesicles were found in juxtaposition to needle-like structures removed by demineralization with ethylenediaminetetraacetate and identified by electron diffraction as hydroxyapatite and/or fluorapatite crystal structure—the former being indistinguishable from the latter for most cases in which electron diffraction methods are employed. Decalcification also revealed electron-opaque, partially membrane-bounded structures within previously calcified cartilage of the epiphyseal plate and underlying metaphysis which corresponded in size and distribution to matrix vesicles. It is suggested that matrix vesicles are derived from cells and that they may play a role in initiating calcification at the epiphysis.