Ultrastructure of human osteoblasts and associated matrix in culture.

The ultrastructure, as visualized by transmission electron microscopy, of cells obtained from human bone explants and subsequently cultured is described along with the electron microscopic appearance of the associated intercellular matrix. The cells were characterized as osteoblasts on the basis of immunohistochemical, enzymatic, and functional criteria. Although the osteoblasts could be cultured in standard culture media and always appeared singly, not forming syncytia, the cultures were eventually confluent and formed multilayers. The cells were fusiform or cuboidal with diameters ranging between 10-15 microns. The cytoplasm was characterized by numerous large mitochondria, and especially by a very prominent RER. The intercellular matrix was woven with collagen fibres surrounding large numbers of matrix vesicles. In areas with matrix vesicles, evidence for osteoblast activity, i.e. mineralization related to matrix vesicles, could be observed after incubation with beta-glycerophosphate. In conclusion, we provide evidence that human osteoblasts cultured in vitro synthesize collagen and produce a matrix with vesicles capable of initiating mineralization processes.