Mineralization in in vitro cultures of rabbit marrow stromal cells.

Localized regions of mineralization were found in confluent cultures of rabbit marrow fibroblastic cells. The mineralized tissue developed within clusters of giant fat cells in the spaces between the cells. Investigations with light and electron microscopy demonstrated that in these sites there was some differentiation of the fibroblastic cells in an osteogenic direction, shown by changes to more polygonal shapes, and the synthesis of well-banded collagen similar to that found in bone tissue. Differentiation may be due, in part, to increased cell density in a confined space. Growth of the mineralized tissue was observed in the living cultures with a fluorescence microscope. Electron probe microanalysis confirmed that the mineral formed was hydroxyapatite. Initiating sites of mineralization included membranous vesicular bodies, lipid, and products of cellular degeneration. Once initiated, mineralization appeared to spread rapidly into adjacent collagenous and other structures, suggesting the appearance of a mixture of skeletal-type and dystrophic mineralization.