ONTOGENY OF ULTRASTRUCTURAL FEATURES DURING OSTEOGENIC DIFFERENTIATION IN DIFFUSION CHAMBER CULTURES OF MARROW-CELLS

Three stages of osteogenic differentiation can be identified in in viv o diffusion chamber cultures (DCC) of unselected marrow cells, namely, proliferation, differentiation, and maturation (mineralization). Thes e stages were characterized correlatively by in situ differential cell counts, alkaline phosphatase activity, and mineral accumulation. In t he present study, the ultrastructure of marrow cell DCC was examined a fter incubation for 3-21 days. Features characteristic of osteoblastic and chondroblastic differentiation were first noted in 12 day DCC. Si tes of osteoblastic differentiation showed cell-cell contacts associat ed with an increased cell density. The osteoblastic cells had long pro cesses and were embedded in matrix with prominent fiber bundles remini scent of collagen type I. The chondroblastic cells appeared solitary i n areas of lesser cell density. By contrast to the long osteoblastic c ell processes, they had short plasmalemmal projections and the matrix surrounding them contained single, thin, short fibers reminiscent of c ollagen type II, as well as proteoglycan granules. Both cell types sho wed prominent cytoskeletal elements, rough endoplasmic reticulum, and Golgi. One finding, previously unnoted in differentiating osteogenic c ells, was mitochondria with condensed cristae that represent an increa sed rate of energy metabolism. These mitochondria were particularly ab undant in the differentiation stage and declined as the cultures matur ed. These findings, together with previous reports in the epiphyseal g rowth plate, suggest that mineralization is associated with an optimal level of energy metabolism rather than extreme hypo- or hyperoxia. Th e set of ultrastructural parameters defined here in the marrow cell DC C may serve as useful markers for cells undergoing osteogenic differen tiation.