DIFFERENT BEHAVIOR OF HUMAN OSTEOBLAST-LIKE CELLS ISOLATED FROM NORMAL AND HETEROTOPIC BONE IN-VITRO

In this study, a characterization of human bone-forming cells responsi ble for heterotopic ossification was carried out in vitro. The biologi cal and biochemical cell characteristics of the heterotopic osteoblast -like (HOB) cells were compared with those of orthotopic osteoblast-li ke (OB) cells from normal bone and stromal bone marrow cells believed to contain a subpopulation of osteogenic precursor cells. We found tha t HOB's from the spongiosa of heterotopic ossification required less t ime until the beginning of migration and the achievement of confluence in vitro compared with OBs from femoral shaft spongiosa. The fraction of mitotically active cells assessed by a clonogenic assay was higher as well in HOB cells. The in vitro studies of mitogenesis and the eff iciency of colony formation of osteogenic cells indicate that with inc reasing differentiation and relative age they become more dependent on growth factors in the medium, otherwise the morphology of osteoblast- like cells changes and they pass irreversibly into the postmitotic sta ge of the cell cycle. The activity of the alkaline phosphatase is dist inctly higher in the HOB than in the OB cells, HOB cells exhibit a low er level of osteocalcin expression compared with OB cells. No signific ant difference was found between OB and HOB cells in the amount of pro collagen of type I sequestered by the cells. After 30 days, HOB and OB cells formed a mineralized matrix on exposure to 2 mM beta-glyceropho sphate. Since HOBs were isolated from heterotopic bone that had develo ped within 3-6 months after hip surgery, the differences in cellular b ehavior compared with OBs may be attributed to the relatively young ag e of HOB cells.