SKELETAL UNLOADING INHIBITS THE IN-VITRO PROLIFERATION AND DIFFERENTIATION OF RAT OSTEOPROGENITOR CELLS

Loss of weight bearing in the growing rat decreases bone formation, os teoblast numbers, and bone maturation in unloaded bones. These respons es suggest an impairment of osteoblast proliferation and differentiati on. To test this assumption, we assessed the effects of skeletal unloa ding using an in vitro model of osteoprogenitor cell differentiation. Rats were hindlimb elevated for 0 (control), 2, or 5 days, after which their tibial bone marrow stromal cells (BMSCs) were harvested and cul tured. Five days of hindlimb elevation led to significant decreases in proliferation, alkaline phosphatase (AP) enzyme activity, and mineral ization of BMSC cultures. Differentiation of BMSCs was analyzed by qua ntitative competitive polymerase chain reaction of cDNA after 10, 15, 20, and 28 days of culture. cDNA pools were analyzed for the expressio n of c-fos (an index of proliferation), AP (an index of early osteobla st differentiation), and osteocalcin (a marker of late differentiation ). BMSCs from 5-day unloaded rats expressed 50% less c-fos, 61% more A P, and 35% less osteocalcin mRNA compared with controls. These data de monstrate that cultured osteoprogenitor cells retain a memory of their in vivo loading history and indicate that skeletal unloading inhibits proliferation and differentiation of osteoprogenitor cells in vitro.