Effect of estradiol benzoate on the proliferation of osteogenic cells in fetal rat fibulae cultured in vitro.

The influence of estradiol benzoate on the proliferation of bone cells was investigated in vitro by radioautography. The fibulae from 19-day fetal rats were pulse-labeled with 3H-thymidine after various periods of culture on estrogen-supplemented (5 microgram/ml) or control media. The analysis of radioautographs revealed that the cell division was restricted to osteogenic cells of undifferentiated appearance. No label was found in osteoblasts or in osteoclasts. At the time of explantation, about 30% of osteogenic cells were labeled. However, this percentage varied considerably after different intervals of culture. In the control bones, it decreased during first 12 hours to 15% and then rose slowly to attain a preculture value after 48 hours. In the presence of estradiol, the initial decline of labeled cells was less pronounced. The percentage recovered to a preculture level already after 24 hours and continued to increase thereafter. The numbers of labeled osteogenic cells, in estrogen-treated bones, were significantly higher than in controls already after 12 hours of culture. It is concluded that the fetal rat bones undergo in vitro a period of adaptation to a new environment as indicated by a transient decrease of dividing cells. Estradiol benzoate appears to shorten this period by stimulating the proliferation of undifferentiated cells.