Rat thecal/interstitial cells produce a mitogenic activity that promotes the growth of granulosa cells.

To test the hypothesis that the cells outside the basal lamina of the follicle secrete paracrine factors that influence the cells on the inside of the follicle, two ovarian cell populations were obtained from diethylstilbestrol-treated rats. Granulosa cells were obtained by extrusion from the follicles and an ovarian cell preparation, termed thecal/interstitial, was derived from the granulosa cell-depleted ovaries. Light microscopy showed that each cell population in culture had distinctive morphologies. Electrophoretic examination of the radiolabeled proteins secreted by the two ovarian cell preparations revealed that each secreted unique protein components into the culture medium. Rat thecal/interstitial cell-conditioned medium promoted [3H]thymidine incorporation into normal rat kidney cell line (NRK) DNA and into bovine granulosa cell DNA. The growth-promoting activity was stable to heating at 70 degrees C for 5 min whereas native fibroblast growth factor (FGF) lost its activity, showing that the factor was not characteristic of FGF. To further characterize the growth-promoting activity thecal/interstitial cell-conditioned medium was concentrated and the proteins separated by size exclusion high performance liquid chromatography. The growth-promoting activity eluted with an apparent molecular weight between 15,000 and 25,000. The finding that thecal/interstitial cells in culture secrete growth-promoting factors suggests that those cells that are in close proximity to the granulosa cells may secrete protein factors that diffuse into the follicular antrum and influence granulosa cell proliferation.