PROLIFERATIVE AND STEROIDOGENIC CAPABILITIES OF RAT GRANULOSA-CELLS OF DIFFERENT SIZES

Equine chorionic gonadotrophin stimulates both rat granulosa cell mito ses and oestradiol secretion. However, the mitotic potential of oestra diol-secreting granulosa cells is not known. In the first study, granu losa cells of different sizes were isolated and their ability to secre te oestradiol and proliferate in vitro was determined. Granulosa cells were harvested from equine chorionic gonadotrophin-primed immature ra ts, separated on a 15-45% Percoll gradient, and collected in 12 fracti ons. An enriched population of small granulosa cells (44 +/- 1 mu m(2) ) was collected in fractions 3 and 4 and an enriched population of lar ge granulosa cells (97 +/- 2 mu m(2)) in fractions 6-8. When granulosa cells hom each fraction were cultured for 24 h in the presence of tes tosterone, the large cells secreted 50% more oestradiol than did the s mall cells (P < 0.05). Aromatase was shown, by immunocytochemistry, to be expressed mainly by granulosa cells larger than 73 mu m(2), with t he relative amount of aromatase expressed per cell increasing with inc reasing cell size. However, not all large granulosa cells expressed ar omatase. To test proliferative capacity, cells from each fraction were cultured with testosterone and the mitogen, insulin. This study showe d that only small cells were able to undergo insulin-induced mitosis. In a second study, follicles of different sizes were isolated from imm ature and equine chorionic gonadotrophin-primed immature rats and the granulosa cell size distribution determined for each follicle size. Th is study confirmed that equine chorionic gonadotrophin altered the siz e distribution from principally small mitotically competent cells to l arge oestradiol-secreting cells. Studies in vitro further demonstrated that FSH in the presence of 8-bromo-cAMP stimulated small granulosa c ells to differentiate into large cells. It is proposed that changes in the population of granulosa cells could account for both the slower g rowth rate of large antral follicles compared with small antral follic les and the inverse relationship between follicular oestradiol secreti on and DNA synthesis of granulosa cells.