Kj. Lederer et al., PROLIFERATIVE AND STEROIDOGENIC CAPABILITIES OF RAT GRANULOSA-CELLS OF DIFFERENT SIZES, Journal of Reproduction and Fertility, 103(1), 1995, pp. 47-54
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.