Although it has been shown that granulosa cells regulate the growth and meiotic maturation
of mammalian oocytes, there is little evidence of a role for the oocyte in the differentiation
or function of granulosa cells. To test the hypothesis that the oocyte participates
in the regulation of granulosa cell function, oocytes were removed from isolated oocyte-cumulus
cell complexes by a microsurgical procedure and oocytectomized complexes were tested
for their ability to undergo expansion in response to follicle-stimulating hormone
(FSH). FSH increased the levels of intracellular cAMP, the activity of the hyaluronic
acid-synthesizing enzyme system, and induced cumulus expansion in intact complexes.
In contrast, FSH did not induce increased hyaluronic acid-synthesizing enzyme activity
or cumulus expansion in oocytectomized complexes. Therefore, the participation of
the oocyte is necessary for the cumulus cells to synthesize hyaluronic acid and undergo
cumulus expansion in vitro in response to stimulation with FSH. FSH induced the elevation
of intracellular cAMP to the same extent in both intact and oocytectomized complexes
and the cAMP analog 8-bromo cyclic adenosine monophosphate (8Br-cAMP) did not stimulate
expansion in oocytectomized complexes. Therefore, the influence of the oocyte on cumulus
expansion occurs downstream from the elevation of cAMP levels in the cumulus cells.
Epidermal growth factor (EGF), a potent stimulator of cumulus expansion in intact
complexes, which probably acts by a mechanism at least initially different from FSH,
failed to stimulate cumulus expansion after oocytectomy. Next, oocytectomized complexes
were either cocultured with germinal vesicle stage denuded oocytes or cultured in
medium conditioned by denuded oocytes. In both cases, FSH or EGF stimulated expansion
by oocytectomized complexes. The degree of expansion was directly correlated to the
number of oocytes used to condition the medium. Contact between the oocyte and the
cumulus cells is not necessary for cumulus expansion. Rather, a factor(s) secreted
by the oocyte is necessary for the cumulus cells to undergo expansion in response
to either FSH or EGF. FSH did not induce expansion of oocytectomized complexes in
media conditioned by various somatic cells such as granulosa cells, fibroblasts, and
Sertoli cells; by a mixed population of male germ cells; or by spermatozoa. This suggests
that the expansion enabling activity is specific to the oocyte. These results demonstrate
that the oocyte participates in the regulation of cumulus cell function.