Oocytes are required for the preantral granulosa cell to cumulus cell transition in mice

Preantral granulosa cells (PAGCs) differentiate into cumulus cells following antrum formation. Cumulus cells, but not PAGCs, are competent to undergo expansion. Experiments reported here tested the respective roles of both oocytes and FSH in the transition of preantral granulosa cells to cumulus cells competent to undergo expansion. PAGC-oocyte complexes were cultured with or without a low dose of FSH (0.005 IU/ml) and isolated PAGCs were cultured with or without oocytes. At the end of culture, complexes or isolated PAGCs were tested for their ability to undergo cumulus expansion and upregulate expansion transcripts in response to EGF or FSH (0.5 IU/ml). The ability to undergo expansion in response to EGF required the presence of oocytes but not FSH during the culture period. Likewise, complexes isolated from the ovaries of hypogonadal mice, which lack circulating gonadotropins, underwent expansion in response to EGF, but not FSH. In contrast, the ability to activate MAPK3/1 and MAPK14 and undergo expansion in response to FSH required prior exposure to low doses of FSH. However, these low levels (0.005 or 0.025 IU FSH/ml) suppressed expression of Slc38a3 and Amh, two transcripts highly expressed in cumulus cells, suggesting opposing effects of FSH on cumulus cell differentiation. In conclusion, the ability to undergo expansion in response to FSH requires prior exposure to FSH during development, while oocyte-derived factors alone are sufficient to promote the ability to undergo expansion in response to EGF. These results highlight the crucial role of oocytes in driving the differentiation of PAGCs into cumulus cells during the preantral to antral follicle transition.