Oocyte control of granulosa cell development: how and why.

Interaction between oocytes and granulosa cells is complex and involves both gap junctions and paracrine signalling factors. Oocyte development in antral follicles is highly dependent on communication with cumulus cells, a subset of granulosa cells that is intimately associated with oocytes. Cumulus cells express characteristics distinct from the mural granulosa cells of preovulatory follicles. The thesis of this paper is that, without the influence of oocytes, the pathway of granulosa cell differentiation in antral follicles leads to the establishment of the mural granulosa cell phenotype. Oocytes in antral follicles abrogate that pathway of granulosa cell differentiation and promote the development of the cumulus cell phenotype. Oocytes may do this in order to control their own microenvironment by regulating differentiation of the supporting cells that are in direct communication with them. Possibly, some aspects of the mural granulosa cell phenotype are antagonistic to, or insufficient for, supporting the final stages of oocyte development. We present evidence that oocytes control their environment by suppressing differentiation of the mural granulosa cell phenotype and promoting differentiation of the cumulus cell phenotype. They achieve this suppression via the secretion of labile paracrine signalling factors. Errors in this regulatory mechanism, whether instigated by defects in the production of oocyte-derived ligands or granulosa cell responses to them, may result in the production of oocytes unable to undergo embryo development or that undergo abnormal follicular development.