Dialogue between oocytes and somatic cells
There are two populations of GCs in large antral follicles: mural granulosa cells (MGCs) that line the ovarian follicle wall, and cumulus cells (CCs) closely associated with the oocyte. Among the genes expressed more highly in CCs was one encoding an amino acid transporter (Slc38a3). Slc38a3 mRNA was not detected in oocytes. Expression of Slc38a3 mRNA was reduced in the CCs after removal of the oocyte and restored by co-culturing CCs with fully-grown oocytes. Alanine is one of the amino acids transported by SLC38A3. This amino acid is poorly transported across the oocyte plasma membrane, but gains access to the oocyte from the cumulus cells via gap junctional communication. Alanine transport into cumulus cells is promoted by paracrine factors secreted by fully-grown oocytes (FGOs), but not by growing oocytes (GOs) from preantral follicles. Thus FGOs promote the transport of alanine into CCs, and this amino acid is then passed on to the oocyte via gap junctions.
Transcripts encoding enzymes in the glycolytic pathway are also more highly expressed in CCs than MGCs. FGOs, but not GOs, promote elevated expression of some of these transcripts. Likewise, FGOs promote both glycolysis and oxidative phosphorylation by isolated CCs and MGCs. Oocytes do not effectively utilize glucose as an energy source, and oocytes require the presence of CCs to resume meiosis when glucose is the only energy source present. In contrast, oocytes can resume meiosis in the absence of CCs when pyruvate is the sole energy source. Thus oocytes apparently promote glycolysis by their companion granulosa cells to provide energy for their own development. In addition, this may be one way that oocytes coordinate their development with that of follicular somatic components. (Supported by Grants HD23839 and HD44416 from the NICHD)