Bidirectional communication between cumulus cells and the oocyte: Old hands and new players?

Cumulus cell-oocyte communication is an essential feature of mammalian reproduction. Established mechanisms involve the bidirectional transfer of ions and small molecules through gap junctions that fundamentally regulate the process of oocyte maturation. Also, well established is the paracrine signaling from the oocyte to the cumulus, which regulates much of the flow of ions and molecules to the oocyte and orchestrates many of the associated local signaling events around ovulation, which is the key to establishing oocyte competence to sustain early embryo development. Less well-characterized and new potential players include exosomal transfer of noncoding RNAs from cumulus to oocytes and the recent observations of the presence of hemoglobin in oocytes and cumulus cells. The impact of these new communication pathways is either poorly defined or even unknown. Finally, signaling between the two cell types most likely continues after ovulation and even fertilization; however, this too is largely undefined but may play roles in substrate transport, sperm chemotaxis and "trapping", and potential signaling to the rest of the reproductive tract.