Interactions between oocytes and cumulus cells during in vitro maturation of porcine cumulus-oocyte complexes in a chemically defined medium: effect of denuded oocytes on cumulus expansion and oocyte maturation

Abstract

The aim of the present study was to clarify interactions between oocytes and cumulus cells (CCs) on the level of cumulus expansion and oocyte maturation during IVM of cumulus-oocyte complexes (COCs) in a chemically defined medium using a system that allows individual tracking of oocytes. Especially, the influence of oocyte-secreted factors was investigated by the aid of addition of denuded oocytes (DOs) as a possible approach to improve the IVM system. The basic maturation medium was porcine oocyte medium with addition of gonadotropins only during the first 20 hours of IVM. During IVM, COCs were kept fixed to the bottom of culture dish by adhesive Cell-Tak coating, which enabled individual tracking of COCs during IVM. Size changes in COCs during IVM were measured by digital image analysis. Cumulus expansion in a porcine oocyte medium of intact COCs increased in a typical manner until 20 hours and decreased in size subsequently until 48 hours of IVM (P < 0.05). Removal of oocytes from COCs by oocytectomy allowed the expansion of CCs to some extent, although their expansion ability was lower than that of COCs (P < 0.05). Addition of DOs (COCs to DOs ratio of 9:16) did not improve cumulus expansion and oocyte maturation rates of intact COCs (P > 0.05) but did enhance cumulus expansion of oocytectomized complexes (P < 0.05). Furthermore, removal of CCs before IVM increased oocyte maturation rates compared with COCs (52.3% and 32.9%, respectively) (P < 0.05) and a similar effect was observed in COCs when the gap junction inhibitor carbenoxolone was added to the IVM medium: carbenoxolone repressed the expansion of COCs at 20 hours of IVM. In conclusion, the porcine oocyte enhances cumulus expansion both by gap junctional communications and presumably by oocyte-secreted factor production. Nevertheless, the presence of oocytes is not a prerequisite for this process. In return, CCs maintain meiotic arrest in cumulus-enclosed oocytes during the initial culture through gap junctions. On the basis of these findings, future research could investigate if coculture with DOs during IVM is beneficial for fertilization and embryo development.

Keywords: Cumulus expansion; Defined medium; Denuded oocyte; IVM; Nuclear maturation; Porcine.