Macromolecular crowded conditions strengthen contacts between mouse oocytes and companion granulosa cells during in vitro growth

Macromolecular crowded culture medium formed by addition of polyvinylpyrrolidone (PVP; molecular weight = 360 000), positively influences the viability, growth, and development of bovine oocytes. Owing to its apparently various effects, uncovering the specific mechanisms of crowding responsible for these outcomes is important. The present study was conducted to determine the effects of crowding on oocytes with a particular focus on the intimacy of contacts between oocyte and cumulus/granulosa cells. Growing mouse oocyte-granulosa cell complexes were cultured for 10 days in a modified α-minimum essential medium, supplemented with PVP at a concentration of 0%, 1%, 2%, or 3% (w/v). Although the complexes developed in all groups, 2% and 3% PVP medium induced a substantial morphological modification, and a larger proportion of oocytes associated with cumulus cells survived in 3% PVP medium than in the 0% or 1% PVP medium. No significant difference was found in the frequencies of polar body extrusion (78–88%) and blastocyst formation (approximately 40%) after in vitro fertilization among the experimental groups. Confocal laser scanning microscopy indicated a higher number of transzonal processes reaching the oocyte from cumulus cells in 2% PVP medium than in 0% PVP medium. Transmission electron microscopy depicted close adhesion of the oocyte with cumulus cells in 2% PVP medium —bearing a resemblance to their in vivo counterparts— and loose adhesion in 0% PVP medium. In conclusion, we found that a mechanism for the action of crowded conditions involves the strengthening of contacts and communication between oocytes and companion cumulus/granulosa cells.