Follicular cells affect the fertilizability and developmental competency of bovine oocytes in vitro

The present study examined the time-dependent effects of follicular cells on the fertilizability of oocytes and their subsequent development to blastocysts. The percentages of oocytes reaching the metaphase-II stage of maturation rose from 51.3% after 16 h of culture to 86.2% at 28 h (cumulus-intact oocytes; CIO) and, for the same time points, from 65.4% to 83.3% (corona-enclosed oocytes; CO) and 54.3% to 88.9% (denuded oocytes; DO), respectively. When DO were cultured for more than 24 h before insemination, fertilization rates were significantly lower compared with CIO and CO. The maximum rates of development to blastocysts were observed when the oocytes were cultured for 24 h in the CIO group (22.1%), 20 h in the CO group (19.7%) and 18 h in the DO group (9.2%), respectively. These results suggest that (i) the presence of cumulus cells or corona cells during maturation is not necessary for nuclear maturation of oocytes; (ii) the attachment of corona cells to the oocytes during maturation is important for the further development to the blastocyst stage, and (iii) the presence of attached cumulus and/or corona cells during maturation in vitro extends the maturation period required for further development to the blastocyst stage.