Involvement of meiotic resumption in the disruption of gap junctions between cumulus cells attached to pig oocytes.

The present study was undertaken to examine the mechanisms by which cumulus cells regulate meiotic resumption in pig oocytes using microinjections of lucifer yellow into cumulus-oocyte complexes combined with a fluorescent assay. Some cumulus-oocyte complexes cultured for 0, 8, 16, 24, 32, 40 and 48 h were denuded to assess the nuclear status of oocytes; the remaining complexes were injected with lucifer yellow and monitored for the transfer from the oocyte to the surrounding cumulus cells using confocal laser scanning microscopy. The proportion of oocytes undergoing germinal vesicle breakdown at 16, 24 and 32 h of cultivation was much higher than that of cumulus-oocyte complexes in which all gap junctions within cumulus cells and between cumulus cells and oocyte were disrupted (16 h: 22.7% versus 8.8%, 24 h: 66.7% versus 40.3%, 32 h: 84.0% versus 69.4%), showing that the disruption does not trigger meiotic resumption in the pig oocyte. A significant positive correlation (r = 0.99, P < 0.01) was established between the proportion of germinal vesicle breakdown oocytes and that of cumulus-oocyte complexes exhibiting loss of gap junctions within all cumulus cell layers excluding the innermost layer. From these results, it is concluded that meiotic resumption in pig oocytes is induced by the disruption of gap junctions within cumulus cells, rather than that between the oocyte and cumulus cells, which blocks the conduction of meiosis inhibitory signals from the outer cumulus cells to the oocyte.