Oestrous cycle stage influences the morphology and maturation of porcine oocytes in vitro

The objective of the study was to characterize the effect of the oestrous cycle stage on the yield, morphology and meiotic competence of porcine oocytes. A total of 46 cycling gilts, at 8.5-9 months of age, were used as oocyte donors. Their oestrous cycle was synchronized by Regumate and the onset of oestrus was checked (Day 0). The gilts were slaughtered at the early (Days 1-5), middle (6-10) and late (11-14) luteal or early (Days 15-16), middle (17-19) and late (20-21) follicular phase. Oocytes were isolated separately from medium (5-9 mm) and small (≤ 4 mm) follicles. Cumulus-oocyte complexes with dark, evenly granulated cytoplasm and at least two compact layers of cumulus cells were selected as useful for maturation. They were matured by a standard protocol, denuded from cumulus cells, fixed in glutaraldehyde, stained with 33258-Hoechst and examined by epifluorescence. The oocytes collected from small and medium follicles differed in their yield, morphology and meiotic competence regardless of the phase. The mean number (± S.E.M.) of oocytes isolated per donor was higher (187.7 ± 48.4 vs. 16.9 ± 6.1) but the mean percentage (± S.E.M.) of useful oocytes was lower (22.4 ± 7.5% vs. 80.2 ± 6.8%; P < 0.01) for small than for medium follicles. The mean number (± S.E.M.) of useful oocytes per donor was significantly (P < 0.01) higher (42.1 ± 16.8 vs. 11.9 ± 4.3) but the mean percentage of matured oocytes was significantly (P < 0.01) lower (48.4 ± 17.8% vs. 79.9 ± 7.9%) for small than for medium follicles. The oocyte population collected from small follicles varied during the oestrous cycle. The mean number (± S.E.M.) of oocytes isolated per donor from small follicles increased during the luteal and decreased during the follicular phase, except for the late follicular phase when it increased again. The mean percentage (± S.E.M.) of useful oocytes did not differ too much during this period, except for the late follicular phase when it decreased significantly (P < 0.01). The mean number (± S.E.M.) of useful oocytes per donor increased during the luteal and decreased during the follicular phase, but the differences were not significant except for the late follicular phase (P < 0.01). Similarly, the mean percentage (± S.E.M.) of matured oocytes increased during the luteal and decreased during the follicular phase, and the differences were significant. Compared with the oocyte population from small follicles, the oocyte population from medium follicles was less variable in the period from the middle luteal to middle follicular phase, when these follicles were present on the ovaries. It can be concluded that the porcine oocyte population changes in terms of quantity, morphological quality and meiotic competence according to the stage of follicular development. The late luteal and early follicular phases appeared to be most productive for oocyte recovery, because more morphologically normal oocytes with greater meiotic competence were collected, as compared with the other stages of the oestrous cycle.