Oocyte quality in small antral follicles in the presence or absence of a large dominant follicle in cattle.

The aim of these experiments was to determine whether the presence of a dominant follicle affects the developmental competence of oocytes from small antral follicles of cattle. In Expt 1, oocytes or follicular fluid samples were collected from follicles 2-7 mm in diameter before (day 3 of the oestrous cycle; n = 4) or after (days 6 and 7 of the oestrous cycle; n = 6) emergence of the first wave dominant follicle (verified by rectal ultrasonography). Five to ten follicles were aspirated for the determination of individual follicular fluid concentrations of oestradiol, progesterone, testosterone and dimeric inhibin; oocytes from the remaining follicles from each cow were pooled and developmental capacity assessed by in vitro fertilization and maturation. In Expt 2, ovaries containing a young corpus luteum and with or without a large oestrogen-active follicle were collected from the abattoir. Follicular aspirates from small follicles in each pair of ovaries were pooled, and oocyte quality and steroid concentrations were determined. In Expt 1, small follicles obtained before emergence of the dominant follicle contained significantly more oestradiol than they did after emergence (19.5 +/- 1.5 versus 0.7 +/- 1.1 ng ml-1, respectively; P < 0.05), but there were no significant differences in concentrations of progesterone, testosterone or dimeric inhibin. The percentage of blastocysts obtained from oocytes collected before (12.1 +/- 9.0) or after emergence (11.8 +/- 7.0) of the dominant follicle did not differ significantly (P > 0.05). In Expt 2, follicular steroid concentrations did not differ in small follicles taken in the presence versus the absence of a large oestrogen-active follicle, and there were no differences in the developmental capacity of the oocytes. There were significant negative correlations between follicular oestradiol concentration and the percentage of blastocysts formed from two-cell (r = -0.90; P < 0.01) and eight-cell embryos (r = -0.65; P < 0.05). These data suggest that in cattle the developmental competence of oocytes from small antral follicles is not adversely affected by the presence of a dominant follicle.