The Steroidal Environment of Estradiol and Progesterone in Dominant Follicles does not Affect the Competence of Bovine Oocytes to Develop from Small Antral Follicles

The aim of this study was to examine the effect of the endocrine environment of dominant follicles on the competence of bovine oocytes to develop from small antral follicles 2-5 mm in diameter. Twenty-seven pairs of ovaries with mature corpora lutea were obtained from a local abattoir. The mean diameter and wet weight of the corpora lutea of individual cows were 23.3 ± 2.7 mm and 4.7 ± 1.0 g, respectively. Follicular fluid was aspirated from large follicles more than 6 mm in diameter, and estradiol (E2) and progesterone (P4) concentrations were determined by a RIA. Ovaries were classified into each group of E2-active-dominant (E2-A-dom; E2≥50 ng/ml and E2:P4 ratio>1); 12 pairs of ovaries, E2-dominant (E2-dom; E2 1); 6 pairs of ovaries or P4-dominant (P4-dom; E2:P4 ratio<1); 9 pairs of ovaries. The concentrations of E2 and P4 in E2-A-dom were 132.5 ± 97.6 ng/ml and 8.4 ± 3.5 ng/ml, in E2-dom were 27.7 ± 9.7 ng/ml and 9.2 ± 3.7 ng/ml, and in P4-dom were 4.2 ± 4.7 ng/ml and 98.5 ± 208.4 ng/ml, respectively. The mean number of small antral follicles, the mean number of oocytes fertilized, the rate of cleavage and of blastocysts in E2-A-dom were 44.4 ± 20.3, 24.2 ± 10.9, 68.2%, 26.0%, in E2-dom were 48.5 ± 11.9, 28.7 ± 8.6, 70.4%, 20.2%, in P4-dom were 41.7 ± 24.0, 23.0 ± 15.3, 70.9%, 20.0%, respectively. There were no significant differences in the number of small antral follicles, the number of oocytes fertilized and developmental capacity of the oocytes among the groups. Results of the present study indicate that in cattle E2 and P4 in a dominant follicle do not affect the competence of the development of oocytes derived from small antral follicles.