Effect of Ovulatory Follicle Size on Steroidogenic Capacity and Molecular Markers of Oocyte Competence prior to GnRH-Induced Ovulation in Nonlactating Beef Cows.

Abstract

Gonadotropin releasing hormone (GnRH)-induced ovulation of small dominant follicles decreased pregnancy rates and increased late embryonic/fetal mortality in beef cows. Inadequate oocyte competence, as affected by the physiological status of the dominant follicle, is a potential explanation for the reduction in pregnancy rates and late embryonic/fetal survival. Molecular markers of oocyte competence in both the oocyte (i.e. inhibin beta A [INHBA], inhibin beta B [INHBB] and their binding protein follistatin [FST]) and the surrounding cumulus cells (cathepsins B [CTSB], S [CTSS], and Z [CTSZ]) have been reported. The objective was to examine the relationship between preovulatory follicle diameter, steroidogenic capacity of the preovulatory follicle, and the following measures of oocyte competence: relative abundance of FST, INHBA, and INHBB mRNA in the oocyte, relative abundance of cathepsin (CTSB, CTSS, CTSK, and CTSZ) mRNA in cumulus cells, and the fatty acid profile (total monounsaturated fatty acids, polyunsaturated fatty acids, and saturated fatty acids) in follicular fluid. Non-lactating beef cows (n = 40) were administered GnRH on d −9, prostaglandin F2a (PG) on d −2, and ovaries harvested 48 hr after PG administration (d 0). Blood samples were collected every 8 hr from PG until ovary collection to characterize serum concentrations of estradiol, progesterone, insulin like growth factor 1 (IGF1), and luteinizing hormone (LH). None of the animals had a spontaneous LH surge by the time of ovary collection. The dominant follicle was dissected from the stroma and the cumulus-oocyte complex retrieved along with the corresponding follicular fluid. Only data from cows (n = 16) that did not express estrus and that had an estrogenic follicle at harvest (estradiol to progesterone ratio = 1 in follicular fluid) were included in the simple linear regression analysis. Dominant follicle diameter ranged from 9 to 14 mm at 48 hr after PG and the correlation between dominant follicle size and serum estradiol and follicular fluid estradiol was 0.54 (P < 0.05) and 0.50 (P < 0.04), respectively. As dominant follicle diameter increased, oocyte mRNA abundance (n = 16) of FST (P < 0.02), INHBA (P < 0.01) and INHBB (P < 0.03) decreased. However, as dominant follicle diameter increased cumulus cells (n = 12) had lower mRNA abundance of CTSB (P < 0.04), and tended to have lower abundance of CTSZ (P < 0.07). Furthermore, as follicular fluid concentrations of estradiol increased, CTSS abundance increased (P < 0.04) and CTSZ abundance tended to decrease (P < 0.06). Fatty acid profiles of total monounsaturated fatty acids, polyunsaturated fatty acids, and saturated fatty acids were not affected by dominant follicle size. In summary, relative abundance of INHBA, INHBB and FST mRNA in the oocyte were all negatively correlated with follicle diameter 48 hr after PG. Furthermore, relative abundance of CTSB mRNA in the cumulus cells from the ooctyes collected was also negatively associated with follicle diameter. As follicular diameter increased there was a correlated increase in serum and intrafollicular estradiol at ovary collection, but no effect on total fatty acids. In conclusion, there was no compelling data from the study to indicate that larger dominant follicles contained more competent oocytes. Supported by USDA NRI grant 2006-35203-17284 from USDA-CSREES.