Selection of the dominant follicle in cattle occurs in the absence of differences in the expression of messenger ribonucleic acid for gonadotropin receptors.

Abstract

Mechanisms that allow selection of a dominant ovarian follicle from a cohort of growing follicles are unknown. Large healthy, estrogen-active follicles contain more LH receptors than atretic estrogen-inactive follicles, and levels of messenger RNA (mRNA) for LH receptor increase in the granulosa cells of dominant follicles as growth progresses. The aim of the present study was to test the hypothesis that changes in the temporal pattern of expression of mRNA for LH and FSH receptors are associated with selection of dominant follicles in cattle. Based on size, the dominant and two largest subordinate follicles were collected from the ovaries of heifers on days 2 (n = 3) or 3 (n = 3) of a follicular wave. On day 2, the dominant follicle was 1 mm larger than the largest subordinate follicle, but by day 3 of the wave the dominant follicle was 2-4 mm larger than the largest subordinate. Follicular fluid concentrations of estradiol and estradiol secretion in vitro by pieces of follicle wall (granulosa and theca cells) were greatest by the dominant compared with the subordinate follicles (P < 0.05). These data indicate that selection of a dominant follicle had occurred by the second day of the follicular wave. By in situ hybridization, mRNAs for LH and FSH receptors, P450 aromatase and P450 17alpha-hydroxylase (17alpha-OH) were localized in frozen sections from each follicle. The expression of mRNA for LH receptor in granulosa cells was always at or near background and was not different between days or follicle types (P = 0.63). In contrast, the expression of mRNA for LH receptor in theca cells of the same sections was readily detectable; there was no difference between follicle types on the second day of the follicular wave, but by the third day expression in the subordinate follicles had decreased (P < 0.05). The expression of mRNA for FSH receptor was highest in granulosa cells of dominant follicles collected on day 3 of the follicular wave (P < 0.05) and was not different between dominant and subordinate follicles on day 2 of the wave (P > 0.05). The expression of mRNA for aromatase in granulosa cells was similar (P > 0.05) between the dominant follicles on days 2 and 3 and the largest subordinate follicle on day 2 of the follicular wave and was much lower in the remaining follicles (P < 0.01). On day 2 of the wave, the expression of mRNA for 17alpha-OH was not different between the dominant and subordinate follicles, but by day 3 the dominant follicles had more mRNA for 17alpha-OH than the subordinate follicles (P < 0.05). These data show that the dominant follicle had been selected by the second day of the follicular wave (based on diameter and estradiol secretion) and that selection occurred in the absence of detectable levels of mRNA for LH receptor in the granulosa cells or differences between dominant and subordinate follicles in mRNA for LH receptor in theca cells or FSH receptor in granulosa cells. However, the divergent pattern of growth between dominant and subordinate follicles (after follicle selection) was associated with higher levels of mRNA for gonadotropin receptors and steroidogenic enzymes in dominant compared with subordinate follicles. Therefore, selection of the dominant follicle in cattle does not appear to involve the regulation of expression of mRNA for gonadotropin receptors, although such regulation may be important at other stages of differentiation of the dominant follicle.