SELECTION OF THE DOMINANT FOLLICLE IN CATTLE OCCURS IN THE ABSENCE OFDIFFERENCES IN THE EXPRESSION OF MESSENGER-RIBONUCLEIC-ACID FOR GONADOTROPIN RECEPTORS

Mechanisms that allow selection of a dominant ovarian follicle from a cohort of growing follicles are unknown. Large healthy, estrogen-activ e follicles contain more LH receptors than atretic estrogen-inactive f ollicles, 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 t he temporal pattern of expression of mRNA for LH and FSH receptors are associated with selection of dominant follicles in cattle. Based on s ize, 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 folli cular wave. On day 2, the dominant follicle was 1 mm larger than the l argest subordinate follicle, but by day 3 of the wave the dominant fol licle was 2-4 mm larger than the largest subordinate. Follicular fluid concentrations of estradiol and estradiol secretion in vitro by piece s of follicle wall (granulosa and theca cells) were greatest by the do minant compared with the subordinate follicles (P < 0.05). These data indicate that selection of a dominant follicle had occurred by the sec ond day of the follicular wave. By in situ hybridization, mRNAs for LH and FSH receptors, P450 aromatase and P450 17 alpha-hydroxylase (17 a lpha-OH) were localized in frozen sections from each follicle. The exp ression of mRNA for LH receptor in granulosa cells was always at or ne ar 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 differ ence between follicle types on the second day of the follicular wave, but by the third day expression in the subordinate follicles had decre ased (P < 0.05). The expression of mRNA for FSH receptor was highest i n granulosa cells of dominant follicles collected on day 3 of the foll icular wave (P < 0.05) and was not different between dominant and subo rdinate follicles on day 2 of the wave (P > 0.05). The expression of m RNA for aromatase in granulosa cells was similar (P > 0.05) between th e dominant follicles on days 2 and 3 and the largest subordinate folli cle on day 2 of the follicular wave and was much lower in the remainin g follicles (P < 0.01). On day 2 of the wave, the expression of mRNA f or 17 alpha-OH was not different between the dominant and subordinate follicles, but by day 3 the dominant follicles had more mRNA for 17 al pha-OH than the subordinate follicles (P < 0.05). These data show that the dominant follicle had been selected by the second day of the foll icular wave (based on diameter and estradiol secretion) and that selec tion occurred in the absence of detectable levels of mRNA for LH recep tor in the granulosa cells or differences between dominant and subordi nate follicles in mRNA for LH receptor in theca cells or FSH receptor in granulosa cells. However, the divergent pattern of growth between d ominant and subordinate follicles (after follicle selection) was assoc iated with higher levels of mRNA for gonadotropin receptors and steroi dogenic enzymes in dominant compared with subordinate follicles. There fore, selection of the dominant follicle in cattle does not appear to involve the regulation of expression of mRNA for gonadotropin receptor s, although such regulation may be important at other stages of differ entiation of the dominant follicle.