Expression of messenger ribonucleic acid encoding cytochrome P450 side-chain cleavage, cytochrome p450 17 alpha-hydroxylase, and cytochrome P450 aromatase in bovine follicles during the first follicular wave.

The objective of the present study was to investigate changes in the expression of messenger RNAs (mRNAs) encoding cytochrome P450 side-chain cleavage (P450scc), cytochrome P450 17 alpha-hydroxylase (P450c17), and cytochrome P450 aromatase (P450arom) at different stages of the first follicular wave of the bovine estrous cycle. Groups of heifers (three to five per group) were ovariectomized on the day of initiation of the first follicular wave (as determined by ultrasonography, day 0) or on days 2, 4, 6, 8, and 10 after initiation of the first follicular wave following estrus. Expression of mRNAs encoding P450scc, P450c17, and P450arom was detected by in situ hybridization and quantified by image analysis. P450scc mRNA was localized to theca interna cells of large preantral follicles and also to granulosa cells of follicles 4 mm or greater in diameter. mRNA for P450c17 was localized exclusively to theca interna cells, whereas P450arom mRNA was localized to granulosa cells of follicles 4 mm or greater in diameter. There were changes in mRNA levels for all three enzymes in thecal and/or granulosa cells at different times of the first follicular wave. Before identification of the dominant follicle (i.e. on days 0 and 2), there was no change in expression of P450scc and P450c17 mRNAs, whereas expression of P450arom mRNA was higher on day 2 than on day 0. Maximal mRNA levels for all three enzymes were observed on day 4. By day 6, P450scc and P450c17 mRNA levels were reduced compared to those on day 4, whereas P450arom mRNA levels remained elevated. On day 8, mRNA levels for all three enzymes were reduced. After initiation of the second follicular wave (day 10), dominant follicles from the first wave were at an advanced stage of atresia. P450scc and P450arom mRNAs were undetectable in granulosa cells, and very low levels of P450scc and P450c17 mRNAs were observed in theca interna cells. Before identification of the dominant follicle, mRNA levels for all three enzymes were similar within a cohort of follicles. Therefore, expression of these enzymes may not be associated with the mechanism of selection of the dominant follicle during a follicular wave.