Expression of messenger ribonucleic acid encoding for steroidogenic acute regulatory protein and enzymes, and luteinizing hormone receptor during the spring transitional season in equine follicles.

The period of spring transition, from the anovulatory to the ovulatory season, is characterized in many mares by cyclical growth and regression of large dominant follicles. These follicles produce only low concentrations of estradiol and it is thought that acquisition of steroidogenic competence by large follicles during spring transition is prerequisite in stimulating LH prior to first ovulation. In situ hybridization was used to localize and quantify expression of factors that play a key role in follicular steroidogenesis: StAR, P450scc (CYP11A1), P450c17 (CYP17), P450arom (CYP19), and LH receptor (LHr). One ovary was obtained from mares on the day after detection of an actively growing 30 mm transitional anovulatory follicle (defined as the transitional follicle), and the remaining ovary was removed at the third estrus of the breeding season on the day after the preovulatory follicle reached 30 mm in diameter (defined as the preovulatory follicle). Messenger RNAs encoding StAR, CYP11A1, and CYP17 were detected only in theca cells and CYP19 mRNA was confined to the granulosa layer. There was significantly lower expression of mRNAs for the steroidogenic enzymes, StAR (P<0.001) and LHr (P<0.05) in transitional follicles than in preovulatory follicles. In conclusion, large equine follicles during spring transition have low levels of mRNA encoding steroidogenic enzymes, StAR and LHr which will contribute to the steroidogenic incompetence of dominant follicles during spring transition and their subsequent regression.