CHANGES IN MESSENGER-RIBONUCLEIC-ACID ENCODING LUTEINIZING-HORMONE RECEPTOR, CYTOCHROME-P450 SIDE-CHAIN CLEAVAGE, AND AROMATASE ARE ASSOCIATED WITH RECRUITMENT AND SELECTION OF BOVINE OVARIAN FOLLICLES

The objective of this study was to investigate changes in expression o f mRNAs encoding FSH receptor (FSHr), LH receptor (LHr), cytochrome P4 50 side-chain cleavage (P450(scc)), cytochrome P450 17 alpha-hydroxyla se (P450(c17)), and cytochrome P450 aromatase (P450(arom)) during recr uitment and selection of bovine ovarian follicles. Dairy heifers (4-5 per group) were ovariectomized at 12, 24, 36, 48, 60, 72, 84, or 96 h after initiation of the first follicular wave following estrus as dete rmined by ultrasonography (Time 0 = initiation of follicular wave; mea n +/- SEM = 42.0 +/- 2.6 h after estrus). Expression of mRNAs encoding FSHr, LHr, P450(scc), P450(c17), and P450(arom) was detected by in si tu hybridization and quantified by image analysis. Antral follicles we re classified as healthy or atretic, Healthy follicles expressed highe r (p < 0.01) amounts of mRNAs for gonadotropin receptors and steroidog enic enzymes than did atretic follicles, and expression of LHr, FSHr, P450(scc), P450(c17), and P450(arom) increased (p < 0.01) with follicu lar size and stage of the follicular wave. Expression of mRNAs for P45 0(scc), P450(arom), and LHr was time- and size-dependent during recrui tment and selection. During recruitment, expression of mRNAs for P450( scc) and P450(arom) was first detected in granulosa cells of 16 of 21 of the follicles 4-6 mm in diameter at 12 h. At 24 and 36 h, almost al l follicles 6-9 mm in diameter, but not those 4-5 mm in diameter, expr essed both P450(scc) and P450(arom) mRNA in the granulosa cells. At 48 h and thereafter, P450(scc) and P450(arom) mRNA were expressed predom inantly in one healthy large follicle per cow with a few exceptions. E xpression of LHr mRNA was first detected in granulosa cells at 36 h an d was always found in granulosa cells of one follicle greater than or equal to 8 mm per cow with exception of one cow at 36 h (no expression ) and another two cows, one each at 36 and at 84 h (expression in 2 fo llicles). In addition, LHr mRNA expression in the granulosa cell layer was limited to follicles that also expressed mRNAs for P450(scc) and P450(arom) in the granulosa cells. In summary, follicular recruitment in cattle was associated with expression of P450(scc) and P450(arom) m RNA within granulosa cells, and the process of follicular selection wa s associated with initiation of LHr mRNA expression in granulosa cells .