EXPRESSION OF FOLLICLE-STIMULATING-HORMONE AND LUTEINIZING-HORMONE RECEPTOR MESSENGER RIBONUCLEIC-ACIDS IN BOVINE FOLLICLES DURING THE FIRST FOLLICULAR WAVE

The objective of the present study was to characterize expression of mRNAs encoding FSH and LH receptors during follicular development and at different stages of the first follicular wave in cattle. Following estrus, groups of heifers (3-5 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, or 10 after initiation of the first wave. FSH and LH receptor mRNAs were detected within follicles greater than or equal to 4 mm and in some smaller follicles by in situ hybridization and were quantified by image analysis. FSH receptor mRNA was expressed in granulosa cells of all growing follicles, starting in some follicles with only one layer of granulosa cells. Irrespective of day of the follicular wave, the level of expression of FSH receptor mRNA in granulosa cells of healthy antral follicles ranging from 0.5 to 14 mm in diameter did not vary significantly with follicular size (r = 0.02, p > 0.10). Expression of LH receptor mRNA was first observed in theca interna cells of follicles shortly after antral formation. Irrespective of day of the follicular wave, the levels of LH receptor mRNA in theca interna cells of healthy antral follicles ranging from 0.5 to 14 mm increased with follicular size (r = 0.39, p < 0.01). In granulosa cells, LH receptor mRNA was expressed only in healthy follicles > 9 mm in diameter and was first observed in the dominant follicles collected on Day 4. Expression of mRNA for LH receptor, but not for FSH receptor, changed (p < 0.01) with the stage of the first follicular wave. LH receptor mRNA levels were highest in theca interna cells of dominant follicles collected on Day 4 and did not differ on other days. Only the healthy dominant follicles collected on Days 4, 6, and 8 expressed LH receptor mRNA in the granulosa cells. Levels of LH receptor mRNA in granulosa cells were similar on Day 4 and Day 6 but were reduced (p < 0.05) on Day 8. Atresia was associated with a loss of LH receptor mRNA in granulosa cells and with reduced expression of LH receptor mRNA in theca cells and FSH receptor mRNA in granulosa cells. In summary, acquisition of LH receptors in granulosa cells between Day 2 and Day 4 may be important in the establishment of follicular dominance during the first follicular wave.