The objective of the present study was to examine changes in expression of mRNA encoding 3 beta-hydroxysteroid dehydrogenase Delta(4),Delta(5) isomerase (3 beta-HSD) during recruitment and selection of bovine ovarian follicles. Dairy heifers (4-5/time period) were ovariectomized at 12, 24, 36, 48, 60, 72, 84, or 96 h after initiation of the first follicular wave (Time 0) following estrus. Expression of 3 beta-HSD mRNA was localized by in situ hybridization and quantified by image analysis. Expression of 3 beta-HSD mRNA was first detected in theca interna cells of preantral follicles with a well-developed theca layer and in granulosa cells of follicles greater than or equal to 8 mm in diameter. Regardless of stage of follicular wave, expression of 3 beta-HSD mRNA in granulosa cells of follicles 8 mm was correlated with follicular size (r = 0.665; p < 0.01). The 36-h time period appeared to be a transition period for selection since dominant follicles were detected by size and expression of 3 beta-HSD mRNA in some cows but not in others. By 48 h after wave initiation, dominant follicles could be identified by both size and expression of 3 beta-HSD mRNA. Expression of mRNA for 3 beta-HSD in theca cells was higher (p < 0.05) at 24 h than at 12 h and remained elevated thereafter through 96 h. In contrast to theca cells, expression of mRNA for 3 beta-HSD was undetectable within granulosa cells at 12 and 24 h. At 36 h, 3 beta-HSD mRNA was expressed in granulosa cells of healthy follicles greater than or equal to 8 mm, and expression was higher (p < 0.05) at 48 h compared with 36 h. Expression of 3 beta-HSD mRNA levels increased further in granulosa cells (p < 0.05) at 84 and 96 h compared to 48 h. Upon detection of mRNA for 3 beta-HSD in granulosa cells, high levels of expression were always found in one (dominant) follicle/cow with the exception of two cows at 36 and 84 h that expressed 3 beta-HSD mRNA in two large healthy follicles. Expression of 3 beta-HSD mRNA was also detectable in granulosa cells of a few large atretic follicles in which remnant granulosa cells appeared to be luteinized. Healthy follicles expressed higher (p < 0.05) levels of 3 beta-HSD mRNA in both theca and granulosa cells than did atretic follicles. Expression of 3 beta-HSD mRNA in theca cells was higher (p < 0.01) in dominant follicles than in other subordinate healthy follicles. These results indicate that only selected dominant follicles express 3 beta-HSD mRNA within granulosa cells, and expression increased in both thecal and granulosa cells during the follicular wave. Therefore, expression of 3 beta-HSD mRNA within granulosa cells may be associated with the mechanism of selection of the dominant follicle during a follicular wave and may be required for maximum steroid production during follicular dominance.
