Abstract:
The steroidogenic enzymes, P450 aromatase(P450 <emph type="8">arom ) and P450 17a-hydroxylase (P450<emph type="8">17<emph type="8">a ), were precisely locatedwithin the healthy porcine follicle by immunohistochemistry. Enzymedistribution was examined throughout follicular development during naturaloestrous cycles (n = 14 gilts) and was comparedwith steroid production by healthy whole and theca-only follicles. Allfollicles 2 mm or more in diameter were either fixed for immunohistochemistry(n = 380 of which 197 were assessed as healthy)or incubated as whole (n = 110) or theca-only(n = 110) follicles to measure steroidogenesis.P450 <emph type="8">17<emph type="8">a was confined to the thecalayer. The number of positive cells and staining intensity increased withfollicle size. P450 <emph type="8">arom was consistently detected inthe granulosa layer of follicles measuring 6 mm or more in diameter and thosecells furthest from the antrum were most strongly stained.P450<emph type="8">arom was also detected in the theca layer of theselarge follicles. Whole and theca-only follicles produced oestradiol andandrostenedione, and the levels of both hormones increased with follicle size(P<0.001). Whole follicles produced more oestradiol(P<0.001), but less androstenedione(P = 0.01) than theca-only follicles of the samesize. Although granulosa cells contained P450 <emph type="8">arom andsynthesized oestradiol, only theca cells contained P450<emph type="8">17<emph type="8">a. Theca cells thereforeprovided granulosa cells with androgen substrate. In addition, theca cellspossessed P450 <emph type="8">arom , making them capable of independentoestradiol production, which may be required to trigger the LH surge. Thisstudy confirms the vital role of theca cells in follicular steroidogenesis inthe pig.