STEROIDOGENIC ENZYME EXPRESSION DURING PREOVULATORY FOLLICLE MATURATION IN PIGS

During preovulatory maturation, follicles destined to ovulate exhibit a marked increase in steroidogenesis, while nonovulatory, steroidogeni cally inactive follicles are lost by atresia. The purpose of this stud y was to assess the expression of the steroidogenic enzymes cytochrome P450 aromatase (P450(arom)), cytochrome P450 17 alpha-hydroxylase/C17 -20 lyase (P450(c17)), and 3 beta hydroxysteroid dehydrogenase (3 beta HSD) and that of the tell proliferation-associated nuclear antigen Ki -67 by immunohistochemistry in both nonatretic and atretic follicles d eveloped during altrenogest-synchronized preovulatory maturation. Fift een cyclic gilts were slaughtered on Days 1, 3, 5 (n = 3 per day), and 7 (n = 6) following the withdrawal of altrenogest. Staining intensity was assigned a numeric value and was related to follicle size and day following altrenogest withdrawal. Follicle atresia was assessed by th e extent of in situ 3' end-labeling of DNA in apoptotic cells. The ove rall incidence of atresia was 41%, 17%, and 7% in small (< 3 mm), medi um (3-5 mm), and large (> 5 mm) follicles, respectively. Expression of granulosa cell P450(arom), Ki-67 antigen, and thecal P450(c17), was s ignificantly less (p < 0.001) in atretic compared to nonatretic follic les. Thecal 3 beta HSD was expressed predominantly in medium and large nonatretic follicles; expression was nondetectable in 88% of small fo llicles irrespective of follicle health. Ki-67 expression in granulosa cells was greater in small and medium nonatretic than in large nonatr etic follicles (p < 0.005). In large presumptive ovulatory follicles, P450(arom) was maximally expressed on Days 3 and 5 and decreased on Da y 7 (p < 0.005), while P450(c17) was unchanged between Days 3 and 7. I n contrast, 3 beta HSD increased linearly between Days 3 and 7 (p < 0. 005). Staining intensities of P450(arom), P450(c17), and 3 beta HSD we re correlated with each other and with follicular fluid concentrations of the steroids estradiol, androstenedione, and progesterone, whose p roduction they catalyzed. These data show that selection of ovulatory follicles is associated with a low incidence of apoptosis, a reduction in cell proliferation, maintenance of high levels of P450(arom), and increased expression of 3 beta HSD to provide substrate for androgen a nd estrogen production. During the period of development investigated in this study, changes in follicular steroid production in vivo are ex plained in large measure by changes in steroidogenic enzyme expression .