Differential steroid secretion and gonadotropin response by individual tertiary porcine follicles in vitro. Possible physiologic role of atretic follicles

Since atretic follicles contain significant amounts of androgen and/or progesterone in their follicular fluid, we examined whether they also contribute to ovarian steroid secretion. Steroid secretion by atretic porcine follicles and their responsiveness to FSH was assessed by a perifusion system that allows for separate dynamic incubation of whole follicles in vitro. Identically treated nonatretic follicles of comparable size served as a reference group. The extent of granulosal pyknosis, on which the staging of atresia was based, was inversely related to follicular estradiol (E2) secretion and its responsiveness to FSH. Both basal and FSH-stimulated secretion of testosterone (T), androstenedione (A), and progesterone (P) were maintained by follicles in all stages of atresia. Secretion of A by late atretic follicles was greater than that in earlier stages or by nonatretic follicles. Atretic follicles may therefore release comparable or larger amounts of androgen and P into their intraovarian environment than do nonatretic follicles. We examined whether steroids secreted by atretic follicles in vitro could be utilized by nonatretic follicles. A static incubation system was used that allows for simultaneous incubation of a number of individual follicles. When nonatretic follicles were exposed to A, T, or P in physiologic concentrations (10(-7)-10(-5) M), their secretion of E2 increased 2-8-fold. Doses of FSH or LH that stimulated follicular steroid secretion in vitro had no additional stimulatory effect when combined with A or P treatment, respectively. In conclusion, atretic follicles may contribute significantly to intraovarian levels of androgen and P. It is hypothesized that nonovulatory follicles are an important intraovarian source of steroid substrate, since nonatretic follicles can utilize these steroids in vitro to enhance E2 synthesis.