Immature hypophysectomized (HYPOXD) rats develop large, polyfollicular
ovarian cysts in response to unabated, combined stimulation by subovulatory doses of
human chorionic gonadotropin (hCG) and highly purified ovine
follicle-stimulating hormone (FSH). Further, circulating amounts of
androstenedione (A4) and
estradiol (E2), but not
testosterone or
dihydrotestosterone (DHT), change in parallel with the development of these
cysts. To determine the potential roles of either A4 or E2 at the level of the ovary in the induction of
ovarian cysts, pellets containing either (1)
cholesterol (placebo; controls); (2) A4; or (3) E2 were administered subcutaneously (sc) to immature HYPOXD rats. Some of these animals also received either twice-daily sc
injections of 1 IU hCG, or daily s.c.
injections of 2 microg FSH, for 13 days. Ovaries and sera were harvested from all treatment groups on the morning of day 14 of the combined-
hormone treatment schedule. As expected, ovaries from HYPOXD rats treated with placebo, A4, or E2 pellets (with or without hCG) failed to display
antral follicles. Ovaries from HYPOXD rats treated with FSH and a placebo pellet displayed polyfollicular, atretic, small
antral follicles with unstimulated thecal shells. In addition, the ovarian stromal-interstitial tissue had an unstimulated appearance. In contrast, ovaries from HYPOXD rats treated with FSH plus either A4 or E2 implants displayed stimulated stromal-interstitial tissue as well as small
follicular cysts and precysts with stimulated thecal shells. The number of
cysts and precysts observed in the largest ovarian cross-sections for animals treated with FSH + A4 (17.0 +/- 3.0) was less than that observed in the largest ovarian cross-sections for HYPOXD rats treated with FSH + E2 (40.2 +/- 10.1; p < 0.05). To determine if the development of
ovarian cysts in response to FSH + A4 was due, at least in part, to the metabolism of A4 to E2, HYPOXD rats were treated with either (1) placebo pellets; (2) pellets containing
dihydrotestosterone (DHT) which cannot be metabolized to
estrogen; (3) E2 pellets plus DHT pellets (E2 + DHT); (4) FSH + DHT; or (5) FSH + E2 + DHT. The largest ovarian cross-sections from FSH + DHT-treated HYPOXD rats displayed 18.3 +/- 4.1 small follicles with a mean diameter of approximately 0.437 mm which possessed few granulosa cells. The thecal and stromal-interstitial tissues in these ovaries were unstimulated, which indicates that these small degenerating follicles were atretic rather than cystic. In contrast, the largest ovarian cross-sections from FSH + E2 + DHT-treated HYPOXD rats displayed 51.6 +/- 2.4
cysts with stimulated thecal shells and a mean diameter of approximately 0.634 mm. Further, these
cysts were arranged in a "string of pearls" pattern and the ovarian stromal-interstitial tissue possessed a stimulated appearance. These data demonstrate a direct, unambiguous role at the level of the ovary for unabated tonic stimulation by FSH plus
estrogen in the development of small polyfollicular
cysts in HYPOXD rats. Further, the data also indicate that, at least in HYPOXD rats, combined, tonic stimulation by FSH plus
estrogen and
androgen is sufficient for the development of small, polyfollicular
ovarian cysts in a "string of pearls" pattern. These observations are in distinct contrast to our previous observations that tonic stimulation by FSH + hCG results in the induction of large
ovarian cysts in HYPOXD rats and provide tantalizing new insights regarding the potential importance of specific
hormones at the level of the ovary in the induction of specific types of cystic follicles.