Changes in follicular populations, in serum estrogen and progesterone, and in ovarian steroid secretion in vitro during the guinea pig estrous cycle

Abstract

It has been proposed that the guinea pig estrous cycle manifests biphasic follicular development. The follicles of one cohort apparently achieve their greatest diameter by approximately Day 10 of the cycle and then undergo atresia while the second cohort ovulates; this constitutes an uninvestigated and novel model for the evaluation of atresia. In this study, follicular development was evaluated in vivo and in vitro to confirm this pattern. On cycle Day 3, 5, 7, 10, 12, 15, or 0 (ovulation), guinea pigs were killed and trunk blood was collected; ovaries were excised, weighed, and measured, and size and number of large (>40 μm) follicles were determined. Ovaries were quartered and placed into culture dishes for incubation. Culture variables were presence or absence of human follicle-stimulating hormone (100 ng/ml) and time. Ovarian fragments were processed for histology. Estrogen (E) and progesterone (P) in sera and culture media were analyzed by radioimmunoassay. The abundance of large follicles on both the ovarian surface (>635 μm) and in histologic section (>500 μm) relative to all follicles observed was high on Day 7 and Day 10, respectively; lower on Day 10 and Day 12; and high again at Day 12 and Day 15. Mean secretion of E in culture was elevated on Day 10 (253.0 ± 60.3 pg/ml/mg ovary), low on Day 12 (67.9 ± 13.0), and high again on Day 0 (185.8 ± 56.8). Peripheral P reached a maximum of 2.93 ± 0.09 ng/m (Day 5), and then declined to Day 15. Mean ovarian P output in vitro was low on Day 3 (2.31 ± 0.13 ng/ml), highest on Day 5 (4.54 ± 0.30), reduced on Day 10 and augmented again by Day 15. These results confirm that follicle development in the guinea pig consists of two waves that are initially of the same size and steroidogenic capacity. The first cohort of follicles manifests enhanced E production on Day 10 and reduced steroidogenic capability thereafter, presumably indicative of atresia, compared with the preovulatory cohort.