258. Characterisation of ovarian follicular growth in the brushtail possum

Abstract

The number and size of follicles selected for ovulation differ between species. The aim of this study was to characterise antral follicular growth and determine the size when selection of the ovulatory follicle occurs in the monovular brushtail possum. For this study, antral follicles ≥ 1 mm were dissected from the ovaries of 31 adult female possums at different reproductive states and follicular fluid and granulosa cells were harvested from each individual follicle. Selection of the ovulatory follicle in the brushtail possum occurred when follicles reached between 2.5 and 2.8 mm in diameter. Based on the analysis of steroids in follicular fluid, before selection, most follicles produced varying amounts of oestradiol (E2), but very few if any produced progesterone (P4). After selection, the selected follicle continued to produce increasing amounts of E2 and P4, whereas most other follicles were steroidogenically inactive. Near the time of ovulation, presumably after the LH surge, P4 became the predominant steroid produced by the selected follicle and most other follicles once again produced varying amounts of E2. The number of granulosa cells per follicle was highly variable, but tended to increase with increasing diameter. Cell viability was very high, averaging about 95%. Interestingly, the morphology of granulosa cells changed markedly after selection becoming larger and granular in appearance. The weights of the vaginal cul-de-sac and uteri correlated well with the presence of a selected follicle.  In ovaries from pregnant animals (n = 3), follicles grew up to 3.5 mm, and although they reached the size of a selected follicle during the follicular phase, E2 production by the other follicles was not suppressed and the weights of the cul-de-sac were less than those from non-pregnant animals with similar sized follicles. During anoestrus (n = 4), follicles did not grow beyond 2 mm and produced very little steroids.