Preovulatory follicle development and ovulation in the brushtail possum (Trichosurus vulpecula) monitored by repeated...

The common method for synchronizing oestrus in brushtail possums is by removal of their pouch young (RPY). However, there is little information on the ovarian response to this treatment, the timing and incidence of ovulation is poorly defined, and methods of identifying oestrus are unreliable. In this study, the development of preovulatory follicles, ovulation and reproductive tract changes following RPY were monitored by repeated laparoscopic observation. A total of 120 adult female possums underwent laparoscopy at intervals of 1-4 days over the period from 0 to 21 days after RPY. Tissue was collected from a further 30 animals for correlative histology of ovarian structures, and to quantify changes in reproductive tract organs. Only 80 of 120 animals ovulated, and the time of ovulation ranged from 7 to 18 days following RPY. In most animals, enlargement of vaginal cul-de-sac and uterine tissue occurred within 10 days. Correlative histology supported the macroscopic classification of ovarian structures, and healthy and atretic follicles could be identified by laparoscopy. Vaginal smears and plasma progesterone concentrations verified the occurrence of ovulation as observed by laparoscopy. A 'presumptive' preovulatory follicle, first identifiable approximately 5 days before ovulation, was recorded in all animals that ovulated and in none that failed to ovulate. Changes to its surface morphology indicated impending ovulation. This study has enabled the day of ovulation to be identified accurately for the first time in this species. It has also shown that there is wide variation in follicle development, lack of synchrony in the time of ovulation in the brushtail possum, and that some animals fail to ovulate following RPY.