PREOVULATORY FOLLICLE DEVELOPMENT AND OVULATION IN THE BRUSHTAIL POSSUM (TRICHOSURUS-VULPECULA) MONITORED BY REPEATED LAPAROSCOPY

The common method for synchronizing oestrus in brushtail possums is by removal of their pouch young (RPY). However, there is little informat ion on the ovarian response to this treatment, the timing and incidenc e of ovulation is poorly defined, and methods of identifying oestrus a re unreliable. In this study, the development of preovulatory follicle s, ovulation and reproductive tract changes following RPY were monitor ed by repeated laparoscopic observation. A total of 120 adult female p ossums 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 an imals 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 oc curred within 10 days. Correlative histology supported the macroscopic classification of ovarian structures, and healthy and atretic follicl es could be identified by laparoscopy. Vaginal smears and plasma proge sterone concentrations verified the occurrence of ovulation as observe d by laparoscopy. A 'presumptive' preovulatory follicle, first identif iable approximately 5 days before ovulation, was recorded in all anima ls that ovulated and in none that failed to ovulate. Changes to its su rface morphology indicated impending ovulation. This study has enabled the day of ovulation to be identified accurately for the first time i n this species. It has also shown that there is wide variation in foll icle development, Lack of synchrony in the time of ovulation in the br ushtail possum, and that some animals fail to ovulate following RPY.