Follicle development in cyclic, anoestrous and FSH-treated brushtail possums (Trichosurus vulpecula)

The common brushtail possum (Trichosurus vulpecula) is a pest of considerable economic importance in New Zealand. Attempts to develop methods of suppressing reproduction in this species are currently hampered by the lack of reliable methods to synchronise oestrus and ovulation in this species. The objective of this study was to compare antral follicle populations in anoestrous and cyclic brushtail possums and to assess the efficacy of exogenous FSH to induce follicle development in anoestrous animals. Ovaries were recovered from anoestrous possums after administration of either exogenous FSH (1.0 mg/injection) or the saline vehicle alone (0.5 ml/injection) at 12-h intervals for 3 days (n=6/group), and from cyclic animals (n=6) that were euthanised in mid-follicular phase (5 days after removal of their pouch young). All antral follicles ≥1.0 mm in diameter were dissected free of extraneous tissue, incubated in vitro to measure oestradiol production, and then processed for histological assessment of health status. Mean weight of ovaries and vaginal cul-de-sac tissues were both significantly greater (P<0.001) in FSH-treated anoestrous females (24.2±5.1 mg and 6.50±1.34 g, respectively), but did not differ significantly between saline-treated anoestrous possums (12.4±3.0 mg and 1.31±0.27 g) and cyclic animals (13.5±1.6 mg and 2.62±0.95 g). Mean uterine weights in both cyclic (889±161 mg) and FSH-treated (1098±184 mg) animals were significantly heavier (P<0.001) than those of anoestrous possums (414±61 mg). The mean number of follicles (≥1.0-mm diameter) present was significantly greater (P<0.001) in FSH-treated, than in cyclic and anoestrous possums (38.0±4.4, 23.2±3.2 and 10.7±3.4 follicles/animal, respectively). Cyclic animals had significantly more (P<0.01) follicles than anoestrous possums. The proportion of follicles that were classified as healthy, was significantly lower (P<0.01) in cyclic possums (38%) than in anoestrous (69%) and FSH-treated (88%) animals. The mean diameter of the largest healthy follicle present was 2.5±0.41, 2.1±0.08, and 3.1±0.15 mm for cyclic, anoestrous and FSH-treated animals, respectively. None of the follicles harvested from saline-treated anoestrous possums produced measurable levels of oestradiol in vitro, whereas 7% and 59% of those from cyclic and FSH-treated animals did so. In summary, cyclic possums had more antral follicles present than anoestrous animals, but a lower percentage of these follicles were healthy. Less than 10% of healthy follicles from cyclic possums, and none of those from anoestrous animals, were capable of producing oestradiol when incubated in vitro. Treatment with ovine FSH promoted follicle development in anoestrous possums, to significantly increase the number of follicles present, the proportion that were healthy and the percentage capable of producing oestradiol.