Follicle size-dependent changes in follicular fluid components and oocyte diameter in Antarctic minke whales (Balaenoptera bonaerensis)

The concentrations of various components of follicular fluid were compared among three groups of follicles (small, <5 mm; medium: 5-10 mm; large, >10 mm) with a control that consisted of the components of umbilical serum using seven pregnant Antarctic minke whales. Follicular oocytes recovered from the follicles were also used for measurement of oocyte diameter after removing the cumulus cells. The mean diameter of the ooplasm in the oocytes from the large follicles (143.2 microm) was significantly greater than those from the small (127.1 microm) and medium (131.7 microm) follicles, although there were no significant differences in diameter of the whole oocyte and thickness of the zona pellucida among the three follicular sizes. The osmolarity of the follicular fluid from the small follicles (363.3 mOsmol) was significantly lower than that of the medium follicles (388.9 mOsmol) and tended to be lower than that of large (381.9 mOsmol) follicles, respectively, both of which were similar to that of the umbilical serum (379.5 mOsmol). There was no significant difference in the concentrations of all components of the follicular fluid between the medium and large follicles. As compared with the values of the umbilical serum, the total-protein, glucose, albumin and chlorine concentrations of the follicular fluid from the medium and large follicles were significantly higher, and the total cholesterol and calcium concentrations were significantly lower. The concentrations of lactic acid (85.3-136.0 mg/dl) of the follicular fluid from the three groups of follicles were significantly lower than that of the umbilical serum (360.0 mg/dl). Among the follicles, the follicular fluid from the small follicles (136.0 mg/dl) contained a significantly higher concentration of lactic acid than that from the large follicles (85.3 mg/dl). The progesterone concentrations were not significantly different among the fluid from the three group of follicles and the umbilical serum: however, the estradiol 17-beta concentrations of the follicular fluid increased with the size of the follicle (14.3 and 34.6 ng/ml for small and large follicles, respectively). These results offer new information concerning whale reproductive physiology, especially for improvement of in vitro oocyte maturation and related technologies in whales.