Content of meiosis activating sterols in equine follicular fluids: correlation to follicular size and dominance.

Meiosis activating sterols (MAS) are pre-cholesterol sterols that can be isolated from follicular fluid (FF-MAS) or testes (T-MAS). Meiosis activating sterols trigger the resumption of meiosis in cultured meiotically competent oocytes. In the present work MAS, cholesterol and progesterone were assayed by HPLC in follicular fluids collected from pony mares at fixed days after the last ovulation. Follicles were divided into two groups according to whether they were aspirated before or after Day 17 after the last ovulation. The latter group was further divided according to whether the follicle diameter was < or = 22 mm or > 27 mm. Both FF-MAS and T-MAS were detected in almost all samples. Overall, the total amount of MAS in the follicular fluids increased with the size of the follicles but was accompanied by a decrease in the amount of free cholesterol. The amounts of MAS and progesterone in > 27 mm follicles aspirated after Day 17 were significantly higher as compared to the other groups. A transversal cohort analysis showed that the largest follicle at the time of aspiration had the highest level of MAS after day 17 of the cycle, which was not always true for follicle samples aspirated before Day 17 of the cycle. The study demonstrates that the content of MAS in equine follicular fluids increased during follicular maturation concomitant with a decrease in the concentration of free cholesterol. Moreover, MAS concentration is higher in dominant follicles than in subordinate follicles. The MAS may therefore play an as yet unknown physiological role during pre-ovulatory maturation.