Effect of administering a crude equine gonadotrophin preparation to mares on follicular development, oocyte recovery rate and oocyte maturation in vivo

In mares, the shortage of oocytes and the variability in nuclear maturation at a certain time of the oestrous cycle hinders the optimization of methods for in vitro maturation and in vitro fertilization. Increasing the number of small-to-medium-sized follicles available for aspiration in vivo may increase the overall oocyte yield. The aims of the present study were to investigate whether administration of crude equine gonadotrophins affects follicular development, oocyte recovery rate, in vivo oocyte maturation and follicular concentrations of meiosis-activating sterols. During oestrus, all follicles greater than or equal to 4 mm were aspirated from 19 pony mares (first aspiration: Al). Over the next 8 days, the mares were treated daily with either 25 mg crude equine gonadotrophins (n = 10) or physiological saline (n = 9). Between day 1 and day 8, follicular growth was monitored by ultrasonography. On day 8, all follicles greater than or equal to 4 mm were evacuated (second aspiration: A2) and nuclear maturation of the recovered oocytes was assessed after orcein staining. Follicular growth between Al and A2, as well as the number and size of follicles at A2 were similar for control mares and mares treated with crude equine gonadotrophins. The oocyte recovery rates at Al and A2 were similar. At A2, the oocyte recovery rate and oocyte maturation in vivo were not affected by treatment with crude equine gonadotrophins. The number of expanded cumulus oophorus complexes recovered from follicles less than or equal to 29 mm was significantly higher at Al than at A2. The number of oocytes at the germinal vesicle stage was significantly higher at A2 (41.5%) than at Al (17.8%). Meiosis-activating sterols (FF-MAS and T-MAS) were identified in follicular fluid recovered at A2. Follicular concentrations of FF-MAS and T-MAS were unaffected by treatment with crude equine gonadotrophins. The present study demonstrates that follicular aspiration during oestrus allowed a new follicular population to develop and resulted in a higher degree of synchronization of oocyte development with respect to cumulus expansion and nuclear maturation. The availability of a more homogeneous population of oocytes might facilitate a better optimization of in vitro maturation and in vitro fertilization techniques in mares. Administration of crude equine gonadotrophins during early dioestrus did not affect the growth of small follicles, the oocyte yield after aspiration or oocyte maturation in vivo.