Effect of homologous follicular fluid from medium-sized and large follicles on in vitro maturation of equine cumulus-oocyte complexes.

The in vitro maturation (IVM) of equine oocytes is typically performed using various synthetic media; however, an optimal IVM system for equine oocytes has not been developed. The aim of the present study was to evaluate the effects of two types of follicular fluid (FF) obtained from cyclic mares and two incubation intervals for the IVM of equine cumulus-oocyte complexes ( COCs). Follicular fluid was collected from medium- sized (20-29mm diameter) and large (= 30 mm; post-human chorionic gonadotrophin administration) follicles using transvaginal ultrasound-guided follicle aspiration. Compact (n= 232) and non- compact ( n= 183) COCs obtained from a slaughterhouse were incubated separately in the following groups: (1) FF from medium follicles for 24 h; (2) FF from large follicles for 24 h; (3) control ( synthetic) medium for 24 h; (4) FF from medium follicles for 24 h then FF from large follicles for an additional 24 h; (5) FF from large follicles for 48 h; and (6) control medium for 48 h. For compact COCs, there was a tendency (P = 0.06) for more COCs incubated in FF from large follicles for 24 h to reach metaphase II compared with those incubated in control medium for 24 h (58% v. 35%, respectively). More (P < 0.05) compact COCs had degenerated after incubation in control medium for 48 h compared with all other groups (51% v. 14-24%, respectively). For non- compact COCs, incubation in FF from medium follicles for 24 h resulted in more ( P = 0.05) COCs at metaphase II compared with control medium for 48 h ( 58% v. 29%, respectively). These results indicate that homologous FF from cyclic mares is a suitable alternative for the IVM of equine COCs and that it may be superior to conventional media for longer ( i. e. > 24 h) incubation intervals.