FOLLICLE DEVELOPMENT AND OOCYTE MATURATION DURING THE IMMEDIATE PREOVULATORY PERIOD IN MEISHAN AND WHITE HYBRID GILTS

A study was carried out to compare preovulatory ovarian events in prol ific Meishan gilts (which have high levels of embryo survival) with co ntemporary white hybrid control gilts. Gilts of similar reproductive a ges (second or third oestrous cycle) were observed three times a day f or oestrous behaviour and ovaries recovered at a time estimated as wit hin 7 h of ovulation (white hybrid, n = 13; Meishan, n = 16). Preovula tory follicles were recovered (n = 195, white hybrid; n = 252, Meishan ), and oocytes were aspirated and fixed for later identification of me iotic stage, and follicular fluid was frozen for subsequent determinat ion of progesterone. The number of presumed ovulatory follicles recove red per animal was similar to the expected ovulation rate (15.0, white hybrid; 15.75, Meishan; P > 0.1); however, follicles from Meishan gil ts were smaller (8.1 versus 9.0 mm, P < 0.001) and contained less foll icular fluid (139.9 versus 187.6 mu l, P < 0.02) than did white hybrid gilts. Total follicular progesterone (88.7 ng per follicle, white hyb rid; 77.4 ng per follicle, Meishan) and concentration of progesterone in follicular fluid (526 ng ml(-1), white hybrid; 640 ng ml(-1), Meish an) did not differ between the two breeds (P > 0.1). There were no bre ed differences in the extent of heterogeneity in follicular diameter, volume of follicular fluid, and total follicular progesterone and prog esterone concentration (P > 0.1). However, classification of recovered oocytes into seven discrete meiotic stages revealed that more oocytes recovered from Meishan preovulatory follicles were in the more advanc ed stages of meiosis than were those recovered from white hybrid gilts (P < 0.001). In conclusion, it is suggested that the advanced oocyte maturation in follicles of Meishan pigs before ovulation may be import ant for ensuring the prolificacy of this breed.