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 prolific Meishan gilts (which have high levels of embryo survival) with contemporary white hybrid control gilts. Gilts of similar reproductive ages (second or third oestrous cycle) were observed three times a day for oestrous behaviour and ovaries recovered at a time estimated as within 7 h of ovulation (white hybrid, n = 13; Meishan, n = 16). Preovulatory follicles were recovered (n = 195, white hybrid; n = 252, Meishan), and oocytes were aspirated and fixed for later identification of meiotic stage, and follicular fluid was frozen for subsequent determination of progesterone. The number of presumed ovulatory follicles recovered per animal was similar to the expected ovulation rate (15.0, white hybrid; 15.75, Meishan; P > 0.1); however, follicles from Meishan gilts were smaller (8.1 versus 9.0 mm, P < 0.001) and contained less follicular fluid (139.9 versus 187.6 microliters, P < 0.02) than did white hybrid gilts. Total follicular progesterone (88.7 ng per follicle, white hybrid; 77.4 ng per follicle, Meishan) and concentration of progesterone in follicular fluid (526 ng ml-1, white hybrid; 640 ng ml-1, Meishan) did not differ between the two breeds (P > 0.1). There were no breed differences in the extent of heterogeneity in follicular diameter, volume of follicular fluid, and total follicular progesterone and progesterone 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 advanced stages of meiosis than were those recovered from white hybrid gilts (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)