OVARIAN FOLLICULAR DEVELOPMENT DURING EARLY-PREGNANCY IN THE PIG

Follicles were recovered from groups of gilts on Day 12 of the oestrous cycle and on Days 12, 20 and 30 of gestation. Follicular diameter, follicular fluid volume and concentrations of oestradiol and progesterone were determined in individual follicles with diameters > 2 mm (n = 635). There were no differences in the mean total number of follicles recovered per animal. However, the distribution of follicles in cyclic and pregnant pigs differed: the ovaries of pregnant pigs contained a greater number of large follicles ( > 4 mm) than the ovaries of Day 12 cyclic pigs (P < 0.001 ). Small follicles from Day 20 and Day 30 pregnant pigs produced significantly (P < 0.00 1) more progesterone than large follicles. A similar relationship (P < 0.007) was also evident among follicles on Day 12 of the oestrous cycle but not on Day 12 of pregnancy. Furthermore, pregnancy affected follicular maturation by reducing oestrogenic activity in all pigs of the pregnant group and subsequently blocking ovulation; generally oestradiol concentration in follicular fluid did not exceed 30 ng ml-1. In conclusion, the growth of follicles during early pregnancy in the pig is altered in comparison with that during the luteal phase of the oestrous cycle and the process of follicle maturation is suppressed.