Follicular growth and maturation in hyperprolific and large white sows

The mechanisms whereby hyperprolific sows achieve their increased ovulation rate (+5 oocytes on average) compared with normal Large White sows were explored in this study. The following specific questions were asked. Is increased ovulation rate related to 1) increased follicular populations within the ovaries or 2) alterations in terminal follicular growth and maturation? The population of antral follicles in six sows of each genotype was studied using histological techniques on ovaries obtained at the preovulatory stage. No difference between the total number of antral follicles, number of healthy or atretic follicles in specific size classes, and in size of the preovulatory follicles could be detected. The number of granulosa cells contained in preovulatory follicles was also similar between genotypes. Terminal follicular growth and maturation were studied in 15 sows of each genotype killed at d 1, 3, or 5 (n = 5.d-1.genotype-1) after the end of Regumate administration (d 0). Small (< or = 3.5 mm) follicles were counted, and follicles > 3.5 mm were dissected, measured, and incubated in vitro. Steroid concentrations (estradiol and testosterone) in culture medium were then measured. The two genotypes differed in the patterns of growth of their ovulatory follicles, because at d 3 all ovulatory follicles were obvious in Large White sows. In contrast, between d 3 and 5, seven additional ovulatory follicles grew in hyperprolific sows. Differences in follicular maturation between genotypes were also detected. Whereas testosterone concentrations in culture medium were similar in the two genotypes, estradiol concentrations were markedly (P < .01) increased in hyperprolific follicles. This indicates that these follicles may have an increased aromatizing ability. How this generates the altered pattern of follicular growth and the increased ovulation rate of hyperprolific sows remains to be established.