Endocrine dynamics associated with follicle development in pigs: a review

A significant increase in growth of follicles that are selected for ovulation seems to occur on days 14-17 of the estrous cycle. In pigs there is a continuous growth of follicles without appearance of dominant follicles or follicle waves during the estrous cycle. There is a general consensus that a decrease of FSH during the follicular phase is accompanied with the selection of ovulatory follicles and changing from FSH to LH dependence. Development of preovulatory follicles is prevented during lactation mainly due to the inhibition of LH secretion. FSH is not inhibited during lactation and temporary increase in FSH is associated with wave-like follicular growth. Weaning of piglets normally results in increased secretion of LH, which is characterized by a high pulse frequency and low pulse amplitude. The duration of weaning-to-estrus interval is associated with plasma changes in gonadotropins, steroids, inhibin, leptin, IGF-I and insulin. Evidence for a positive role of short elevations in cortisol on LH secretion and ovarian function are accumulating but further studies are still needed to elucidate this issue. The aim of this review was to summarize our current knowledge of endocrinological changes in relation to follicular development during estrous cycle, lactation and after weaning in pigs.