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.
