Control of ovarian follicles activity in the ewe

During the ovine estrous cycles, three waves of follicular growth, closely associated with the FSH secretion pattern, were observed. The parameters of these follicular waves and the ability of follicles to produce steroids in vitro were studied in various conditions. In vivo, the follicular events w ere similar between the breeding season and the anestrus, except for the la ck of ovulation; but at the end of the breeding season and in anestrus, the follicles lose a big part of their aromatization ability. In ewes carrying the Booroola fecundity gene or Cambridge fecundity gene, the reduction in follicular atresia seems to be one of the main follicular features implicat ed in the control of high ovulation rate. In vitro, the most relevant diffe rence is an early acquisition of estrogen production ability of small folli cles in Booroola fecundity gene barring ewes. Fluoro-gestone-acetate (FGA) pessaries reduced the number of growing follicles; despite this effect disa ppearing after the sponge withdrawal, the ovulation rate is significantly r educed. But an equine chorionic gonadotrophin (eCG) treatment restores the ovulation rate (OR) by reducing the atresia rate of pre-ovulatory follicles . In similar conditions, a pretreatment of the ewes with melatonin again re duced the atresia rate of large follicles and resulted in an increased ovul ation rate. In vitro, FGA blocked aromatization ability, and melatonin inhi bited both androstenedione and estradiol production, but a further treatmen t with eCG partly restores the steroid secretion. Immunization against andr ostenedione leads to a higher OR, owning to a reduced atresia of large foll icles. Daily growth hormone injections for a hole cycle resulted in an incr eased follicular population and ovulation rate, while FSH plasma levels dec reased and the follicle sensitivity to gonadotrophins was reduced. (C) 1999 Elsevier Science Inc. All rights reserved.