Induction of ovarian cystic follicles in sheep

Cystic follicles are a significant cause of infertility in women, dairy cat tle and sheep. Sheep were used as a model to identify factors that may elic it formation of cystic follicles. Insulin resistance and elevated LI-I acti vity were tested in overweight ewes because of associations among these fac tors and thr formation of cystic follicles. Sheep were synchronized using a progesterone-releasing pessary and insulin resistance was induced during t he synchronization period through administration of bovine somatotropin. Fo llowing removal of pessaries follicular growth was stimulated by treatment with eCG or eCG and hCG (PG-600). Follicular growth was monitored via daily transrectal ultrasonography and blood samples were collected for hormonal analyses. Six of 18 ewes had a subnormal or absent preovulatory gonadotropi n surge and developed cystic follicles. Neither insulin resistance nor elev ated LH activity were associated with formation of cystic follicles. Ewes t hat developed cystic follicles were heavier (93 +/- 4 kg) than ewes that ov ulated(81 +/- 3 kg; P = 0.02). Furthermore, following pessary removal and i nitiation of daily ultrasonography, ewes that developed cystic follicles lo st body weight (-3 +/- 1%), while ovulatory ewes continued to gain body wei ght (1 +/- 1%; P = 0.005), It is speculated that in heavy ewes metabolic fa ctors associated with acute body weight loss inhibit the positive feedback of estradiol and thereby suppress the preovulatory gonadotropin surge leadi ng to formation of cystic follicles. (C) 2000 Elsevier Science Inc. All rig hts reserved.