CHORIONIC-GONADOTROPIN SECRETION IS ASSOCIATED WITH AN INHIBITION OF FOLLICULAR-GROWTH AND AN IMPROVEMENT IN OOCYTE COMPETENCE FOR IN-VITROMATURATION IN THE MARE

This study reports the follicular growth and oocyte competence for in vitro maturation ant! fertilization under the influence of circulating eCG. Three to 7 successive ultrasound-guided follicular punctures wer e performed on if pregnant mares from Day 23 until Day 75 of pregnancy and on 5 control mares whose embryonic vesicle was crushed on Day 22. All follicles larger than 5 mm were punctured 24 h after the largest follicle reached 18 mm. Expanded cumulus oocyte complexes (COCs) were stained at recovery to analyze the nuclear stage, Compact COCs were cu ltured in vitro for 46 h and either stained or processed for in vitro fertilization (IVF) and stained 26 h after IVF. in the control group, no mares showed an increase in eCG levels, whereas all the pregnant ma res had concentrations higher than 100 ng/ml from Day 37. The number o f follicles flushed during each puncture attempt significantly decreas ed with time for 3 of 4 pregnant mares. No significant change in this number was observed for the 5 control mares, The maturation rate of th e oocytes from follicles 10-14 mm was significantly higher in the preg nant vs, the control group (14 of 17, 82%, vs, 13 of 30, 43%). The dif ference was not significant for the oocytes from follicles smaller tha n 9 mm or larger than 15 mm. After IVF no oocyte was fertilized. The r esults led us to conclude that eCG is associated with an inhibition of follicular growth and an improvement in oocyte competence for in vitr o maturation.