EQUINE OOCYTE COMPETENCE FOR NUCLEAR AND CYTOPLASMIC IN-VITRO MATURATION - EFFECT OF FOLLICLE SIZE AND HORMONAL ENVIRONMENT

Equine oocyte competence after in vitro maturation (IVM) was investiga ted in terms of the diameter of the follicle of origin and the stage o f the estrous cycle, with three criteria of maturation: nuclear stage after DNA Hoechst staining, meiotic spindle morphology after tubulin i mmunocytochemical staining, and cortical granule localization after le ctin labeling. Seven successive in vivo ultrasound-guided follicular p unctures were performed on 10 cyclic saddle mares, alternatively at th e end of the follicular phase (after induction of ovulation with a gon adotropin injection) and in midluteal phase (with or without a gonadot ropin injection). Expanded cumulus-oocyte complexes (COCs) were staine d at collection, and compact COCs were stained after in vitro culture. They were observed under a confocal microscope. Successive punctures on one mare provided 0.9 preovulatory COCs and 8 immature COCs per 22 days. Among the preovulatory oocytes, 55% had completed nuclear and cy toplasmic maturation, 86% of which displayed a normal meiotic spindle. Of the 262 oocytes cultured in vitro, 37% completed nuclear maturatio n. The nuclear and cytoplasmic maturation rate significantly increased with follicle diameter. The IVM rate tended to be higher in follicula r phase and tended to increase in luteal phase with the gonadotropin i njection. The meiotic spindle morphology was not significantly differe nt between the classes of follicular diameters. This study provided th e opportunity to increase the number of characterized oocytes collecte d per cycle and per mare. This is the first report showing the progres sive acquisition of meiotic competence in the equine oocyte during ant ral follicle growth and is the only description of the equine meiotic spindle.