Repeated follicle aspiration in mares: consequences for follicle growth and oocyte quality

Cumulus-oocyte complexes (COCs) recovered from ovaries of mares killed at abattoirs or after in vivo collection have heterogeneous morphologies and meiotic competence as follicles of variable quality are used. It is thought that it should be possible to recover more uniform COCs, with respect to morphology and nuclear maturation, by repeated follicle aspiration. Therefore, the influence of repeated follicle aspiration on the number and diameter of follicles > or =5 mm in diameter, the morphology and recovery rate of COCs, and the chromatin configuration in oocytes was investigated. Repeated ultrasound-guided aspirations were performed on Warmblood mares (n=6) after either a normal cycle ('cyclic' sessions) or at 4-12 day intervals ('consecutive' sessions). In 88 follicle aspiration sessions, 1268 follicles were aspirated and 280 COCs were recovered: the mean number of follicles aspirated and the number of COCs obtained per session per mare were 14.4 and 3.2, respectively. The mean recovery rate was 22.1%; there was no significant difference in the recovery rate between cyclic and consecutive aspirations. However, the mean number of follicles aspirated was significantly different between cyclic and consecutive aspirations (15.3 versus 10.8, respectively) and, hence, fewer COCs were obtained in consecutive aspirations compared with cyclic aspirations (2.2 versus 3.5, respectively). The proportion of compact COCs was higher for consecutive than for cyclic aspirations (51.9 versus 28.8%, respectively; P < or = 0.02). Within consecutive sessions, the proportion of compact COCs decreased with increasing interval between aspirations. Moreover, the proportion of oocytes with a diffuse germinal vesicle chromatin configuration was higher in COCs collected in consecutive aspirations than in COCs collected in cyclic aspirations. Repeated follicle aspiration can be used to induce a more uniform follicular population and to provide more uniform COCs. The optimum interval between aspirations to provide the greatest number of meiotically competent oocytes must be determined.