Follicle aspiration has previously been used in mares as a research tool to remove growing and atretic follicles, in order to study follicular growth. The aim of the present study was to (1) evaluate the fate of the aspirated follicles, and (2) determine the point of selection of the dominant follicle subsequent to follicular aspiration. In six Standardbred mares, all follicles larger than 9 mm were removed by transvaginal ultrasound-guided aspiration (Day 0). Subsequent to follicular aspiration, the growth of follicles larger than 1 mm was monitored daily by ultrasonography from Day 0 to Day 7. Aspirated follicles were monitored to establish whether they refilled and continued to grow, or luteinized. The experiment was conducted in six replicates in each mare. On Day 1 after aspiration the largest and second largest follicles were 10.4 ± 0.8 mm (mean ± SEM) and 7.8 ± 0.6 mm, respectively. On Day 7 the largest follicle and the second largest follicle were 25.7 ± 1.2 mm and 18.6 ± 1.2 mm, respectively. In 10/209 follicles, the follicular cavity refilled subsequent to aspiration with non-echogenic fluid and the follicle diameter increased during the following 7 days. Four of the ten aspirated and refilled follicles grew to become the largest follicles, whereas the remaining six follicles did not reach dominance. A further three aspirated follicles grew and ovulated on Day 5. In one case, an aspirated follicle refilled and continued to grow after an oocyte had been recovered. However, from these preliminary results, the growth pattern of the aspirated follicle can not be predicted on the basis of whether or not the oocyte was removed during the aspiration session. Preliminary results of this study indicate that follicular selection for dominance as determined by follicular size difference may already have occurred on Day 1 after aspiration. Furthermore, follicles that refill with fluid and continue to grow after aspiration may pose a problem when follicular growth and selection are studied.

This research was funded by the Danish Research Agency, project no. 23-02-0133.