123 THE MARE MODEL TO STUDY HOW OVARIAN DYNAMICS AFFECTS PREANTRAL FOLLICLE FEATURES

Abstract

The mare has been strongly advocated by several research groups as an important comparative animal model to study antral follicular dynamics in women due to some similarities in reproductive events. More recently, the mare has also been suggested as a potential model for studies related to preantral follicles. The search for an appropriate animal model for comparative studies of preantral follicle population, density, and distribution has been a major focus of recent ovarian translational studies. The aim of the present study was to investigate the influence of reproductive phase (anestrous v. diestrous) and ovarian structures (antral follicles and corpus luteum) on the quality, class distribution, number, and density of preantral follicles, and stromal cell density. Ovarian fragments were harvested in vivo from young, healthy mares (n = 10) during both reproductive phases using a biopsy pickup method and submitted to histological analysis. All evaluations and measurements were performed by a single operator and only follicles with a visible nucleus was considered for follicle counting. Data were analysed by Kruskal-Wallis test, Wilcoxon-Mann-Whitney test, and Spearman’s correlation. A total of 142 ovarian biopsy fragments were collected (mean, 3.7 fragments per mare in each reproductive phase) and 13 462 histological sections were evaluated. Overall, 1493 preantral follicles were recorded with a mean of 10.5 ± 1.7 follicles per ovarian fragment [range, 0 to 165; mean coefficient of variation (CV) = 195%]. The mean follicle density was 2.7 follicles per cm² (range, 0 to 39; CV = 214%) and differed (P < 0.05) among mares. The mean preantral follicle and ovarian stromal cell densities were greater (P < 0.05) in the diestrous phase and a positive correlation of stromal cell density with the number and density of preantral follicles was observed. The mean area (mm²) of ovarian structures increased (P < 0.05) in the diestrous phase and had positive correlations with number of preantral follicles, follicle density, and stromal cell density. Biopsy fragments collected from ovaries during the diestrous phase had a higher (P < 0.05) follicle density, stromal cell density, and proportion of normal preantral follicles. In conclusion, our results showed (1) the diestrous phase influenced positively the preantral follicle quality, class distribution, and follicle and stromal cell densities; (2) the area of ovarian structures was positively correlated with the follicle and stromal cell densities; and (3) the presence of an active corpus luteum had a positive effect on the quality of preantral follicles and follicle and stromal densities. Therefore, herein we demonstrated that reproductive phases and ovarian structures induce changes in preantral follicle features and stromal cell density favouring the harvesting of ovarian fragments containing an appropriate number of healthy preantral follicles. These findings reinforce the concept of the use of the mare as an appropriate model to provide comparative insights about preantral follicle density and ovarian plasticity.