Sperm dynamics in the female reproductive tract and their role in regulating fertility

The variability observed in bull field fertility cannot be fully explained by conventional semen analysis. Work by our group and others has shown that ejaculates are highly heterogeneous and contain distinct sperm subpopulations (SSPs) that differ in motility, functionality, and susceptibility to processes such as cryopreservation, with recent evidence indicating differential interactions within the female reproductive tract (FRT). Therefore, these SSPs have the potential to influence sperm survival and the uterine and oviductal environments, which not only influence fertilization but also early embryo development success. This review discusses key sperm-related processes within the FRT, including molecular changes, penetration into the uterine glands, sperm reservoir formation, and, in particular, the role of the immune response and endometrial priming in fertility. Comparative studies between high- and low-fertility bulls offer valuable models for the investigation of the mechanisms behind fertility variation, providing indications of the relevance of some SSPs. By summarizing the existing knowledge on SSPs, this review aims to provide a framework for understanding sperm behavior within the FRT and highlights the need to better understand sperm heterogeneity and its role in successful pregnancy establishment.