How does the boar epididymis regulate the emission of fertile spermatozoa?

The epididymis is responsible for peripheral immune tolerance of maturing spermatozoa even though these have xeno-antigens foreign to the male and female immune system. The epididymis also produces factors required for fertilization and serves as a sperm repository until the time of ejaculation. These reproduction-relevant epididymal functions occur in the mesonephros-derived duct-system that is composed of absorptive and secretory epithelial cells with the capacity for merocrine and apocrine secretion of proteins, antioxidative- and electrolyte/pH-regulating enzymes and small, non-coding RNAs (sncRNAs), many stored in epididymosomes for sperm adhesion and long-lasting modifications of sperm functions. This paper provides a review summary of current and new knowledge of how the boar epididymis affects the quality of spermatozoa in the ejaculate of breeding boars. There is a particular focus on sperm maturation, survival, function and the role of signaling to the female immune system in fertility modulation. Furthermore, aspects related to the ductus epithelial contributions regarding electrolyte control, protein production, release of epididymosomes that contain sncRNAs are emphasized as are novel associations with fertility of the male, sperm quiescence during storage in the cauda epididymis, and on changes occurring in sperm subsequent to ejaculation. Funding Agencies|Research Council FORMAS, Stockholm [2017-00946, 2019-00288]; MICINN [PID2020-113493RB-I00]; Madrid, Spain; European Union, Brussels, Belgium [891663]