Epididymal cell secretory activities and the role of proteins in boar sperm maturation

The final stages of sperm differentiation occur outside the gonad, in the epididymal tubule. These last maturation steps, essential to the quality of spermatozoa, are not under the genomic control of the germ cells. A series of sequential interactions with the epididymal fluid, mostly specific proteins present in the lumen of different regions, are believed to induce the final steps of sperm maturation. In order to provide the luminal changes required for this maturation to occur, the epithelium may resort to two basic mechanisms: absorption and secretion. Far from being a uniform channel, the epididymal duct is a canal with highly specialized regional differentiation of its epithelial ultrastructure and its secretory and absorptive functions. This review focuses on the ultrastructural characteristic of the epithelial cells, their specific secretory activity according to the epididymal regions and their eventual role in sperm maturation of the boar. The chronology of the changes that occur in and on the sperm and in the surrounding environment are described. Relationships between the highly regionalized epididymal activities, sperm characteristics linked to their survival and fertility potential are also presented in this review.