All this suggests that epididymal and seminal plasma vesicles may be members of an exosome-vesicle family. This hypothesis is also supported by the fact that the different proteome of the exosomes published to date also contain similar proteins to epididymal and seminal plasma vesicles. It has been suggested that exo-somal vesicles are secreted from cells upon fusion of mul-tivesicular endosomes with the cell membrane by an unknown mechanism.
However, we still do not have enough information on epididymal vesicles to know whether they are a true homogeneous population of exosomes. For example, no V-ATPase subunits (a marker of intracellular acidic vesicles) were found in exosomes, while one is present in the epididymal vesicles. Alternatively, this is in agreement with the suggestion that the composition of exosomes may vary in function with the tissue from which they are derived.
Several types of membrane secretion have been described in the efferent ducts and epididymis, including bleb secretions, which involved a large surface of the apical membrane and then form larger vesicles in the fluid.