However, our data demonstrated that none of the epidi-dymosomes major membrane-bound proteins was present in vivo on cauda epididymal or ejaculated sperm extracts. Thus, if these vesicles are involved in sperm membrane transformations, it is via very subtle exchange mechanisms that remain to be explained but not by a vesicle-fusion mechanism. Moreover, the epididymal exososomes represent only a small percent of the total cauda protein, and the most hydrophobic proteins were mainly present in the caudal fluid bulk phase under a soluble form.
It is possible that these soluble proteins exist in micelles or complexes, which could explain the mechanism of their transfer to the cell membrane. Alternatively, these vesicles could play a role in the female genital tract, where their immunosuppressive and anti-oxidant properties might protect the sperm from degradation.
In conclusion, we have clearly demonstrated that vesicles with a specific protein content exist in the caudal fluid and form the majority of the vesicles retrieved in the seminal plasma of the ram. Using different markers, we have also provided evidence that the majority of these vesicles are mainly secreted in the cauda/corpus regions of the epididymis and accumulated in the caudal fluid. These vesicles, which are similar to the previously described vesicles from the seminal plasma, are physically and biochemically related to exosomes. They represented only a small fraction of the caudal-fluid proteins and do not contain all the hydrophobic compounds of this fluid. We found no evidence for the exchange of the major proteins between vesicles and sperm within the epididymis or during ejaculation. The role of these vesicles in male reproductive physiology requires further investigations.