Moreover, almost all proteins identified in ram epididymal vesicles have also been found in a recent pro-teomic profiling study of human seminal plasma vesicles, and one protein, aldose reductase, has also been described in bull epididymal vesicles (see also Table 2). This suggests that, as in the ram, a large proportion of the vesicles found in seminal plasma in humans (and certainly in bulls) are derived from the epididymis. Alternatively, these different types of vesicles may be shed from the cell membrane by a very similar mechanism, including the same membrane domains containing the same proteins.
Both the epididymal and seminal plasma vesicles also show some physical and biochemical similarities to vesicles secreted by different types of epithelia and cells called ex-osomes. They are in the same size range, they are not retained by 0.22-^m filtration, and they can be purified by size exclusion on gel filtration. Indeed, the ram cauda fluid size-exclusion peak obtained by gel filtration was found to have a similar protein pattern as the vesicle pellet, and this peak was considerably decreased when the fluid was centrifuged at high speed before separation (unpublished data).