Characterization and localization of membrane vesicles in ejaculate fractions from the ram, boar and stallion

Membrane vesicles, separated by differential centrifugation from the seminal plasma, were detected in the sperm-rich ejaculate fractions of four boars and three stallions, and in the whole ejaculates of seven rams. The volume and percentage of vesicles, determined by a stereological technique, were higher in the sperm-rich than in the post-sperm-rich fractions of the boar and stallion ejaculates, and no vesicles were detected in the pre sperm-rich fractions. Vesicles were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). For boar, stallion and ram semen, the mean (+/- s.e.m.) vesicle diameters were 130.9 +/- 3.22 (range 18-577), 164.1 +/- 4.42 (range 15-671) and 159.7 +/- 2.92 nm (range 22-986), respectively, although they were not significantly different (p = 0.709). The vesicles had approximately round (TEM) or spherical shape (SEM), were surrounded by single, double or multi-laminar membranes, and were trapped within ample amorphous material, sometimes containing short, flattened membranous elements. The majority of the vesicles had a clear interior but some contained granule-dense material. Ram membrane vesicles, purified from ultracentrifuged plasma by size exclusion chromatography, kept their round shape and the amorphous material was less evident compared with the sections taken before purification. This is the first report to identify seminal plasma membrane vesicles in the different fractions of ejaculated semen in the boar and stallion, and confirms their presence in ram seminal plasma. The origin and function of these vesicles are yet to be elucidated.