Significance of midpiece vesicles and functional integrity of the membranes of human spermatozoa after osmotic stress

Some human spermatozoa respond to osmotic pressure changes in their surroundings by volume change of vesicles on the plasmalemma in the midpiece domain. We aim to determine how cyclic change in osmotic stress affects motility and permeability of the plasmalemma and whether the response is affected by the presence of a midpiece vesicle. Spermatozoa from normozoospermic men were examined in medium, whose osmolality was adjusted to 276 and 450 mmol kg(-1) with raffinose. Plasmalemma permeability was measured by propidium iodide (PI) uptake and motility assessed by computer-aided semen analysis. Spermatozoa were imaged with fluorescence light and X-ray microscopy. PI staining was significantly increased, and motility decreased only when spermatozoa were cycled from hyper- to hypo-osmotic conditions. A single change in osmotic pressure did not significantly affect PI uptake. As expected, midpiece vesicles swelled and became spherical in hypo-osmotic conditions. However, a significant association existed between osmotically induced plasmalemma damage and absence of a midpiece vesicle. We conclude that the midpiece vesicle is not a consequence of membrane leakage but a normal part of spermatozoon anatomy, conferring resistance to osmotic stress mediated damage. As osmotic stress affects both motility and plasmalemma permeability, the presence of a midpiece vesicle is significant in spermatozoal function and could have implications to understanding specific cases of undetermined male factor infertility.