Andrology. Response of midpiece vesicles on human sperm to osmotic stress

BACKGROUND: We investigated the osmotic response of midpiece vesicles (MPV) on human sperm. METHODS: Light microscopy, transmission X-ray microscopy and computer-aided semen analysis was used to investigate sperm in normozoospermic semen from healthy donors, separated from semen and suspended in hyper- or hypo-osmotic solutions. RESULTS: The mean incidence of MPV-bearing sperm in semen was 31% (range 8-71; n = 47). MPV were morphologically different from cytoplasmic droplets. The incidence of MPV-bearing sperm in separated populations increased reversibly after washing in Sperm Preparation Medium but not after washing in seminal plasma. There was an inverse relationship between medium osmolality and both MPV-bearing sperm incidence and MPV diameter. However, initial osmolality in semen from different donors did not correlate with incidence of MPV-bearing sperm. Furthermore, a direct relationship was observed in semen as osmolality increased with time. No correlation existed between progressive motility and semen osmolality. Progressive motility and the amplitude of lateral head displacement were significantly reduced in sperm with an MPV (three out of four semen samples, 26-32 sperm). The incidence of MPV-bearing sperm in those that had penetrated cervical mucus (75, 46, and 40%) was increased compared with the adjacent semen (24, 35, and 24%). CONCLUSIONS: MPV are ubiquitous and distinct from cytoplasmic droplets. They respond to osmolality changes of the surrounding medium. The presence of an MPV can reduce motility but not survival in cervical mucus. Therefore, they should not be considered detrimental to sperm function.