Relationship of cytoplasmic droplets to motility, migration in mucus, and volume regulation of human spermatozoa.

A significantly greater percentage of motile than immotile spermatozoa bore droplets at the osmolality of semen and cervical mucus. The percentage of spermatozoa with droplets was not significantly correlated with the osmolality of semen or the extent of cell swelling in response to quinine in hypotonic medium. Droplets appeared slightly more frequently (ie, were more obvious) in the presence of quinine, which blocks regulatory volume decrease, indicating that they are the major site of volume expansion. There was no selection for or against droplet-bearing spermatozoa migrating through viscous surrogate mucus at high or low osmolality. Sperm swelling in response to quinine at mucus osmolality was significantly greater in fathers than in patients whose partners had no fertility problem. Therefore, cytoplasmic droplets are not deleterious to sperm motility and may be related to physiological volume regulation, which may be predictive of some forms of human male infertility.