Abstract:
The spermatozoa of most mammals behave as ‘perfect osmometers’.The volume response to osmolality obeys the Boyle-Van’t Hoffrelationship (i.e. volume changes are determined by the osmotically activefraction of the cell volume (solids and water)). Most evaluations of osmoticsperm cell behaviour have been based on the mean volume of the cellpopulation. In the present study, both mean and modal volumes of samples ofsperm were evaluated. Both mean and modal volumes responded to environmentalosmolality via the Boyle-Van’t Hoff relationship; however, themodal volume showed a more sensitive response than the mean volume. This wasconfirmed for both ejaculated and epididymal spermatozoa. After incubationunder capacitating conditions, the difference in modal and mean volumeresponse of ejaculated sperm was considerably diminished and, in epididymalsperm, completely abolished. The sperm osmotic behaviour was still consistentwith the Boyle-Van’t Hoff equation, but the apparent osmoticallyinactive modal cell volume decreased after exposure to capacitating conditionsin both ejaculated and epididymal sperm samples. The changes in epididymalsperm were more intensive. Due to its enhanced sensitivity to environmentalosmolality and incubation under capacitating conditions, the modal volumecould be used as a parameter for evaluating sperm population response, such asfor detecting environmentally or cryopreservation-induced membrane changes.