Time course of hypo-osmotic swellings of human spermatozoa: evidence of ordered transition between swelling subtypes

The hypo-osmotic swelling test (HOST or HOS test) usually takes intoconsideration the total HOS response value with no emphasis either on thevalue of the response subtypes or the response evaluation time. This studyinvestigated the time course of HOS responses and analysed theirphysiological relevance. Raw semen spermatozoa and Percoll washedspermatozoa were used in the experiment. The morphological changes in thesperm tail were monitored by incubating the spermatozoa in the hypo-osmotic solution for 16 different time periods. The HOS reactivespermatozoa and the type of HOS reaction (swelling subtypes) of the samplessubjected to different duration of treatment were identified under a phasecontrast microscope. Also the fate of individual spermatozoa in ahypo-osmotic environment were monitored for 30 min. In spermatozoa exposedto a hypo-osmotic solution, the motility lasted usually less than 2 min andmotility characteristics were uniquely different from that of thespermatozoa under iso-osmotic conditions. The HOS response development waspermanent but the motility loss due to hypo-osmotic shock was reversible upto 1 min of incubation. There was an indication of ordered transition amongthe HOS swelling subtypes apparently initiating with subtype b destined toc, d, e, f and g. Further, the subtypes a and g showed gradual decrease andincrease, respectively, while subtype b showed abrupt initial increase andthen gradual decrease. Transition from b to g could be direct or via one ormore than one subtypes. Ultrastructure based analysis indicated that HOSresponse subtypes are the apparent reflection of the differences in thecytoskeletal assembly of the sperm tail and thus may be identifyingdifferent physiological variants in the sperm population. These resultsindicate that shorter incubation is essential to document the kinetics ofvarious HOS responses but the conventional HOS test misses these importantHOS features because of lengthy incubation. Since the time course ofordered transition of HOS responses will vary more than the total HOSresponse in semen of different aetiologies, the importance of HOS responsesubtypes and response evaluation time should be taken into considerationwhen applying HOS test.