The response of epididymal bull spermatozoa to hypotonic media was investigated. Experiments in which spermatocrits were plotted versus the reciprocal of medium osmolality indicate that the swelling of the spermatozoa takes place linearly with a decrease in medium osmolality.
The mean volume and area of swollen spermatozoa at critical medium osmolality (44 mosmol) were calculated as 126 μm3 and 145 μ2, respectively. Data available on the isotonic volume, area and water content of the spermatozoa indicate that, at critical medium tonicity, the cells have increased their volume 3·78 to 3·94 times without an apparent increase in area and that, according to conventional osmotic theory, they may be regarded as osmometers with a value of the Ponder's `R' correction factor in the range 0·81 to 0·91, i.e. they seem to deviate from the state of a `perfect osmometer' by 9 to 19%.
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