Predictive water loss curves for ram spermatozoa during cryopreservation: comparison with experimental observations

Calculated curves predicting intracellular water loss during cryopreservation at different cooling rates were calculated from published equations. To compute these curves, basic cell parameters specific to ram spermatozoa were measured, i.e., the total surface area (139 microns2), the hydraulic conductivity (0.222 micron3/micron2.atm-1.min-1), and its temperature dependence (0.045/degree C). Cell surface area was derived from measurements of physical dimensions. Hydraulic conductivity was calculated from measurements of the critical medium hypotonicity on exposure of sperm to various hypotonic solutions and the time taken for membrane rupture in sperm exposed to distilled water (spermolysis time). The temperature dependence of the water permeability was derived from measurements of spermolysis time at various temperatures above zero. Several discrepancies were noted between the resulting calculated curves and experimental observations made on the effects of cooling rate on sperm cell survival. These could be due to errors in the estimates of the basic parameters, or to false assumptions in the basic equations used to compute the curves, e.g., the validity of the Boyle-van't Hoff relationship. Nevertheless, this study represents a first attempt to predict intracellular water loss from ram sperm during cooling and may provide a novel approach for the interpretation of the many empirical studies carried out to investigate optimal conditions for the cryopreservation of sperm.