The motility of hamster epididymal spermatozoa has been studied in modified Tyrode's solution; a high level of motility was maintained when bovine serum albumin, used in previous studies, was replaced by polyvinyl pyrrolidone, and when no macromolecule was included. In these culture media, it was found that sperm motility and survival were considerably enhanced if the osmotic pressure was lowered by 20% from the usual level of 290 mosmol. The reduced osmotic pressure caused leakage of a motility-stimulating substance from spermatozoa; the need to lower the osmotic pressure for optimum sperm motility could be eliminated by adding an extract of spermatozoa to the culture medium. With this culture system, spermatozoa could be maintained in a high state of motility for several hours, and the `sperm dilution effect' was substantially reduced.
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