CONTINUOUS ASSESSMENT OF HUMAN SPERMATOZOA VIABILITY DURING CRYOPRESERVATION

Cryomicroscopy has enabled direct observation of freezing and thawing of human spermatozoa. When used with a fluorescent viability kit, sper m membrane damage was not apparent down to temperatures of -5 degrees C, but significant damage occurred after thawing (55% of spermatozoa h ad damaged membranes). Semen samples were cooled or frozen to temperat ures (at decrements of 10 degrees C) from 0 degrees C to -110 degrees C. At all these temperatures the proportion of live to membrane-damage d cells remained constant. Samples held at temperatures above -30 degr ees C were not adversely affected. Below -30 degrees C there was a gra dual increase in the proportion of membrane-damaged cells on thaw and a decrease in the number of live cells recovering motility. At tempera tures between -50 degrees C and -60 degrees C there was an equal propo rtion of live motile, immotile, and membrane-damaged cells. It is conc luded that some irreversible damage to spermatozoa was a result of fre ezing processes in cells frozen to -30 degrees C or less, but most of the cryodamage was incurred during thawing, possibly due to recrystall ization.