Cryopreservation of human spermatozoa decreases the number of motile normal spermatozoa, induces nuclear vacuolization and chromatin decondensation.

Even though cryopreservation of human spermatozoa is known to alter sperm motility and viability, it may also induce nuclear damages. The present study set out to determine whether or not cryopreservation alters motile sperm morphology under high magnification and/or is associated with chromatin decondensation. For 25 infertile men, we used high-magnification microscopy to determine the proportions of various types of motile spermatozoa before and after freezing-thawing: morphometrically normal spermatozoa with no vacuole (grade I), ≤ 2 small vacuoles (grade II), at least 1 large vacuole or >2 small vacuoles (grade III), and morphometrically abnormal spermatozoa (grade IV). The spermatozoa's chromatin condensation and viability were also assessed before and after freezing-thawing. Cryopreservation induced sperm nuclear vacuolization. It decreased the proportion of grade I + II spermatozoa (P < .001). It induced a decrease in the sperm viability rate (P < .001) and increased the proportion of sperm with noncondensed chromatin (P < .001). The latter parameter was strongly correlated with sperm viability (r = 0.71; P < .001). However, even motile sperm presented a failure of chromatin condensation after freezing-thawing, because the proportion of sperm with noncondensed chromatin was correlated with high-magnification morphology (r = -0.49 and 0.49 for the proportions of grade I + II and grades III + IV, respectively; P < .001). Cryopreservation alters the organelle morphology of motile human spermatozoa and induces sperm chromatin decondensation. High-magnification microscopy may be useful for evaluating frozen-thawed spermatozoa before use in assisted reproductive technology procedures (such as intrauterine insemination, in vitro fertilization, and intracytoplasmic sperm injection) and for performing research on cryopreservation methods. If frozen-thawed sperm is to be used for intracytoplasmic sperm injection, morphological selection under high magnification may be of particular value.