Small human sperm vacuoles observed under high magnification are pocket-like nuclear concavities linked to chromatin condensation failure

Since an embryo’s ability to grow to the blastocyst stage and implant can be improved by selection of a normal spermatozoon with a vacuole-free head, this study set out to determine the nature of small sperm vacuoles observed under high magnification (>×6300). For 15 infertile men with various sperm profiles, high-magnification microscopy was used to select motile, morphometrically normal spermatozoa with no vacuoles (n = 450) or more than two small vacuoles (each of which occupied less than 4% of the head’s area; n = 450). Spermatozoa acrosome reaction status and degree of chromatin condensation were analysed. Three-dimensional deconvolution microscopy was used to accurately image the nucleus and acrosome at all depths in all spermatozoa. In all 450 spermatozoa with small vacuoles, the latter were seen to be abnormal, DNA-free nuclear concavities. Spermatozoa with small vacuoles were significantly more likely than vacuole-free spermatozoa to have noncondensed chromatin (39.8% versus 9.3%, respectively; P < 0.0001). There was no significant difference between the two groups of spermatozoa in terms of acrosome reaction status. No association between chromatin condensation and acrosome reaction status was observed. Small human sperm vacuoles observed under high magnification are pocket-like nuclear concavities related to failure of chromatin condensation.The present study set out to determine the nature of small sperm vacuoles observed under high magnification (>×6300), since these structures reportedly have a harmful effect on embryo development and implantation. For 15 infertile men with various sperm profiles, high-magnification microscopy was used to select a total of 900 motile, morphometrically normal spermatozoa with no vacuoles (n = 450) or more than two small vacuoles (each of which occupied less than 4% of the head’s area; n = 450). We subsequently analysed the spermatozoa’s acrosome reaction status and degree of chromatin condensation. Three-dimensional deconvolution microscopy was used to accurately image both the nucleus and the acrosome at all depths in each of the 900 spermatozoa. In all 450 spermatozoa with small vacuoles, the latter were seen to be abnormal, DNA-free nuclear concavities. Spermatozoa with small vacuoles were significantly more likely than vacuole-free spermatozoa to have non-condensed chromatin (39.8 ± 2.4% versus 9.3 ± 1.3%, respectively; P < 0.0001). There was no significant difference between the two groups of spermatozoa in terms of the acrosome reaction status. No association between chromatin condensation and acrosome reaction status was observed. Small human sperm vacuoles observed under high magnification are pocket-like nuclear concavities related to the failure of chromatin condensation.