Morphological alterations in protamine-deficient spermatozoa

STUDY QUESTION: How are protamine deficiencies associated with sperm head morphology in subfertile men? SUMMARY ANSWER: The prevalence of morphological variations and large nuclear vacuoles was slightly higher in protamine-deficient spermatozoa than in non-deficient spermatozoa. WHAT IS KNOWN ALREADY: A protamine deficiency was previously reported to be associated with an abnormal sperm morphology; however, how they are related to each other remains unclear. This is further confounded by a number of protamine-deficient spermatozoa having a normal head morphology. STUDY DESIGN, SIZE, DURATION: This is a cross-sectional study, including 36 men diagnosed with male factor infertility or participating in an assisted reproduction program. To assess sperm head morphology, this study analyzed 2400 spermatozoa with a protamine deficiency and 2400 spermatozoa with a normal protamine status. An additional 21 men were analyzed to examine DNA fragmentation and its relationship with protamine deficiencies and sperm head morphologies. PARTICIPANTS/MATERIALS, SETTING, METHODS: The morphology of the sperm head was evaluated based on its shape, size and nuclear vacuoles at a magnification of >6000×. Using elliptic Fourier analysis, the shape was summarized into four numeric variables. The protamine status was evaluated with chromomycin A3 (CMA3). Sperm head size, vacuoles and shape were compared between protamine-deficient and non-deficient spermatozoa. DNA fragmentation was evaluated with the terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling (TUNEL) assay. The percentages of protamine-deficient spermatozoa and DNA fragmentation were compared between spermatozoa with morphologically normal heads and those with abnormal heads. MAIN RESULTS AND THE ROLE OF CHANCE: Variations in head size (P < 0.0001) and shape (P < 0.0001) were significantly higher, with narrower (P < 0.001), more fan-shaped (P < 0.01) and more square-shaped forms (P < 0.001) in protamine-deficient spermatozoa than in non-deficient spermatozoa; however, the distribution of morphological variations markedly overlapped. Protamine deficiencies were more frequently observed in spermatozoa with large nuclear vacuoles than in those without them (32.0 ± 3.1 versus 39.4 ± 2.9%, P < 0.001). The percentage of protamine-deficient spermatozoa was significantly lower in spermatozoa with a normal head morphology than in those with an abnormal head morphology (25.4 ± 2.6 versus 38.0 ± 2.5%, P < 0.001). The percentage of DNA fragmentation was significantly higher in protamine-deficient spermatozoa than in non-deficient spermatozoa (11.3 ± 2.1 versus 1.6 ± 0.6%, P < 0.001), and was lower in spermatozoa with a normal head morphology than in those with an abnormal head morphology (2.6 ± 0.7 versus 6.4 ± 0.2%, P < 0.001). LIMITATIONS, REASONS FOR CAUTION: We were unable to discriminate the kind of protamines or quantify the extent of the protamine deficiency in spermatozoa using the CMA3 staining method. WIDER IMPLICATIONS OF THE FINDINGS: This study provided a novel insight into how abnormal protamination affects sperm head morphology as well as the relationship between sperm head morphology and its own molecular integrity. Our results will contribute to a deeper understanding of the benefits and limitations of the morphological selection of spermatozoa for ICSI. STUDY FUNDING/COMPETING INTERESTS: This study was supported by a JSPS Grant-in-Aid for the Encouragement of Scientists (25931009, 26931010). All authors have no conflicts of interest to disclose. TRIAL REGISTRATION NUMBER: N/A.