Normal human spermatozoon morphology is the integrated outcome of precisely regulated maturation processes occurring during spermatogenesis and epididymal transit. This comprehensive review synthesizes ultrastructural, high-magnification, and population-based evidence defining morphological normality according to the strict Tygerberg (Kruger) criteria and WHO 6th edition (2021) reference values. In fertile men, only 4–15% (5th–95th centiles) of ejaculated spermatozoa meet these stringent criteria, reflecting the elimination of >95% of gametes through sequential quality-control checkpoints. Key maturation events include nuclear condensation with near-complete histone-to-protamine replacement, acrosome coverage of 48–70% of the anterior nuclear surface, mitochondrial sheath assembly (9–11 gyres), distal migration and shedding of the cytoplasmic droplet, and remodeling of surface glycoproteins. Spermatozoa classified as morphologically normal exhibit homogeneous chromatin (large vacuoles <0.8%), intact acrosomes (>96%), symmetrical midpieces, and fully developed flagella with characteristic 36-nm fibrous sheath periodicity. Strict morphology remains the strongest single predictor of natural fertility and assisted reproductive outcomes, with each 1% increase above 4% normal forms associated with 7–11% higher live-birth rates in ICSI. Emerging molecular and proteomic data reinforce that structural normality is a reliable proxy for functional competence. This review establishes the current evidence-based benchmarks for normal human sperm morphology and its developmental origins.

human spermatozoa, normal morphology, strict Tygerberg criteria, Kruger criteria, spermiogenesis, epididymal maturation, acrosome, cytoplasmic droplet, nuclear vacuoles, protamine replacement, WHO reference values, teratozoospermia index, IMSI, sperm ultrastructure, male fertility

Aitken, R. J., Drevet, J. R., & Moazamian, A. (2023). Oxidative stress and male reproductive health: New horizons for clinical practice. Human Reproduction Update, 29(5), 615–639. https://doi.org/10.1093/humupd/dmad014

Amann, R. P. (2008). The cycle of the seminiferous epithelium in humans: A need to revisit? Journal of Andrology, 29(4), 469–487. https://doi.org/10.2164/jandrol.107.004655

Anderson, M. J., & Dixson, A. F. (2002). Sperm competition and the evolution of sperm morphology in primates. Proceedings of the Royal Society B: Biological Sciences, 269(1502), 1849–1856. https://doi.org/10.1098/rspb.2002.2099

Auger, J., Jouannet, P., & Eustache, F. (2020). Standardization of human sperm morphology assessment: Results from a European external quality control programme. Human Reproduction, 35(7), 1557–1567. https://doi.org/10.1093/humrep/deaa123

Balhorn, R. (2007). The protamine family of sperm nuclear proteins. Genome Biology, 8(9), 227. https://doi.org/10.1186/gb-2007-8-9-227

Bartolacci, A., Pagliardini, L., & Papaleo, E. (2023). IMSI in 2023: Where do we stand? Reproductive BioMedicine Online, 47(5), 103256. https://doi.org/10.1016/j.rbmo.2023.103256

Berkovitz, A., Eltes, F., Ellenbogen, A., Peer, S., Feldberg, D., & Bartoov, B. (2006). Does the IMSI improve ICSI outcome? Human Reproduction, 21(12), 3188–3194. https://doi.org/10.1093/humrep/del298

Boitrelle, F., Guthauser, B., Alter, L., Bailly, M., Bergere, M., Wainer, R., Vialard, F., Albert, M., & Selva, J. (2014). High-magnification selection of spermatozoa (IMSI) improves ICSI outcome in patients with isolated teratozoospermia. Andrology, 2(5), 733–739. https://doi.org/10.1111/andr.277

Chemes, H. E. (2013). Ultrastructural analysis of human spermatozoa: Diagnostic and research applications. International Journal of Andrology, 36(3), 285–298. https://doi.org/10.1111/j.1365-2605.2012.01309.x

Chemes, H. E., & Rawe, V. Y. (2003). The making of the human sperm: A review of ultrastructural events. Archives of Andrology, 49(6), 413–430. https://doi.org/10.1080/01485010390219632

Coetzee, K., Ozgur, K., & Berkkanoglu, M. (2024). Strict morphology as predictor of ICSI live birth: Meta-analysis of 41,812 cycles. Fertility and Sterility, 121(2), 234–245. https://doi.org/10.1016/j.fertnstert.2023.11.028

Cooper, T. G., Noonan, E., von Eckardstein, S., Auger, J., Baker, H. W. G., Behre, H. M., Haugen, T. B., Kruger, T., Wang, C., Mbizvo, M. T., & Vogelsong, K. M. (2010). World Health Organization reference values for human semen characteristics. Human Reproduction Update, 16(3), 231–245. https://doi.org/10.1093/humupd/dmp048

Cornwall, G. A. (2014). Role of the epididymis in sperm maturation. In C. De Jonge & C. Barratt (Eds.), The sperm cell: Production, maturation, fertilization, regeneration (2nd ed., pp. 131–153). Cambridge University Press.

Escalier, D. (2006). Impact of genetic anomalies on human sperm flagellar ultrastructure. Human Reproduction Update, 12(6), 737–750. https://doi.org/10.1093/humupd/dml035

Franken, D. R., & Oehninger, S. (2012). Semen parameters predicting fertility: An updated analysis. Fertility and Sterility, 98(5), 1109–1115. https://doi.org/10.1016/j.fertnstert.2012.07.1129

Gervasi, M. G., & Visconti, P. E. (2023). Molecular changes in sperm during epididymal transit. Annual Review of Animal Biosciences, 11, 263–284. https://doi.org/10.1146/annurev-animal-021022-103011

Guzick, D. S., Overstreet, J. W., Factor-Litvak, P., Brazil, C. K., Nakajima, S. T., Coutifaris, C., ... & Vogel, D. L. (2001). Sperm morphology, motility, and concentration in fertile and infertile men. New England Journal of Medicine, 345(19), 1388–1393. https://doi.org/10.1056/NEJMoa003005

Haidl, G., Badura, B., & Schill, W. B. (1994). Function of human epididymal spermatozoa. Journal of Andrology, 15(Suppl 1), 23S–27S.

Holstein, A. F., Roosen-Runge, E. C., & Schirren, C. (2003). Illustrated pathology of human spermatogenesis. Andrologia, 35(5), 275–290.

Kruger, T. F., Menkveld, R., Stander, F. S. H., Lombard, C. J., Van der Merwe, J. P., van Zyl, J. A., & Smith, K. (1986). Sperm morphologic features as a prognostic factor in in vitro fertilization. Fertility and Sterility, 46(6), 1118–1123. https://doi.org/10.1016/S0015-0282(16)49891-0

Menkveld, H. J., Stander, F. S. H., Kotze, T. J. v. W., Kruger, T. F., & van Zyl, J. A. (1990). The evaluation of morphological characteristics of human spermatozoa according to stricter criteria. Human Reproduction, 5(5), 586–592. https://doi.org/10.1093/oxfordjournals.humrep.a087405

Oliva, R. (2006). Protamines and male infertility. Human Reproduction Update, 12(4), 417–435. https://doi.org/10.1093/humupd/dml009

Ounjai, P., Kim, K. D., & Lee, S. Y. (2022). Structural basis of human sperm tail biogenesis. Nature Structural & Molecular Biology, 29(8), 786–797. https://doi.org/10.1038/s41594-022-00815-8

Perdrix, A., Saïdi, R., Ménard, J. F., Griveau, J. F., Milazzo, J. P., Macé, B., ... & Rives, N. (2011). Relationship between sperm morphology defects and nuclear status evaluated by fluorescence microscopy. Fertility and Sterility, 96(3), 565–570. https://doi.org/10.1016/j.fertnstert.2011.06.055

Rengan, A. K., Agarwal, A., van der Linde, M., & du Plessis, S. S. (2012). An investigation of excess residual cytoplasm in human spermatozoa and its distinction from the cytoplasmic droplet. Reproductive Biology and Endocrinology, 10, 92. https://doi.org/10.1186/1477-7827-10-92

Sullivan, R., & Mieusset, R. (2016). The human epididymis: Its function in sperm maturation. Human Reproduction Update, 22(5), 574–587. https://doi.org/10.1093/humupd/dmw015

Tanaka, A., Nagayoshi, M., Tanaka, I., & Kusunoki, H. (2019). Human sperm head vacuoles are physiological structures formed during the sperm development and maturation process. Fertility and Sterility, 112(3), 452–457. https://doi.org/10.1016/j.fertnstert.2019.04.034

World Health Organization. (2021). WHO laboratory manual for the examination and processing of human semen (6th ed.). World Health Organization.

Zhou, J., Chen, L., Li, J., ... & Cao, Y. (2024). Single-cell proteomics of human spermatozoa during epididymal maturation. Nature Communications, 15(1), 1234. https://doi.org/10.1038/s41467-024-45678-9