Over the past 50 years, several approaches have been used to describe human sperm morphology (Eliasson 1971; WHO 1984, 1999, 2010, 2022). Morphological criteria for semen normality have evolved towards more precise definitions, which has led to a decrease in normality thresholds from 50% to 4% over the same period. As semen quality has been affected by lifestyle and involuntary exposure to toxic substances, it is impossible to know how much of the decline in normal morphology is attributable to the tightening of the analytical criteria or the impacts of modern life.
The presence of sperm in the ejaculate is a necessary, but not sufficient condition to guarantee a man's fertility. The quality of the gametes (sperm and egg) and the timing of sexual intercourse are decisive for fertilization to occur. The sperm analysis only reveals the necessary elements for the process of fertilization, such as the sperm number, their motility, and the shape of the head, midpiece, and tail. The crucial morphological aspects of normal and fertile spermatozoa have led to a large debate (Menkveld et al., 2011). They have been included in the WHO recommendation in 2010. Let us have a look at them.
Definition of a normal spermatozoon according to WHO (2010)
For a spermatozoon to be considered normal, both its head and tail must be normal. All borderline forms should be considered abnormal.
- The head should be smooth, regularly contoured and generally oval in shape. There should be a well-defined acrosomal region comprising 40–70%of the head area (Menkveld et al. 2001).
- The acrosomal region should contain no large vacuoles, and not more than two small vacuoles, which should not occupy more than 20% of the sperm head. The post-acrosomal region should not contain any vacuoles.
- The midpiece should be slender, regular, and about the same length as the sperm head. The major axis of the midpiece should be aligned with the major axis of the sperm head. Residual cytoplasm is considered an anomaly only when in excess, i.e. when it exceeds one-third of the sperm head size (Mortimer and Menkveld 2001).
- The principal piece should have a uniform caliber along its length, be thinner than the midpiece, and be approximately 45 µm long (about 10 times the head length). It may be looped back on itself, provided there is no sharp angle indicative of a flagellar break.
Head sizes have been shown to depend on the stain used (Papanicolaou, Diff-Quick). Here are examples of dimensions published by various authors after Papanicolaou staining. For a more complete overview consult the review by Menkveld et al. (2011).
Head sizes Katz (1986) Soler (2003) WHO (2010) Length (µm) 3.50 - 4.00 4.28 ± 0.50 4.12 ± 0.70 Width (µm) 2.50 - 5.00 2.69 ± 0.31 2.68 ± 0.41
Parameters of the EQC test
The EQC test will present you with several possible answers for the head, midpiece, and tail. You can make only one choice. Head shape is an important parameter to monitor in an External Quality Control scheme.
Sperm head: size and shape
|Normal||the head has a regular oval shape|
|Macrocephaly||the head is clearly above the normal size|
|Microcephaly||the head is clearly smaller than the normal size|
|Elongated||the head has the form of a cigar|
|Pyriform||the head has the form of a pear|
|Amorphous||the head has a slight to strong deviation from the oval|
|Double||two heads attached to one or two midpieces and tails|
|Pin||the head is absent, but the cell is usually alive|
|Round||the head is perfectly spheric and has no acrosome|
The acrosomal region should be well defined and be comprising 40-70% of the head area.
|Normal||40-70% of the head’s area|
|Abnormal||small or large acrosome, presence of >2 vacuoles|
The midpiece should be slender (<1 µm thick) and about 1.5 times the head length. The cytoplasmic droplet should be less than half the width of the head.
|Normal||none of the defects below|
|Angulated||angulation between midpiece and head axis|
|Asymmetrical||asymmetrical insertion of the midpiece into the head|
|Thick||thick or irregular midpiece|
|Thin||abnormally thin midpiece (no mitochondrial sheath)|
|Cytoplasmic droplet||cytoplasmic remnant larger than half head’s width|
The tail should be straight, uniform, thinner than the midpiece, uncoiled and about 45 µm long. Tail defects are presented below.
|Normal||about 40-50 µm long, regular|
|Short||short or incompletely formed|
|Multiple||more than one tail|
|Bent||sharp angle, generally due to smearing artifact|
|Coiled||coiled around the head, swelling due to osmotic effect|
- Katz, D.F., Overstreet, J.W., Samuels, S.J., Niswander, P.W., Bloom, T.D., Lewis, E.L. (1986) Morphometric analysis of spermatozoa in the assessment of human male fertility. Journal of Andrology, 7, 203-212.
- Kruger, T.F., Franken, D.F. Atlas of Human Sperm Morphology Evaluation. Francis & Talor. A Parthenon Book, ISBN 1842142771
- Soler, C., De Montserrat, J.J., Gutiérrez, R., Nunez, J., Nunez, M., Sancho, M., Pérez-Sanchez, F., Cooper, T.G. (2003) Use of the Sperm-Class Analyser for objective assessment of human sperm morphology. International Journal of Andrology, 26, 262-270
- Menkveld R, Holleboom CAG, Rhemrev JPT. 2011. Measurement and significance of sperm morphology. Asian J. Androl. 13, 59-68.
- Kruger TF, Menkveld R, Stander FS, Lombard CJ, Van der Merwe JP, van Zyl JA, Smith K. 1986. Sperm morphologic features as a prognostic factor in vitro fertilization. Fertil Steril 46: 1118-1123.
- Menkveld R, Wong WY, Lombard CJ, Wetzels AM, Thomas CM, Merkus HM, Steegers-Theunissen RP. 2001. Semen parameters, including WHO and strict criteria morphology, in a fertile and subfertile population: an effort towards standardization of in-vivo thresholds. Human Reproduction 16: 1165-1171.
- Mortimer D, Menkveld R. 2001. Sperm morphology assessment–historical perspectives and current opinions. Journal of Andrology 22: 192-205.
- WHO. 1999, 2010, 2022. WHO Laboratory Manual for the Examination of Human Semen and Sperm-Cervical Mucus Interaction. Cambridge: Cambridge University Press.