Sperm morphology refers to the size and shape of individual sperm. It is one of, but by no means the only, contributing factor to male fertility.
Not all of an individual's sperm look exactly alike. Abnormalities in sperm size and shape can occur in the head, midpiece, or tail.
In some cases, these mutations or changes do not impact the sperm's overall functionality. In other situations, the sperm may not be able to move properly or quickly enough to reach, puncture, or enter the egg's membrane.
Doctors usually assess sperm morphology during a general semen analysis or fertility test. On its own, an abnormal or low sperm morphology score is typically not an indication of infertility.
Sperm morphology tests examine semen samples under a microscope and calculate the percentage of sperm with a normal form (NF) in the total sample.
Requirements for a normally formed sperm include:
- a smooth rimmed, oval shaped head
- a head that is between 2.5 to 3.5 micrometers (μm) wide and 5 to 6 μm long
- an acrosome (a membrane with enzymes capable of penetrating the egg's membrane) tip that covers between 40 and 70 percent of the sperm head
- a head free of large vacuoles (fluid filled organelles) and have no more than two small vacuoles that take up less than 20 percent of the total head
- midpiece of the sperm (the segment between the head and tail) should be about the same length as the head but much slimmer
- an uncoiled, 45 μm-long tail that should be thinner than the sperm head and midpiece
- free from head or tail defects
Lab technicians usually process sperm morphology tests by putting a small portion of the semen on a glass slide, letting it air dry, and then staining it with a dye that makes individual sperm easier to see under the microscope.
Technicians typically calculate the percentage of NF sperm out of 200 or more sperm in one segment of the sample.
Sperm morphology tests can also be assessed using image analysis technology, such as a computer-aided sperm morphometric assessment (CASA).
According to the World Health Organization, computer based tests are more precise and reliable than manual tests because of the removed risk of human error.
One round of testing is not enough to fully assess semen or sperm quality.
There is a high level of variation between different people, as well as between samples from the same individual. Factors such as human error, contamination, mislabeling, and time before processing also make multiple tests necessary.
It typically takes three or more tests that produce the same or similar results to confirm qualities such as morphology, vitality, and mobility.
Having a large number of abnormally formed sperm in a sample and a low NF score are signs of a condition called teratozoospermia.
The precise range can vary, but typically a normal or healthy sperm morphology range is between 4 and 14 percent NF. A score below 4 percent may mean it takes longer than normal to achieve pregnancy.
A result of 0 percent NF usually means in vitro fertilization (IVF) may be necessary for conception. IVF involves the collection of an egg and sperm sample to be combined in a petri dish in the hopes of fertilization.
The criteria for sperm morphology was developed less than a decade ago, so there are still a lot of discrepancies in how the test is conducted and results interpreted.
It is also important to remember that sperm that is abnormal in size or shape usually still carries healthy genetic material. Many fertile men have a high percentage of abnormal sperm.
Most of the studies that have found a link between low sperm morphology scores and reduced fertility rates used IVF subjects and settings.
A man whose NF is score below 4 percent should talk to his doctor to rule out the potential for complications and additional health conditions.
Doctors specializing in male infertility may help identify a cause for abnormal morphology rates, and in some cases, recommend a course of treatment to improve sperm quality.
Sperm morphology tests are only one component of a general semen analysis.
Different laboratories, medical practices, and doctors may include different processes as part of their sperm analysis testing.
Other factors and hormones levels typically assessed during semen analysis include:
- vitality, or the percentage of living sperm
- motility, or general movement patterns and moving ability of sperm
- the concentration of the semen
- the total fluid volume of semen
- liquefaction, or how quickly semen liquefies to facilitate sperm travel
- total sperm number or count
- semen thickness (viscosity)
- the appearance of the semen
- semen pH
- other foreign cells in semen, most often bacteria
- sex hormone-binding globulin (SHBG)
- follicle stimulating hormone (FSH)
- DNA form and function
The presence of additional body cells in semen, such as epithelial cells (cells from the male ducts), immune cells such as leukocytes (white blood cells), and macrophages (scavenger immune cells) may also be assessed.
The exact role and influence of sperm morphology on fertility is controversial. The specific requirements for what doctors consider a normal sperm form also vary.
Traditionally, if a man had low sperm morphology scores, doctors would likely use assisted reproductive technologies.
Doctors also thought that low sperm morphology scores also caused a decrease in overall semen quality and an increase in sperm damage.
Increasingly, researchers are finding that morphology may not play as much of a role in infertility as once thought.
A 2017 study found that men with 0 percent NF scores were capable of near normal fertility rates. Research has also found that IVF therapies are often ineffective in males with lower than 4 percent NF.
Recently developed DNA tests are considered by many researchers and doctors to be a better indicator of infertility and semen quality than a general semen analysis test.
Some of the newer DNA tests available to assess the quality of semen include:
- sperm chromatin structure assay (SCSA)
- comet assay
- tunnel assay
- DNA fragmentation test (Reprosource®)
If assisted technologies are used, intracytoplasmic sperm injection, ICSI, where a normal sperm is isolated from a semen sample and injected into an egg, may be a good option for men with low morphology scores.