Sperm Morphology -The importance of being in shape.
The shape (morphology) of a sperm is an important determinant of its fertilizability. The egg is enclosed in a protein coat called the zona pellucida (ZP). The ZP performs many functions, but its first task is to select which sperm will fertilize the egg. The ZP bases its selection on the shape of the sperm head. In order to pass through the ZP, the sperm must be vigorously motile and the sperm head must be a symmetrical, oval shape of the appropriate size. Sperm possessing heads that are irregular in shape, too round, too long, too big or too small are prevented from passing through the ZP. It turns out theres a pretty good reason for this.
Abnormally shaped sperm often contain abnormal DNA.
That is, an abnormally shaped sperm may be short a chromosome or have an extra chromosome or the DNA is packed into the sperm head in such a way so that, if that abnormally shaped sperm were to enter the egg, the DNA would get all tangled up when it tries to form chromosomes inside the egg. Therefore, nature has evolved a way to keep abnormally shaped sperm from getting inside the egg and thereby maximizing the genetic development potential of the embryo.
A sperm morphology score is usually determined during the course of a semen analysis.
A microscope slide is prepared with a very thin coating of semen. The slide is stained to make the sperm clearly visible and several hundred sperm are then viewed under high magnification and individually scored normal or abnormal based on their shape. The percentage of normally shaped sperm is then calculated.
There are two criteria currently in use to evaluate the normalcy of the sperm morphology.
The World Health Organization developed the original criteria. Using these criteria, sperm with borderline morphology (slightly abnormal) were considered good enough and scored as normal. Unfortunately, sperm morphology scores using WHO criteria were not predictive of the fertilizability of the sperm.
In 1987, new stricter criteria (Kruger Strict) were introduced. These new criteria classified borderline sperm as abnormal. The sperm morphology scores using the strict criteria classification system were highly predictive of the fertilizability of the sperm. Based on results from in vitro fertilization studies where a fixed number of sperm were used to inseminate each egg, when the percentage of normally shaped sperm dropped below 14%, fertilization began to decline.
When the percentage of normally shaped sperm dropped below 4%, fertilization was very poor or non-existent. Needless to say, poor sperm morphology may have a significant impact on the outcome of therapeutic procedures such as intrauterine insemination (IUI) and in vitro fertilization (IVF). However, recent studies suggest that below normal sperm morphology appears to have less of an impact on the success of IUI. This is probably because of the number of sperm used to inseminate. The normal morphology score is, after all, a percentage of the total number of sperm. Because it is common to places tens of millions of sperm in the uterus when performing an IUI, theres probably enough normally shaped sperm to get the job done.
IVF and Sperm Morphology
To compensate for when the normal morphology is between 4% and 14%, more sperm can be placed with each egg. This usually overcomes the problem, unless there is a concomitant problem with the sperm motility. However, when normal morphology falls below 4%, the problem can no longer be overcome by simply adding more sperm. In these cases, intracytoplasmic sperm injection (ICSI) is usually performed to ensure adequate fertilization. However, this is not a panacea since fewer embryos continue development to the blastocyst stage following ICSI with sperm from individuals with poor sperm morphology (even when great care is taken to inject only normally shaped sperm).
When I report semen analysis results to patients that have below normal sperm morphology scores, invariably I’m asked, What can I do about it?
There are three main causes of poor sperm morphology:
- a genetic trait
- exposure to toxic chemicals
- increased testicular temperature
Nothing can be done if it is a genetic trait, however, the other two causes may be reversible. People can be exposed to toxic chemicals in the workplace or at home. For example, people working in automotive paint shops or as janitors are often exposed to a myriad of chemicals known to be detrimental to fertility. Individuals with occupational exposure to toxic chemicals need to be mindful of the impact of these chemicals on their fertility and follow all safety guidelines. Many household items, if used without proper ventilation, can also cause problems with sperm morphology.
Increased testicular temperature is another common cause of poor sperm morphology.
Seemingly benign occupations that require individuals to spend the majority of their day sitting at a desk can also be problematic. When sitting for a prolonged time, the testicles are drawn up close to the body resulting in an increase in testicular temperature. Those individuals may be advised to get up and walk around periodically to return the testiclular temperature to normal. Sleeping in tight fitting clothing (like jockey shorts) can increase scrotal temperature to a point where sperm morphology is affected. A varicose vein in the scrotum (varicocele) will increase scrotal temperature. The urologist can usually repair a varicocele surgically. However, improvement in sperm morphology is seen in only about half the cases and it may take up to 18 months to see an improvement. As a final note, sperm morphology scores can change a few percent from month to month and vary considerably amongst labs evaluating the same semen specimen. A single abnormal sperm morphology score is not conclusive. This is true for all seminal parameters. When the results of semen analysis are abnormal, the test should be repeated in 1-2 months to confirm the abnormality.