Morphology is tricky. The average pair of male gonads is capable of producing millions of sperm every day and while they may be prolific, they are woefully underwhelming when it comes to quality. According to the World Health Organization, the median percentage of normal sperm morphology in fertile men is 15%. Fortunately, the low percent is balanced out by sheer numbers and in the end, you only need one. Ok, enough of that overused movie reference. Sperm morphology distills down to 2 things. Does the sperm in question look like a sperm that can make it to the egg and it if doesn’t what part is different?
What is normal sperm morphology? Well, a sperm’s single overriding goal is to find the egg and fertilize it, so researchers defined “normal” by collecting sperm that either made it into the female reproductive tract or made it all the way to the egg and embedded in the zona pellucida (the coating around the egg). From there they looked at the cells under a microscope, compared them against contenders that didn’t make the cut and made a list of characteristics a sperm had to meet in order to be considered normal.
Using WHO classification the lower cutoff is 4% normal morphology which is taken from the 5th percentile of fertile men. As mentioned above, the median fertile man only has 15% normal morphology. The upper reference value is 40% normal which is the 95th percentile. What you should take away from this is that what may seem like a low percent isn’t that bad. More is better of course, but even with only 4%, men are still able to father children.
Parts of the Sperm
A sperm has 5 main parts which are grouped into the head and tail regions. The head is composed of the head itself and the neck. There is also a cap on the head which is called the acrosome and allows the sperm to enter the egg during fertilization. The tail attaches to the neck and is composed of the midpiece, principal piece and endpiece. The midpiece is the part that looks like a collar underneath the neck and contains mitochondria which power the sperm, the principal piece is the main part of the tail and the endpiece is the end of the tail. All parts must be present, no parts may be duplicated and all parts must have normal morphology for the entirety of the sperm to be considered normal.
You wouldn’t think sperm characterization would be so detailed but we want to be specific here to address specific questions. A normal head is oval in shape, smooth, and regular, about 5um long and about 3um wide. The acrosome should cover 40-70% of the head. There should be no large vacuoles and no more than 2 small ones. A vacuole is a membrane bound compartment that contains water and looks like a bubble inside the sperm. The maximum amount of space occupied by vacuoles cannot exceed 20% and there should also be no vacuoles post acrosome. The midpiece should have a regular shape, meaning it doesn’t get wider or narrower along its length and be the same length as the sperm head. It should also be aligned with the sperm head, not sticking out sideways at an angle or off center. The principal piece should also have a regular width like the midpiece but is thinner and longer, about 10 times the length of the head. There should also be no sharp angles in the tail which indicates a break. If a sperm meets all of these criteria, then, and only then, can it be considered normal.
In short, any sperm that doesn’t meet all the criteria of a normal sperm, even if it’s borderline, is considered abnormal. Previous classifications were more relaxed, but the current, strict method correlates better with fertility outcomes, specifically IVF. On a semen analysis report you will often see abnormalities classified under the major regions of the sperm: the head, neck, or tail. Because a sperm can have multiple defects the percentages won’t necessarily add up. You might have 50% abnormal heads, 50% abnormal tails and 75% abnormal overall due to overlap.
The analysis is done by hand using a microscope or with the help of a computer. Sperm cells are collected, stained, fixed, and samples are placed on 2 microscope slides. The stain helps technicians differentiate parts of the sperm, specifically the acrosome, and fixing the cells stops them from swimming. 100-200 cells are examined per slide and from there the percentages are calculated. As a note, morphology analysis is vulnerable to operator interpretation.
How does this affect me and what can I do?
What this means for you is a couple of things. Firstly, remember that the median value for normal morphology is only 15 percent. Half of all fathers fall below this value so don’t worry too much if you’re a little low. Secondly, there are things you can do to improve your semen parameters. Sperm cells are constantly generated and it is well known that heat and toxins negatively affect spermatogenesis. Identifying sources of toxins such as cigarettes or excessive heat like saunas and eliminating them from your lifestyle now will improve the quality of sperm produced in the future. However, not all conditions can be cured. Genetic conditions are the underlying cause of a number of abnormal morphologies and try as you might, you cannot change your genes. There are treatment options however such as IVF or ICSI. And finally, remember that morphology is only one aspect of a semen analysis and it’s hard to measure consistently. What one technician or site might consider low, another might consider average. By itself, morphology is not the deciding factor in conception. There is motility to consider, concentration, total count, timing. All of these factors are parts of the bigger picture.