The Genetics of Headless Sperm
You know to not judge a book by its cover. The best read might be wrapped in the crappiest jacket. And so it goes with sperm. Abnormalities of sperm shape don’t necessarily make them “bad” sperm. Hey, the ugly duckling could easily be the beautiful swan.
It’s in the Details
The study of sperm shape, or morphology is a half a century old. From this body of work, here are the truths that we have come to know and love:
- Among fertile men, 4% of sperm look “normal” to the trained eye. That means that the other 96% of sperm look terrible. In dolphins and walruses and other wild species, 99% of sperm look normal. Go figure.
- Sperm shape is largely determined as it is made within the factory, also called the testicle.
- The most common abnormalities in sperm shape involve the head, followed by the midpiece, followed by the tail.
- For the most part, there’s no relationship between the shape of a sperm and its genetic payload inside.
- The most common abnormal morphologic abnormality is the “stress pattern” in which sperm heads are amorphous or tapering along with immature sperm forms. This pattern is a minor deviation from normal and typically due to heat stress (varicocele, hot baths, flu, fevers, illness).
- If < 4% of sperm look “normal”, then the best data suggests that this sperm at the time of in vitro fertilization (IVF) might fail to fertilize eggs.
- However, it is not clear that men with <4% normal sperm will be naturally infertile or fail to conceive with low-tech treatments such as intrauterine insemination (IUI).
Genetics, Genetics, Genetics
So, sperm shape is, well…a complicated matter. Not many sperm look “normal” to begin with and sperm shape does not correlate well with performance. And, of course, as with anything in biology, there are exceptions. And here’s one:
When all the sperm in a semen sample are abnormal and they all look similar (i.e. are “syndromic”), then there is likely a real and significant effect on male fertility.
That’s right, when 100% of sperm look abnormal and they all exhibit the same abnormality, then flags go up. Globoozospermia or “lollipop sperm” is a well-described example. Sperm sheath dysplasia is another. Not only that, “syndromically” affected sperm are much more likely to have genetic origins than be caused by environmental influences.
Indeed, this idea gained new truth recently when Dr. Yan in Nevada discovered a gene that causes another kind of sperm shape disorder: headless sperm. Not kidding, it does exist. Straight out of Sleepy Hollow. And the gene, Spata6, appears to be the master controller of sperm neck assembly. Shows us as clear as day that genetics plays a very important role in male fertility. Dr. Oz may be right on target when it comes to sperm: “Your genetics load the gun. Your lifestyle pulls the trigger.”