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Nearly everyone who makes sperm does so constantly, from puberty until death, at a rate of millions of sperm per day, and people in their 70s, 80s, or even 90s have been known to impregnate their partners, which might make it seem like a person's ability to get someone pregnant is the same whether they're 20 or 85, but medical professionals have come to realize that that's simply not true, even if all of the parts involved are still working well in an older person, it turns out time does take a toll on their sperm.

A person's age matters because the stem cells that make sperm are constantly making copies of themselves, and therefore, their genomes, but no biological process is perfect, so when these stem cells make copies of their DNA, mistakes can occur and with all of the cell divisions needed to churn out hundreds of billions of sperm, mutations accumulate over a person's lifetime.  Plus, these copies go on to become sperm through a multi-step process called meiosis, where other errors, like improper chromosome sorting, can occur.

One of the most obvious effects of these mutations and errors is how sperm look.  Sperm-makers of any age end up with a few sperm with misformed parts, but these abnormal sperm become more common the older they are.  You might see more sperm with mis-shapen heads or tails as a person ages, morphologies which can impair the sperm's ability to fertilize an egg, and older people also tend to have more sperm with large nuclear vacuules.  These are mostly empty pockets inside the sperm which take up a substantial portion of the sperm's head, and they seem to indicate that DNA damage is severe enough to likely impair the development of an embryo, if that sperm does successfully fertilize an egg.

Sperm from older people is also less motile, meaning they struggle to swim around and ultimately to fertilize eggs.  That's likely because of age-related changes in the epididymis, an organ that contains the narrow, tightly coiled, meters long tube where sperm finish maturing.  Some evidence suggests that the older a person is, the harsher the environment inside that tube is, and that can damage the sperm's DNA and ultimately hamper their ability to produce the energy they need to move, and even if sperm look normal and can find and fertilize eggs, they're not out of the woods.  

All of these aging related effects on sperm DNA can contribute to increased risks of infertility, difficult pregnancies, and health conditions in the child like rare birth defects, childhood leukemia, schizophrenia, and autism.  Damage to sperm DNA can also increase the risk of miscarriage in the first half of pregnancy, because most miscarriages during that time occur because the fetus has abnormal genes or chromosomes, though it's important to note the age associated increase in risk for miscarriages, pregnancy complications, and childhood health conditions are small and overall the risks are low, and this is a relatively new field of study, so there's a lot left to learn about the effects of age on sperm, but from what we know so far, it's clear that sperm do change as a person gets older.  I guess time takes its toll on everything.

Thanks for watching this episode of SciShow.  If you liked this episode, you might like the one that we have on why sperm count is dropping, so you can check that out next and just so you know, we put out a new episode every single day, so make sure you see them all by clicking that subscribe button and ringing the notification bell.

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