Why does it take millions of sperm to fertilize an egg?

Dear Reader,  

Great question! Sperm are the individual reproductive cells responsible for fertilizing egg cells. These powerful swimmers are housed in semen, which is what comes out of the body during ejaculation. An individual ejaculate contains anywhere from 15 million to 200 million sperm per milliliter. However, while large numbers of sperm may begin the journey to fertilization together, it technically only takes one sperm to fertilize an egg. That said, the chances of this happening are higher if more sperm can shoot their shot!  

How does sperm reach the egg? 

The path for sperm to reach an egg isn’t easy. It must travel a long distance to reach and ultimately fertilize an egg. After ejaculation, millions of sperm are released into the vaginal canal where the journey begins.  

The prostate releases enzymes that help create a protective gel on the sperm. This helps protect it in the vagina’s naturally acidic environment. Semen initially forms a gel that helps protect sperm from the vagina’s naturally acidic environment. The most mobile sperm then pass through the cervix and surrounding cervical mucus—this acts as a barrier to the uterus. Muscle contractions push sperm through the uterus toward the fallopian tubes where it can reach an egg at random. This randomization is because only one fallopian tube has a fertile egg available at a given time. Each sperm releases enzymes that change its shape to be able to penetrate the layers surrounding an egg. If a fertile egg isn’t yet ready or a match hasn’t been made, these sperm can live in the uterus, hiding in the uterine walls for up to five days. This pool of sperm in waiting is sometimes called the sperm reservoir.  

What parts of sperm influence its ability to reach an egg? 

The shape and movement of sperm affect how it reaches an egg. Healthy, “typical” sperm have an oval shaped head and a long tail that aids in its ability to swim. Sperm with abnormal heads or tails are often left behind as they can’t move as well. In many cases, this prevents them from making it past the cervix and into the uterus.   

How does the uterine environment influence sperm reaching an egg? 

Ovulation often marks the fertile window of a person with a uterus and influences whether the sperm can successfully reach the egg. During ovulation, hormones fluctuate and encourage cervical mucus that normally protects the uterus to thin. This creates a friendlier, more supportive environment for the sperm to enter and move. Additionally, these hormones signal to the ovaries that it’s time to release an egg into the fallopian tubes. After the ovary releases the egg, it’s only able to be fertilized for up to 24 hours, so timing is pretty important! 

The egg has a strong influence on attracting sperm in its quest to become fertilized. Interestingly, eggs don’t attract all sperm equally. Chemical signals in the fluid surrounding the egg appear to attract or even repel sperm, biasing which sperm matches with it. Some eggs are more responsive to sperm from certain individuals or a specific sperm from the same individual. It’s still not completely understood what the egg’s criteria are for selecting individual sperm cells. However, evidence points to the egg as a key player in fertilization! 

So, while it technically only takes one sperm to fertilize an egg, getting there is a complex process involving millions of contenders and a whole lot of biological teamwork. From the environment of the reproductive tract to the egg’s own role in selection, many factors play into whether fertilization happens. It’s a fascinating and finely tuned system designed to give the right sperm the best shot at success!