Whenever a man ejaculates millions of sperm are produced, yet the average pair of testicles produces billions of sperm in a lifetime. So, why aren’t all of these possible sperm cells created at once?

Testicles have a built in safety device that stops their owners from becoming infertile

Sperm cells come from stem cells deep inside the testicles. When these stem cells divide, they can become another pair of stem cells that stay inside the testicles, or they can start to become sperm cells. In order for men to remain fertile, their testicles need to keep these two options well balanced. If too many cells change, the reserve of stem cells could run out so that no new sperm can be made. If too few cells change, the sperm count would drop because the testicles are filled with stem cells. Either scenario would result in infertility.

One “switch” keeps the testicles from making all the sperm cells at once

A new study by Kanako Ikami et al at the National institute for Basic Biology in Japan states that a single difference inside the cells determines whether or not the stem cells go on to become sperm cells. Ikami conducted a study with mice, and she found that genes the cells were using consist of two specific groups. One group makes a protein that can dock with retinoic acid, the other group cannot not.

When it is time to make new sperm, a pulse of retinoid acid is released into the testes. All cells receive this pulse, but not all cells transform to sperm cells. Ikami found that the cells that could bind with the acid became sperm cells, the cells that remained blind to the pulse continued making stem cells. When Ikami forced the “blind” cells to make the protein necessary to bind with the acid, and then introduced them to the retinoic acid, they started to become sperm cells.

All of the cells inside the testicles are capable of making sperm. but one “switch” prevents them from doing it all in one go.