Researchers have now found how males manage to accomplish the seminal feat of how they produce 1,500 sperm in a single second.
For a long time, it is believed that stem cells in the testicles-also called germline stem cells--become sperm only through a simple, two-step process.
However, the researchers found that germline stem cells apparently can become sperm in several different ways, according to new experiments with mice.
"What we're saying is there isn't a strict linear progression from a stem cell to a [sperm] cell. Sometimes the stem cells go through several cell divisions to get there, sometimes they don't," National Geographic News quoted study co-author Robert Braun, associate director at the Jackson Laboratory in Bar Harbor, Maine, as saying.
The scientists also found that a cell on the way to become a sperm cell can revert back to being a germline stem cell-previously thought impossible.
For the new study, scientists genetically engineered mice so that their germline stem cells appeared fluorescent, allowing the team to watch the cells' development.
The researchers also "labeled" specific cells within the mouse germline stem cells a certain colour and observed what happened to them over a period of several days.
The research also revealed that sperm develop from a smaller subset of specialized germline stem cells in the testes than previously thought.
Braun noted that since sperm are short-lived, they must constantly be replenished-which explains the 1,500-per-second production rate.
"In addition, fertilization is surprisingly inefficient. There has to be a large initial payload [for those] few cells to make it to the final destination"-the woman's egg, he said.
However, men need to have a very delicate germ cell balance to be able to continuously pumping out a stream of sperm cells from puberty to old age.
For instance, if germline stem cells stay stem cells for a long time and don't change into sperm cells, a man may be at risk of getting testicular cancer.
But if germline stem cells too often develop into sperm, a man may become infertile.
The study appears in the journal Science.