Attention, men: In the last second your bodies each produced at least
1,500 sperm cells. Now researchers have unlocked just how this seminal
feat is possible. For the past 40 years scientists have thought stem cells in the
testicles—also called germline stem cells—become sperm only through a
simple, two-step process.
Not so. Germline stem cells, it seems, can become sperm in several
different ways, according to new experiments with mice. (Explore an
interactive of the human body.)
"What we're saying is there isn't a strict linear progression from a
stem cell to a [sperm] cell," said study co-author Robert Braun,
associate director at the Jackson Laboratory in Bar Harbor, Maine.
"Sometimes the stem cells go through several cell divisions to get
there, sometimes they don't."
What's more, the researchers found that a cell that's partway to
becoming 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. (For more on this technique, see
"Chemistry Nobel Prize Awarded for Glowing Protein Work.")
The scientists also "labeled" specific cells within the mouse germline
stem cells a certain color and observed what happened to them over a
period of several days. (Related: "'Brainbows' Illuminate the Mind's
Wiring.")
The research also revealed that sperm develop from a smaller subset of
specialized germline stem cells in the testes than previously thought.
Sperm Live Fast, Die Young
Since sperm are short-lived, they must constantly be replenished,
Braun noted—hence that 1,500-per-second production rate.
"In addition, fertiization is surprisingly inefficent," he said.
"There has to be a large initial payload [for those] few cells to make
it to the final destination"—the woman's egg.
But continuously pumping out a stream of sperm cells from puberty to
old age requires a man's body to maintain a very delicate germ cell
balance.
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.
Sperm Research May Lead to Male Birth Control
Unlocking such mysteries of sperm development could someday lead to
infertility treatments or even the elusive male birth control pill,
said Braun, whose research appears tomorrow in the journal Science.
(Related: "New Sex Hormone Found—May Lead to Male Birth Control?")
For instance, scientists may learn how to keep germline stem cells
from becoming sperm.
"The more we learn about the normal behavior of cells," Braun said,
"the more we know how to manipulate them."