By Janelle Weaver
Researchers in Japan have made fertile mammalian sperm in a culture dish, a feat long thought to be impossible. The technique, reported March 23 in Nature, could help to reveal the molecular steps involved in sperm formation and might even lead to treatments for male infertility.
Biologists have been trying to make sperm outside the body for almost a century. Failure has often struck at the stage of meiosis, a type of cell division during which paired chromosomes swap DNA and the number of chromosomes per cell is halved. The result of this process is sperm cells ready to fuse with an egg.
Takehiko Ogawa and colleagues at Yokohama City University discovered that the key to getting sperm through meiosis lay in a simple change to standard culture conditions.
"The report is quite exciting because it represents the fulfilment of a goal held by many reproductive biologists over many years," says Mary Ann Handel, an expert in reproductive genetics at the Jackson Laboratory in Bar Harbor, Maine.
By trial and error, Ogawa's team worked out which culture methods allowed sperm in tissue fragments from neonatal mouse testes to mature. To track sperm development, they used a fluorescent protein that marked cells undergoing--or that had undergone--meiosis.
Initially, the researchers placed the fragments on a gel and soaked them in fetal bovine serum, a typical ingredient of cell cultures. But nothing they added to this mix worked, not even factors known to stimulate sperm maturation.
The authors' success came when they replaced the fetal bovine serum with a serum-free medium, KnockOut Serum Replacement, which is often used to grow embryonic stem cells.
After several weeks of bathing in this mixture, almost all tissue samples contained some cells with the same number of chromosomes found in sperm. Nearly half of the samples contained cells with flagella, tail-like projections that sperm use to swim. Sperm formation peaked after about a month, although it lasted for more than two months.
The researchers injected the sperm into egg cells. A few weeks later, surrogates delivered a dozen live, fertile offspring. The team also grew sperm from neonatal testis tissue that had been frozen for days or weeks.
Matter of time
Ali Honaramooz, a reproductive biologist at the University of Saskatchewan in Saskatoon, Canada, says that the technique could aid prepubescent boys about to undergo cancer therapies that destroy fertility. It could also protect the reproductive potential of endangered animals that might die before reaching sexual maturity, he adds.
The procedure will also be useful for studying the molecular events that underlie sperm production, says Martin Dym, a cell biologist at Georgetown University in Washington, D.C. But before the technique can be used in treatments for male infertility, researchers will have to generate millions of sperm cells and translate the work to humans, Dym adds.
Honaramooz says that is just a matter of time. "If the same methodology can be applied, with many minor changes, to other species, that's great," He says. "If not, then it would take almost the same amount of work, but at least now you know that eventually it's going to work."