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Stem cells toward sperm cells and back again

n the testis because we can detect proteins that distinguish them, and we know exactly where the stem cells are supposed to be," says Matunis. "In fruit flies, we also know the signal that keeps the stem cells in their primitive state, and we can turn it off and on. We don't have any of this information for people, or even for mice."

In the fruit flies, sperm are made by a system that consists of a raspberry-like cluster of sperm-making stem cells centered around a "hub."

The hub emits a chemical signal that tells only the closest cells to maintain their stem cell status. When one of these stem cells divides in two, the "daughter" cell nearest the hub remains a stem cell while the one slightly farther away gets one step closer to being sperm. These more-distant daughter cells, called gonialblasts, then become cells called spermatogonia, each of which divides a number of times before becoming spermatocytes, precursors to sperm.

To see whether lost stem cells could come back, Brawley put the temperature-sensitive fruit flies in warmer climes of 29 degrees Celsius (a balmy 84 degrees Fahrenheit) for two days, and discovered that only 22 percent of testes still contained sperm-making stem cells. After four days, almost no stem cells remained at all.

Brawley then used a special microscope to examine whether stem cells returned in flies allowed to "recover" at the cooler temperature of 18 degrees Celsius (about 64 degrees Fahrenheit) for two days after spending two days at the signal-squelching warmer temperature.

"We were very happy to see that stem cells did return," says Matunis, who discovered the hubs' signal in 2001 with Natasha Tulina.

But much to their surprise, while only 22 percent of testes had remaining stem cells going into the recovery period, 76 percent of them had stem cells after two days at 18 degrees.

"We had expected that remaining stem cells replenished the supply, but even in testes with none le
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Contact: Joanna Downer
jdowner1@jhmi.edu
410-614-5105
Johns Hopkins Medical Institutions
18-May-2004


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