Cells in bloodstream don't refill ovaries.

Blood doesn't carry cells that replenish a female animal's supply of eggs, a new study suggests. The finding contradicts a surprising report last year suggesting that scenario.

Scientists had long held that females are born with a supply of eggs in their ovaries that isn't replenished. However, Jonathan Tilly of Harvard Medical School in Boston and his colleagues reported in the July 29, 2005 Cell that stem cells from bone marrow continually flow to the ovaries and restock the supply of eggs.

"It was a surprise to most people," says stem cell researcher Amy Wagers of the Joslin Diabetes Center in Boston.

To test the hypothesis, Wagers and her colleagues connected the bloodstreams of pairs of mice. Each included a normal animal and an animal with tissues that were engineered to glow green.

After the rodents had spent 6 to 8 months joined, the researchers gave the mice drugs to induce ovulation. If Tilly's hypothesis were correct, the researchers surmised, some eggs produced by the normal mice should glow green and vice versa.

However, the researchers found no evidence that the normal or green-glowing mouse produced any eggs derived from its partner's cells.

If the ovaries are indeed restocked, the stem cells doing it are probably within the ovaries themselves, Wagers' team reports in an upcoming Nature.