Growth factors make an egg grow up.

Reproductive biologists have closed a gap in their understanding of how the female reproductive system works, an advance that could improve in vitro fertilization procedures.

In the middle of a female mammal's reproductive cycle, a surge of luteinizing hormone (LH) floods an ovarian follicle, triggering changes that lead to the maturation and release of an egg. The mystery has been that the immature egg and its surrounding cells don't respond directly to the hormone. These cells don't sport the cell-surface protein that binds LH.

Other cells in an ovarian follicle can respond to LH, however. And now, a research team headed by Marco Conti of Stanford University School of Medicine reports that those cells, when triggered by LH, briefly produce a set of growth factors that directly spurs the maturation of an egg.

"There had to be some intermediary molecules doing the communicating of the LH signal.... This is a very exciting and fulfilling discovery," says reproductive biologist John Eppig of Jackson Laboratory in Bar Harbor, Maine.

Conti and his colleagues, who describe their work in an upcoming Science, suggest that doctors in fertility clinics could use the growth factors to trigger human eggs in the laboratory to mature. Defects in the signaling by these same factors might also underlie some cases of female infertility, the scientists note.--J.T.