Stem cells taken from human ovaries were used to produce early-stage eggs by scientists in Boston who may have created a new method to help infertile women.
Women have a fixed number of eggs from birth that are depleted by the time of menopause. The finding, published today in the journal Nature Medicine, challenges the belief that their ovaries cannot make more. The research was led by Jonathan Tilly, the director of Massachusetts General Hospital’s Vincent Center for Reproductive Biology.
Tilly reported in 2004 that ovarian stem cells in mice create new eggs, or oocytes, in a way similar to how stem cells in male testes produce sperm throughout a man’s life. His latest work, if reproduced, would suggest the same is true for human ovaries, potentially pointing at new ways to aid fertility by delaying when the ovaries stop functioning.
“The 50-year-old belief in our field wasn’t actually based on data proving it was impossible, or not ongoing,” Tilly said in a telephone interview. “It was simply an assumption made because there was no evidence indicating otherwise. We have human cells that can produce new oocytes.”
In the study, healthy ovaries were obtained from consenting patients undergoing sex reassignment surgery. The researchers were able to identify ovarian stem cells because they express a rare protein that is only seen in reproductive cells.
The stem cells from the ovaries were injected into human ovarian tissue that was then grafted under the skin of mice, which provided the blood supply that enabled growth. Within two weeks, early stage human follicles with oocytes had formed.
A woman is most endowed with oocytes, or eggs, as a fetus, when she has about 7 million. That number drops to 1 million by birth, and about 300,000 by puberty. By menopause, the number is zero. Since the 1950s, scientists thought that ovarian stem cells capable of producing new eggs are only active during fetal development.
“This paper essentially opens the door to the ability to control oocyte development in human ovaries,” Tilly said.
About 10 percent of women of child-bearing age in the US, or 6.1 million, have difficulty getting pregnant or staying pregnant, according to the Centers for Disease Control and Prevention. Most cases of female infertility are caused by problems with ovulation, hormone imbalance or age.
The study by Tilley and his colleagues offers “a new model system for understanding the human egg cell,” said David Albertini, director of the Center for Reproductive Services and professor in the department of molecular and integrative physiology at Kansas University, in a telephone interview.
Still, “there’s a long way to go before this has real practical applications. I’ve spent 35 years of my life studying egg cells and this is a cell that is at least as complicated as a neuron in the brain, if not more,” Albertini said.