SUNDAY, Feb. 26 (HealthDay News) -- Researchers report that
they've isolated stem cells from adult human ovaries that can
mature into eggs that may be capable of fertilization.
The lab findings, which upend longstanding scientific theory,
could potentially lead to new reproductive technologies and
possibly extend the years of a woman's fertility.
It was long believed that women were born with a lifetime supply
of eggs, which was depleted by menopause. But a growing body of
research -- including a new paper from Massachusetts General
Hospital -- suggests egg production may continue into adulthood.
The study is published in the March issue of
"Fifty years of thinking, in every aspect of experiments, of interpreting the results, and of the clinical management of ovarian function and fertility in women was dictated by one simple belief that turns out to be incorrect," said lead study author Jonathan Tilly, director of the hospital's Vincent Center for Reproductive Biology. "That belief was the egg cell pool endowed at birth is a fixed entity that cannot be renewed."
Dr. Avner Hershlag, chief of the Center for Human Reproduction
at North Shore-LIJ Health System in Manhasset, N.Y., said the study
is "exciting" but emphasized the work is still very
"This is experimental," Hershlag said. "This is a beginning of perhaps something that could bring in new opportunities, but it's going to be a long time in my estimation until clinically we'll be able to actually have human eggs created from stem cells that make babies."
The same team at Mass General caused a stir in 2004 when it
published a paper in
Nature reporting that female mice retain the ability to make
new egg cells well into adulthood.
In both mice and humans, the vast majority of egg cells die
through a process called programmed cell death, or apoptosis, the
body's way of eliminating unneeded or damaged cells. For humans,
that process is dramatic. Female fetuses have about 6 to 7 million
eggs at about 20 weeks' gestation, a little more than 1 million at
birth, and about 300,000 by puberty.
Studying mice egg cells and follicles, the tiny sacs in which
stem cells become eggs, the Mass General researchers discovered
something that didn't make mathematical sense.
Most prior research had focused on counting the healthy eggs in
the ovaries, and then made assumptions about how many had died from
that, Tilly said. But his lab looked at it the opposite way and
focused on cell death.
"We found far too many eggs were dying than could be accounted for by the net change in the healthy egg pool," Tilly said. "We reasoned that maybe the field had missed something." They wondered if stem, or precursor cells, were repopulating the ovaries with new eggs.
Initially, the findings were met with skepticism, according to
the study authors, but subsequent research bolstered the
Those included a 2009 study from a team in China, published in
Nature Cell Biology, that isolated, purified and cultured egg stem cells from adult mice, and subsequently introduced them into mice ovaries that were rendered infertile. The infertile mice eventually produced mature oocytes that were fertilized and developed into healthy baby mice.
Studies showing that women had the same capacity as mice were
In this study, Tilly's team used tissue from Japanese women in
their 20s and 30s with gender identity disorder, who had their
ovaries removed as part of gender reassignment surgery.
The researchers isolated the egg precursor cells and inserted
into them a gene from a jellyfish that glows green, then inserted
the treated cells into biopsied human ovarian tissue. They then
transplanted the human tissue into mice. The green fluorescence
allowed researchers to see that the stem cells generated new egg
Tilly said the process makes evolutionary sense. "If you look at
this from an evolutionary perspective, males have sperm stem cells
that continually make sperm. Because species propagation is so
important, we want to make sure it's the best sperm, so don't want
sperm sitting around for 60 years waiting to get used," he said. It
makes no sense from an evolutionary perspective that "females will
be born with all the eggs they will have and let them sit there,"
Hershlag, meanwhile, said much remains to be overcome.
"Ultimately, in our field only one thing counts," he said, "and that is if you can make an egg that can make a healthy baby."
U.S. National Library of Medicine has more on how
human embryos develop.
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