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Bone marrow may be source of new egg-cell
generation in adult mammals
Follow-up to landmark 2004 paper
expands new understanding of female reproductive biology
BOSTON - July 27, 2005 - Last year a group of Massachusetts
General Hospital (MGH) researchers announced surprising findings
that female mice - contrary to longstanding theories of mammalian
reproductive physiology - retained the ability to make new egg cells
or oocytes into adulthood. Now the same investigators report new
data supporting the earlier research and identifying a potential
source for the production of these cells - stem cells in the bone
marrow. Their article appears in the July 29 issue of Cell.
"We may be ushering in a new era in the clinical management
of female infertility and menopause," says Jonathan Tilly,
PhD, director of the Vincent
Center for Reproductive Biology at MGH and leader of the research
team. "This could lead to new treatment approaches based not
on drugs but on regenerative medicine through adult stem cells."
The
group's 2004 report in Nature contradicted what had been
regarded as a dogma of mammalian biology: that females are born
with a limited, non-renewable supply of oocytes that are depleted
throughout life. Instead the MGH team found evidence that adult
female mice are constantly turning over their oocyte supply and
producing new oocytes and follicles, the tiny sacs in which eggs
grow. The current study was designed to reinforce the earlier findings
and also to identify the source of the new oocytes.
In their first experiment, the team injected normal adult female
mice with the chemotherapy drug doxorubicin, which is known to be
less likely to cause infertility than other anti-cancer drugs. In
the first 24 hours after the injections, the mice rapidly lost nearly
80 percent of their oocyte-containing follicles. But their follicles
regenerated rapidly, with hundreds appearing over the next 12 to
24 hours. Two months later, ovaries from the treated mice looked
identical to those from untreated controls, suggesting that while
existing oocytes and follicles were killed by doxorubicin, the oocyte
supply was soon replenished by some source not damaged by the drug.
The 2004 study had identified cells on the surface of mouse ovaries
that resembled immature germ cells, which are the source of the
oocytes that develop in embryonic animals. But further investigation
showed that those surface cells began to disappear in adult mice.
In addition, a molecule known to be a marker of embryonic germ cells
was found only in the core of the adult ovary, which is well supplied
with blood vessels but contains neither oocytes nor follicles.
The presence of an embryonic germ cell marker in a blood-vessel-rich
part of the ovary directed the investigators' attention to the bone
marrow, where blood cells and several forms of stem cells are produced.
They then tested marrow from adult females for the presence of several
genes believed to be expressed only in germ cells.
"We found that every germ cell marker we could think of was
expressed in the bone marrow of adult female mice," Tilly says.
"Everyone had missed finding female germline stem cells because
they are not in the ovaries, where everyone would have looked for
them." The same markers were found in bone marrow and blood
samples from reproductive age human females that the research team
also examined.
To verify the presence of functioning germline stem cells in bone
marrow, the investigators ran a series of experiments in two mouse
models: genetically normal females who received extensive chemotherapy
with drugs known to permanently destroy the ovaries and genetically
sterile females that lack a gene essential for the development of
mature oocytes. Some of both groups of mice received transplants
of bone marrow from untreated, genetically normal females.
Two months after the chemotherapy-treated mice received donor marrow,
their ovaries looked identical to those of untreated mice, with
numerous oocytes and follicles. Ovaries of treated mice who had
not received bone marrow were totally lacking in oocytes. Similarly,
the genetically sterile females that received bone marrow from normal
mice also began to produce normal oocyte-containing follicles. In
both instances, restoration of oocyte production by bone marrow
transplantation persisted for the normal reproductive lifespan of
female mice.
If germline stem cells in the bone marrow were the source of new
oocytes, some kind of intermediate cells must travel through the
bloodstream to the ovaries. To search for these potential germline
progenitor cells, the investigators used transgenic mice in which
a green fluorescent protein (GFP) marker is expressed only by germline
cells. Blood cells from these normally fertile transgenic mice were
transfused into both the non-transgenic adult female mice that had
the ovary-destroying chemotherapy regimen and into the genetically
infertile females. In both models, follicles containing GFP-labeled
oocytes - indicating that they were derived from the donor blood
- appeared in the ovaries within two days of the transfusions.
Tilly explains that it now looks like the ovary is part of a three-tiered
system in which germline stem cells in the bone marrow manufacture
progenitor germ cells, which travel thorough the bloodstream to
the ovary, where they mature into oocyte-containing follicles. Measurements
showing that the expression of a germ-cell marker gene in bone marrow
fluctuates according to the female's reproductive cycle suggest
that the ovaries send a biochemical signal back to the bone marrow
to regulate activity of the oocyte-producing stem cells, a possibility
that is supported by the finding that removing the ovaries causes
the gene's expression in marrow to cease altogether.
The possible existence of this sort of system is further reinforced
by the fact that restoration of the oocyte supply by bone marrow
transplantation, which would require the donor marrow to find its
way to the recipient's marrow and begin growing before it could
produce oocyte progenitors, took about two months. In contrast,
the blood transfusions, which would supply more mature progenitors
directly to the ovaries, began to produce new oocytes within two
days.
"These results not only confirm last year's findings that the
old dogma is wrong, they also show we need to think more broadly
about female reproduction - that oocyte production involves more
than just the ovaries," Tilly says. He adds that this new knowledge
may help explain numerous reports in the medical literature of prematurely
menopausal women who unexpectedly regained ovarian function - some
even conceiving - after bone marrow or blood cell transplants involving
pre-treatment with what were expected to be sterilizing doses of
chemotherapy or radiation.
The mouse studies in the current Cell paper did not examine
whether the oocytes that appeared after transplants of blood or
marrow could produce offspring, something the team is currently
investigating. The researchers also hope to identify the molecular
signal the ovary sends back to the bone marrow, which this report
showed was neither estrogen nor progesterone, and to examine potential
applications of their discovery for both fertility treatment and
stem cell research.
Additional authors of the study - all from the MGH - are first authors
Joshua Johnson, PhD, Malgorzata Skaznik-Wikiel, MD, Ho-Joon Lee,
PhD, Yuichi Niikura, PhD, Katherine Tschudy, PhD, and Jacqueline
Canning Tilly of the Vincent Center for Reproductive Biology; Jessamyn
Bagley, PhD, and John Iacomini, PhD, Transplantation Biology Research
Center; Gregor Adams, PhD, Randolf Forkert, PhD, and David Scadden,
MD, Center for Regenerative Medicine and Technology; Maria Cortes,
PhD, Molecular Neurogenetics Unit; and Thomas Spitzer, MD, MGH Cancer
Center. The study was supported by grants from the National Institute
on Aging, National Institute of Environmental Health Sciences, The
Rubin Shulsky Philanthropic Fund, The Sea Breeze Foundation, and
Vincent Memorial Research Funds.
Massachusetts General Hospital, established in 1811, is the original
and largest teaching hospital of Harvard Medical School. The MGH
conducts the largest hospital-based research program in the United
States, with an annual research budget of more than $450 million
and major research centers in AIDS, cardiovascular research, cancer,
cutaneous biology, medical imaging, neurodegenerative disorders,
transplantation biology and photomedicine. In 1994, MGH and Brigham
and Women's Hospital joined to form Partners HealthCare System,
an integrated health care delivery system comprising the two academic
medical centers, specialty and community hospitals, a network of
physician groups, and nonacute and home health services.
Media Contact: Sue
McGreevey, MGH Public Affairs
Physician Referral Service: 1-800-388-4644
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