Women carry ‘egg-making’ stem cells, scientists discover. This discovery of stem cells in human ovaries overturns our knowledge of female reproduction and means that it may someday be possible to give every woman life-long fertility.
An increasing number of women today are putting off childbearing, opting instead to first broaden their horizons, build their careers and gain financial stability.
But for the past 60 years, a woman who made this choice was up against a biological clock that worked against her: Her fertility peaked at 18 or 19 years old, reached optimum in her 20s, then as she approached her 30s, her hormone levels and fertility would start a slow decline, which speeded up after age 35.
Women, as well as all female mammals, are also born with a fixed supply of egg cells that runs out in middle age — that’s at least what scientists and doctors believed, until now.
Stem cell therapy for female infertility: But recent findings from Harvard scientists are challenging this dogma. Their recent discovery of stem cells in human ovaries could someday mean that infertile women could produce new eggs.
It’s been a long-standing scientific belief that females are born with all the oocytes — immature eggs or ‘ova’ — that they will ever have and that after birth no more oocytes are created.
But in recent years, a number of studies in lab mice have disputed this idea. The studies showed that types of cells called “female germline” or “oogonial stem cells” (OSCs) can produce further oocytes in living mice.
And now scientists from the Massachusetts General Hospital and Harvard Medical School in the United States, together with colleagues from the Saitama Medical University in Japan say they’ve discovered that the ovaries of young women hold these same stem cells that can transform into new eggs.
Unlike other cells, stem cells don’t divide to create two identical daughter cells. Their hallmark feature is the ability to continue generating new stem cells. And since 1981, when scientists first grew stem cells from mice embryos, a lot of research has gone into these cells because their special ability to produce new cells give hope that they could be used to grow new tissues and organs to repair or replace those diseased or damaged by sicknesses and accidents.
Female human germline cells or OSCs are specialized cells that can develop into ‘oocytes,’ which can, in turn, develop into mature eggs or ‘ova’.
While previous research — one done by the same lead researcher of this new research — has found that OSCs exist in mice, it’s in this study that scientists discover their existence in human females.
Finding OSCs, growing ova
The Harvard researchers first isolated OSCs from mouse and human ovaries and grew these in the lab. They then transplanted mouse OSCs into the ovaries of mice and prodded these to form oocytes or immature egg cells. Then, they tested these oocytes and found that they could mature into ova or eggs. Further tests also showed that mouse ova could be fertilized to form mouse embryos.
Next, human egg stem cells or OSCs were transplanted into mice to provide them with the blood supply to develop into oocytes. The oocytes matured into ova, but the scientists did not go on to check whether these human oocytes were functional — fertilizing these human ova would have been both unethical and illegal.
The results of this study that overturns parts of our current scientific dogma on female fertility, were reported in the Feb. 26 issue of the journal Nature Medicine.
“Our current views of ovarian aging are incomplete. There’s much more to the story than simply the trickling away of a fixed pool of eggs,” says lead researcher Dr. Jonathan Tilly of Harvard’s Massachusetts General Hospital, who has long searched for these “egg-producing stem cell” in a series of controversial studies.
Dr. Tilly, who is Mass General Hospital’s reproductive biology director, first reported that OSCs were found in the ovaries of adult mice. Recently, a lab in China and another in the US have also reported finding the rare cells in mice.
There’s “clear evidence for the existence of these cells in women,” Dr. Tilly and his team say, adding that this “may offer new opportunities to expand on and enhance current fertility-preservation strategies.”
The researchers also note that they have developed a process or protocol for isolating these cells. More, because the human ovarian tissue that they used for their study was cryopreserved, this will allow the team to produce more functional OSCs and study oogenesis or the egg-forming process in more detail.
If Dr. Tilly’s findings are confirmed, harnessing those egg-producing stem cells might one day lead to better treatments for women who become infertile because of disease — or simply because they’re getting older.
Finding healthy ovarian tissues
In his previous research, Dr. Tilly had isolated a similar population of egg stem cells from mouse ovaries. But critics had questioned if the cells were truly stem cells — or precursors of eggs cells and not very early, immature egg cells themselves.
Dr. Tilly’s group and another team of scientists have since independently verified the results. They showed that the cells found in the ovaries of adult mice were stem cells that were capable of developing further into viable oocytes. These oocytes matured into ova that could be fertilized to produce healthy mouse pups.
Dr. Tilly then wanted to find out if a similar egg stem cell existed in humans.
But obtaining healthy ovarian tissue was difficult. Hoping to spare their tissue from chemotherapy and radiation treatment, cancer patients put some of these aside — and these are what constitute most ovarian samples now available for research.
Meanwhile, a former research fellow from Dr. Tilly’s lab was working at Japan’s Saitama Medical University to perfect a technique to remove and freezing intact ovaries from patients choosing to undergo sex change operations. The Saitama scientists had already frozen ovaries from healthy women between 22 and 33.
By the time Dr. Tilly revealed his egg stem cell research plans to his colleague, Dr. Yasushi Takai, the Japanese doctor had already preserved 30 sets of intact ovaries. “When he told me this, I was jumping for joy. I couldn’t believe we’d have unique access to that amount of tissue,” Dr. Tilly told TIME Healthland.
Stem cells or just very immature eggs?
After isolating the OSCs from the healthy ovarian tissue, they began to grow them in the lab.
To find out if what they were really producing egg stem cells and not just very immature eggs cells, Dr. Tilly’s team first did the obvious: simply letting the cells grow. When the egg stem cells were transferred into a Petri dish, they began to expand rapidly and soon blanketed the surface, even as they continued to divide over and over again. This was one sign that they weren’t immature egg cells, but stem cells that had not yet embarked on the path to becoming oocytes, Dr. Tilly said.
Because eggs and sperm are germ line cells, they lose half their genetic material when they divide.
In humans, eggs and sperm each contain only 23 pairs of chromosomes, and after fertilization all of the cells in the resulting embryo contain a total of 46 pairs of chromosomes — half of which come from the egg and the other half from the sperm.
To shed their extra DNA, eggs and sperm undergo a special cell division called meiosis. Once they enter meiosis, they can’t divide again — and this is how Dr. Tilly knew the cells in the dish, which were still dividing, weren’t egg cells.
Not content with these deductions, the researchers put the cells they produced under a rigorous test. Previous research found that mouse OSCs can be identified by the presence Ddx4 — a certain protein found on their cell surface. The team then latched on to this proteins and fished them out.
After optimizing a procedure for isolating OSCs from mouse ovaries, the investigators then used the same technique to isolate human OSCs from adult human ovaries.
In order to track what happened to these OSCs, the researchers inserted into those cells a piece of DNA that makes some jellyfish glow green. If the stem cells made eggs, those would glow, too, Dr. Tilly explains. They introduced the marked OSCs into the ovaries of normal mice and watched through a microscope as new eggs grew in a lab dish from the oocytes they produced.
“Bang, it worked — cells popped right out” of the human tissue, he enthuses.
Finally, the team did the pivotal experiment: They injected the stem cells into human ovarian tissue, then transplanted this human tissue under the skin of mice to provide it with a nourishing blood supply. Telltale green-tinged egg cells began forming within two weeks, the researchers reported.
The research was funded by the U.S. National Institute on Aging, the Henry and Vivian Rosenberg Philanthropic Fund, the Sea Breeze Foundation and the Vincent Memorial Hospital Research Funds.
The next steps
So far, the Harvard scientists have identified that OSCs do exist in human ovaries and have fully honed their process or protocol of isolating these OSCs. They’ve also been able to their test the growth properties and function of the OSCs both in the lab and in mice.
Because of legal and ethical issues, laboratory and animal-based investigation is the ideal way to answer these scientific questions, says the National Health Service, the United Kingdom’s national provider of health services.
The next step is to see whether other labs and other research teams can verify the work, says Teresa Woodruff, fertility preservation chief at Northwestern University’s Feinberg School of Medicine. “If so, then it would take years of additional research to learn how to use the cells, she adds.
While the research is still a long way from showing that the OSCs can mature into usable and quality human egg cells, Dr. David Albertini, University of Kansas’ Center for Reproductive Sciences director says, “It’s giving us some new directions to work in.”
Dr. Tilly also thinks his research may mean that current ideas about aging and fertility are outdated.
He recounts that recent studies in mice showed that when oocyte stem cells taken from menopausal female mice and transplanted into younger mice, the stem cells were able to make viable eggs.
“When they transferred the tissue into a young ovarian environment, the stem cells woke back up and, lo and behold, a new population of oocytes formed,” says Dr. Tilly. “That tells us that perhaps ovarian failure at menopause isn’t incompatible with the idea of these cells existing.”
“Maybe we need to rethink how menopause is happening and if these cells are still there, but it’s the organs that are failing with age, what does that mean down the road in terms of clinical interventions?” he ends.
Indeed, the NHS calls the new findings an “exciting discovery” that “raises questions about whether new fertility treatments could be developed using the regenerative properties of egg stem cells.
But the national health agency also cautions that “this experimental research is at a very early stage and much further study will be needed before we can understand if it can be safely used to help patients.”
“While this study obtained egg-producing stem cells from both healthy mice and healthy young women, it has only demonstrated that human OSCs can develop into oocytes in the laboratory and when transplanted into mice,” the NHS says.
“This means the study doesn’t provide evidence on whether these human oocytes were healthy, functioned normally, or could be fertilized,” it adds.
There are also many reasons why both men and women may experience fertility problems, the NHS points out, saying it’s unclear how the study’s findings could be put into clinical use to benefit infertile couples.