Are women born with all the eggs they will ever have, or do they possess the ability to make more?
That debate is in full force this week as researchers led by at the University of Gothenburg in Sweden say they have ruled out the tantalising possibility that ovarian stem cells exist.
Back in February, and his colleagues at Massachusetts General Hospital in Boston claimed that they had found stem cells in human ovaries. The news was incredible 鈥� the cells were able to form new immature eggs, and it was hoped that they could be harnessed to improve in vitro fertilisation and help older women to conceive a healthy baby.
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But it hasn鈥檛 been easy to overturn the dogma that women are born with all the eggs that they will ever have.
Brand new eggs
The human ovary contains up to 2聽million immature eggs, and every month one of these matures and is released. It has been long-held that mammals are born with their lifetime鈥檚 supply of eggs. That was until Tilly and various other groups discovered what they believed to be stem cells in mouse ovaries. The groups said that these cells were able to divide into new egg cells.
As these ovarian stem cells mature, a protein called vasa gets pulled from the surface of the cell into the centre, says Tilly. So his team looked for cells with vasa still on their surface in human ovarian tissue. They found a small number and identified them as stem cells because when they were removed from the tissue and placed inside a mouse, they divided into new cells capable of forming early-stage eggs.
Liu鈥檚 team used a different approach. They used a mouse genetically modified to make all its cells glow green. They bred this mouse with another transgenic mouse that carries a piece of DNA that recognises vasa and changes the colour of only those cells that carry it. As a result, all of their offsprings鈥� cells are green except those containing vasa, which appear red, yellow or blue.
The group monitored the cells that weren鈥檛 green for three days. 鈥淭hese cells never proliferate,鈥� says Liu. What鈥檚 more, when his team injected the non-green cells into a piece of mouse ovary, they were not able to make eggs.
鈥淲e鈥檝e found that these cells are not really stem cells,鈥� says Liu. While the cells might look like stem cells, they don鈥檛 act in the same way, he says. 鈥淲e鈥檙e not sure what they are.鈥�
Tilly stands by his discovery. He points out that it is difficult for Liu鈥檚 team to rule out his findings because they did not use the same technique.
A different view
The gene for vasa is expressed in cells at every stage of development, Tilly says. This might mean that the cells that Liu鈥檚 team tried to develop did not include any stem cells, which are rare, he says.
The cells that Liu鈥檚 team found were also about three or four times the size of those that Tilly鈥檚 team identified as stem cells. 鈥淔rom the results shown, Liu was working with nothing but immature eggs, and of course they鈥檙e not going to proliferate,鈥� says Tilly.
He thinks Liu鈥檚 team might have spotted some stem cells if they had monitored the cells samples for longer. 鈥淚t takes several weeks for the cells to start growing, not three days,鈥� he says.
at the University of Edinburgh, UK, is also unconvinced by Liu鈥檚 findings. 鈥淭hey simply do not attempt to look at the population of cells reported by the other group,鈥� she says. 鈥淭hey鈥檙e not comparing like-for-like, they鈥檙e comparing apples and oranges.鈥� Although the role of the stem-like cells that Tilly鈥檚 group identified still needs to be proven, Liu鈥檚 team鈥檚 findings don鈥檛 rule out the cells鈥� existence, says Telfer.
at McGill University in Canada, however, is impressed with Liu鈥檚 study. 鈥淚t鈥檚 a really good study that鈥檚 carefully done,鈥� he says. 鈥淚t provides evidence that ovarian stem cells don鈥檛 exist.鈥�
Instead of settling the debate, the new findings may only have fanned the flames. 鈥淢ultiple groups have found these [stem] cells in multiple species and yet there are still people who refuse to believe it,鈥� says Tilly.
鈥淚 suggest we try to calm down the hype,鈥� says Liu. 鈥淲e need to wait for other labs to repeat [Tilly鈥檚] findings before we throw money into this.鈥�
鈥淚 wouldn鈥檛 waste time looking for something that I don鈥檛 think is there,鈥� says Clarke.
Tilly, however, doesn鈥檛 mind if other groups give up, because then, he says, the most exciting discoveries of what the cells can do will be left to him and his colleagues.
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