Until just a few years ago, scientists thought that all of a woman's eggs were made while she was still in the womb. Before she's born, a certain group of cells called germ cells divides until they are almost mature eggs and then they stop. Once puberty hits, around one egg per month finishes maturing and is released.
When a woman starts to go through puberty, she has around 400,000 of these egg precursor cells. But in the end, only about 400 of these will ever become mature oocytes.
What this meant was that if something bad happened to a woman's eggs, she would not be able to have a baby. She had "all of her eggs in one basket," so to speak.
But turns out that nature is not as stupid as we thought! Adult women still have cells that can become new eggs.
Scientists first had a hint that this was the case about 10 years ago. In 2004, a group of scientists from Harvard University discovered
a set of germ cells in adult mouse ovaries that could become eggs. That's when they named these cells oogonial stem cells or OSCs.
These OSCs could become mature eggs in petri dishes and when injected into mouse ovaries. At the time, this news came as quite a shock, since no one up to this point believed that these cells even existed.
But scientists didn't know if the same thing happens in humans, too. After all, a mouse is not a person.
Then, after eight years of searching, the same group of scientists reported that they've identified OSCs in human ovaries. It took so long because there aren't a lot of these cells around in adults, and they are hard to tell apart from other cells in the ovary.
The first step was to find something that was different about OSCs to help distinguish them from all the other cells in the ovary. They settled on something that's known to only be on the outside of germ cells called Ddx4. They used this knowledge to go on a fishing expedition to isolate the OSCs.
Using an antibody that recognizes this label as bait, they were able to separate out any OSCs with Ddx4 on their surface from the rest of the cells in the ovary. They found that even though OSCs are pretty rare (they make up only about 1/100th of the ovary), they do actually exist.
Next, they wanted to show that these cells were actually OSCs. Just like the mouse OSCs, the cells were able to develop into mature oocytes in petri dishes and even inside mouse ovaries.
But this wasn't easy either. To tell which cells were the OSCs that they added, the researchers used a common lab trick – adding green fluorescent protein
GFP is what makes some jellyfish glow green. Scientists have found that they can pretty much get anything to glow green by adding this gene. This makes cells easy to follow...just look for the glowing green ones!
So by adding the gene for GFP to the OSCs, the scientists could follow the cells after they were injected into a mouse's ovary. They did this for OSCs that they isolated from mice and from humans, and found that both could develop into mature oocytes.
Then the scientists did something even cooler with the OSCs that they isolated from mice. After the mouse OSCs developed into mature oocytes in the ovary, they found that they could be fertilized. And the mouse pups that were born glowed green because they came from an OSC that had GFP in it!