For many years, scientists believed females of human and other species were born with a fixed number of cells, termed oocytes, capable of developing into eggs and that number would decrease throughout their lives until it ceased (Zuckerman 1951). This meant that females could only have offspring as long as they had functional oocytes.
In 2004 (Johnson et al. 2004), however, researchers found a new type of cells – which they termed germ stem cells or oogonial stem cells (Fig. 1) – present in mice ovaries that had the capacity to form new oocytes.
Oogonial stem cells, the new superheroes for fertility research.
Citation: Reproduction and Fertility 5, 2; 10.1530/RAF-24-0004
Still not entirely clear on how and why these cells were there, research continued with what seemed to be a groundbreaking discovery.
Between 2008 and 2010, two new, very promising, findings came to light. First, researchers were able to find these germ stem cells with the same capacity to form oocytes in the ovaries of post-menopausal women and women with premature ovarian failure (POF), a condition in which the ovarian function ceases too early (Virant-Klun et al. 2008).
Secondly, another research group, using adult mice, was able to form oocytes from these germ stem cells and develop mature eggs, which were ovulated and fertilized and eventually produce offspring (Zou et al. 2009).
It then seemed that the initial beliefs of many scientists were, after all, not entirely set in stone. It might just be that there is a chance for females to have more oocytes during their lives, produced from these germ stem cells/oogonial stem cells (Fig. 2).
Women through their lifetime.
Citation: Reproduction and Fertility 5, 2; 10.1530/RAF-24-0004
A new path
Researchers have now been trying to fully characterize these new cells, to discover their properties and to understand the reason why the reproductive system does not take advantage of their existence already.
The aim is to isolate these cells from the ovaries of different species and to develop them into viable oocyte-like cells, by establishing in vitro gametogenesis – or ‘in vitro oocyte-creation’ technique – that could someday produce offspring.
Only a few groups have been working on this subject using ovaries from women and animal models like cow, pig, mice or primates (Bui et al. 2014, Dunlop et al. 2014, Bhartiya 2015, Clarkson et al. 2018, Silvestris et al. 2018, Sequeira et al. 2021) and the truth is that some have been unsuccessful. And, even though promising results have been shown in mice already (Zou et al. 2009), there is still a long way to go with large animal models and women.
My research at the University of Antwerp’s Gamete Research Centre has that exact idea in mind. With ovaries we collect from pigs and cows, we are now trying to find and collect these oogonial stem cells. The first questions are of course: ‘Where are these cells located in the ovary? How can you be sure you are collecting them?’
Although we are not entirely sure of their precise location, we believe these cells are situated in the cortex – the outer portion of the ovary. This is the most organized tissue in the ovary and includes many follicles, which are the structures that hold the developing eggs. Focusing on this part of the ovary, we will get out a cell population with lots of different cell types, which will include the cells we are looking for.
Now we go into action
The main challenge of course is to get just the oogonial stem cells. The previous studies in mice (White et al. 2012) have already showed that it is extremely difficult to isolate them as they represent around 0.014% of that whole cell population you just got out of the ovarian cortex.
As such, with the whole cell population, we will then use antibodies – proteins that can bind to a specific target in a specific location – to detect the cells we want. By using antibodies that will be specific for the oogonial stem cells, we can then add a fluorescent compound that will allow us to see, detect and select them out using a machine (Fig. 3).
Experimental plan on how to obtain oogonial stem cells.
Citation: Reproduction and Fertility 5, 2; 10.1530/RAF-24-0004
Why pig and cow as animal models?
Even though pigs and cows seem to be completely different to human when you look at the three species, for research studies that is not often the case (Fig. 4). To study what happens in women, whose ovaries are not usually available, one should always start with animal models closely related.
Pig and cow, as animal models for human research.
Citation: Reproduction and Fertility 5, 2; 10.1530/RAF-24-0004
Pigs are in fact similar to human in their organ sizes and structures, oocyte sizes and their fertilization mechanisms (Hou et al. 2018, Lunney et al. 2021). Cows, on the other hand, have a remarkable similar reproductive cycle (Langbeen et al. 2015).
The potential of oogonial stem cells
Our research at the Gamete Research Centre will hopefully have an enormous impact on fertility preservation and fertility restoration.
If our novel stem cell technology is successful, it has the potential to be used to preserve fertility for women before they undergo chemotherapy or radiation treatments or to restore fertility in women who already underwent treatment, are menopausal or have POF.
We also aim to use the same technologies in endangered species, as a way to preserve and save species that are on the brink of extinction with too low or no reproductive activity anymore, and thus contribute to the ongoing efforts for wildlife conservation (Bolton et al. 2022).
What is next?
After getting the bovine and porcine oogonial stem cells, the next big step will be to place them in an in vitro culture so they can start to grow and develop them into oocyte-like cells, just as it was done in mice.
Producing bovine and porcine oocyte-like cells from oogonial stem cells will already be a groundbreaking achievement for the field of fertility preservation. But our research will definitely not stop there. The end goal will always be to get those oocytes to form follicles, which can then be matured and fertilized, so that offspring can be produced.
Declaration of interest
The author declares that there is no conflict of interest that could be perceived as prejudicing the impartiality of this review.
Funding
This review was funded by University of Antwerp.
Acknowledgement
Thanks are due to my PI Prof. Dr Ruth Appeltant at the University of Antwerp (Belgium).
References
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