With a discovery that will help to shed light on the grey area of human reproductive biology, scientists in America have developed a technique to convert stem cells into germ cells - the precursors of sperms and eggs.
The process through which humans produce these gametes, which contain only half the amount of DNA found in a normal cell, is poorly understood and cannot be faithfully reproduced in animals, making it very difficult to study. This means that for patients suffering from infertility, research has progressed very slowly in this area.
But now, writing in Nature, Stanford University scientist Renee Reijo Pera and her colleagues have found a way to make germ cells in the dish. The researchers began by incubating human embryonic (ES) stem cells with various growth factors known to encourage them to specialise into reproductive-type cells. They then picked out those cells - about 5% of the total - that had begun to express certain chemical markers associated with germ cells.
The team then confirmed that the resulting cells were doing something else very germ-cell-like - demethylating their DNA. This is where chemical markers called methyl groups that are added to DNA to control gene expression are stripped off to "reset" the DNA to the state seen in sperm and eggs.
Next, to find out what was controlling the process of germ cell production the team repeated the earlier steps but blocked the actions, individually and together, of three different genes called DAZ, DAZL and BOULE, which had previously been linked to the formation of germ cells. These, they found, are critical for stimulating unspecialised stem cells to turn into germ cells.
Even better, when the team used a genetic technique to boost the levels of these genes in the stem cells, they found that they could increase significantly the number of germ cells produced. And when the team examined the DNA contained within the resulting cells they found that a proportion of them contained only half the normal amount of genetic material, just as in mature sperm and eggs.
This means that this new technique can provide scientists with a powerful tool with which to study the previously inpenetrable process of human gamete formation, as well as to test the impact of factors like pollution, toxins or infections, thereby shedding light on some of the factors that underlie infertility.