Stem Cells Coaxed to Make Precursors to Egg, Sperm

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The Stanford team knew that feeding human ESCs growth factors, called bone morphogenetic proteins, would induce the cells to differentiate into germ cells. Led by postdoctoral fellow Kehkooi Kee, they developed a "reporter gene" system to tag and identify the cells they wanted -- those that were differentiating into germ cells -- by making them glow fluorescent. That fluorescence would then enable the team to count developing germ cells, and also purify them.

Using various methods, the team determined that the fluorescent cells tagged by Kee's reporter system were so-called "primordial" germ cells. Accounting for just 5 percent of the cell population in this study overall, these cells are basically early, immature germ cells.

Experimental system in hand, the team then turned to the role of the DAZ protein family in the production of these very early cells. DAZ proteins were already known to be associated with male infertility; that's how Reijo Pera first identified them in the 1990s. The question was: How exactly do they affect germ cell development?

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By alternatively blocking and boosting the expression of the DAZ family members DAZ, DAZL and BOULE, and counting the cells that emerged, the team determined that DAZL is required for differentiation in primordial germ cells, while DAZ and BOULE push the cells further towards the production of true gametes. Importantly, by overexpressing all three proteins, the team produced cells with the same genetic content as gametes.

"We didn't know these genes are major players in germ cell development," said Reijo Pera. "We suspected it, but here we can directly show that we get differences in the pathway. For a biologist, that's incredibly exciting."

Reijo Pera said the system can now help researchers map germ cell development and identify factors that can enhance or inhibit that process. Longer term, her team wants to coax germ cells to differentiate from adult, non-embryonic stem cells -- using cells such as skin cells that are manipulated to regress to an embryo-like state.

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