While induced pluripotent stem cells (iPSCs) may eventually provide an alternative to embryonic stem cells (ESCs), scientists are still trying to figure out how similar iPSCs are to ESCs: do they behave the same? Do they give rise to differentiated cells that look the same or similar to those from ESCs?
The researchers used a technique they developed when they worked with sperm cells last year. In that experiment, they started with mouse ESCs and iPSCs and tinkered with a few genes to eventually transform them into primordial germ cell-like cells, or PGCLCs. When the male PGCLCs were transplanted into the testes of neonatal mice, they differentiated into sperm and contributed to healthy offspring.
In the new experiment, they used the same procedure, but worked with female ESCs and iPSCs, differentiating them first into female epiblast-like cells and then female primordial germ cell-like cells. "We were delighted when we saw for the first time that we get a very nice-looking oocyte from this process," said lead study author Mitinori Saitou, a cell biologist at Kyoto University in Kyoto, Japan.
In contrast to the procedure used with sperm cells, in this experiment researchers collected embryonic ovarian somatic cells from embryonic mouse ovaries and mixed them with female PGCLCs, culturing the two together. The cells "spontaneously self-aggregate," said Saitou, to form a structure called a reconstituted ovary. They transplanted the reconstituted ovary under the ovarian bursa in mice, and waited for them to develop. When they isolated cells from the reconstituted ovary after four weeks, "we found, remarkably, that many oocyte-like follicles had developed," said Saitou.
To see if these were viable, they isolated the oocytes, matured them in vitro, and implanted them using in vitro fertilization into foster mother mice. The mice gave birth to fertile offspring — showing that IPSCs can, in fact, be used to create fully functional egg cells.
However, there were differences between the oocytes that formed from the iPSCs and ESCs when compared to normal mouse egg cells. Some of the oocytes had a more elliptical shape than expected. And usually, when natural egg cells are mechanically isolated in the lab, they are still surrounded by ovarian somatic cells. But in the experiment, the researchers found that many of the oocytes were not entirely surrounded by these cells.
The technique that the authors developed for inducing iPSCs and ESCs to become egg and sperm cells can be used to further study the development of germ cells in both mice and humans, which may eventually lead to insights that help treat human infertility.
1. Hayashi, K., S. Ogushi, K. Kurimoto, S. Shimamoto, H. Ohta, and M. Saitou. 2012. Offspring from oocytes derived from in vitro primordial germ Cell–Like cells in mice. Science (October).