QUEBEC CITY, CANADA--A team of scientists reported here yesterday that they had transformed skin cells collected from fetal pigs into egg cells that went on to become embryos. While scientists say the work, presented here yesterday at the annual meeting of the Society for the Study of Reproduction, is preliminary and more in-depth testing is needed, they agree that it's unusually compelling.
While embryonic stem cells can develop into any kind of tissue, scientists find them difficult to control. So far, few embryonic cells have been successfully directed to form specific tissues. And adult stem cells such as bone marrow cells have rarely been reprogrammed to consistently turn into other cell types. Julang Li and her colleagues at the University of Guelph in Ontario, Canada, decided to take a crack at the problem by selecting skin stem cells as their starting point, with the goal of transforming the cells into oocytes.
The team gathered about 500,000 cells from the skin of pig fetuses and immersed them in a special liquid in a petri dish. Most of the cells stuck to the dish and were discarded. Some, though, floated together and formed aggregates. Roughly a third of the aggregates appeared to have a large cell in the middle. These were transferred to another concoction designed to stimulate oocyte production; Li declined to detail the ingredients because the culture is described in a pending patent application.
A subgroup of those aggregates transferred grew into very large cells that, in their shape and other morphology, closely resembled oocytes. The cells also expressed a half dozen genetic markers of egg cells. Strangely, some of them then dropped these markers and went on to become embryolike structures.
Li "had something that looked a lot like an oocyte," says Hugh Clarke, an expert in mammalian oogenesis at McGill University in Montreal, Canada. He was especially taken with the egglike images, which so closely resemble traditional egg cells that he thinks the cells are expressing "the same genes an egg expresses." Still, say Clarke and others, more must be done to prove the cells are eggs, such as examining their chromosomes and possibly fertilizing the egglike cells to form a traditional embryo. Why the cells seem to become embryos on their own is also a mystery, says Clarke.