26 March 2006

Nature Cell Biology doi: 10.1038/ncb1388

Stem cells isolated from the skin of pig fetuses are capable of giving rise to egg-like cells, according to a paper published in the April issue of Nature Cell Biology. Embryologists have long sought to understand when and where germ cells – which give rise to sperm and eggs – are formed. The stem cells in the early embryo give rise to all the different lineages of the foetus, including the germ-cell lineage. In mammals, germ cells become segregated from non-germ cells (called somatic cells) during embryonic development and migrate into the gonads, where they form sperm or eggs. Now, Julang Li and colleagues show that even post-embryonic somatic stem cells (derived from later stages of fetal development) can also give rise to egg-like cells in vitro. They found that when skin-derived stem cells from pig fetuses were grown in vitro for 30-40 days under specific conditions known to be important for egg formation, they expressed markers normally found in germ cells and produced egg-forming cell aggregates. After a further 10-15 days, these cell aggregates extruded large egg-like cells that expressed markers specific for meiosis – the specialized cell division programme that gives rise to sperm and eggs. Li and colleagues also showed that a subset of these egg-like cells spontaneously underwent parthenogenesis – producing embryo-like structures without fertilisation. Although it remains unclear whether these eggs can be fertilized to form viable embryos, the ability to generate egg-like cells from cells derived from the skin provides new possibilities for tissue therapy and reproductive engineering and offers a powerful in vitro model for the study of egg development.