Scientists discover stage when embryonic cells become stem cells

Scientists at the University of Cambridge have discovered the stage at which some of the cells of fertilised mammalian eggs are fated to become stem cells.

These findings are contrary to the long-held belief that cells are the same until the fourth cleavage of the embryo.

The study, published in Nature magazine, also lists the causes behind this phenomenon.

The researchers say that after fertilisation, the embryonic cells at first undergo equal, symmetrical divisions and unequal, asymmetrical ones that direct smaller daughter cells towards the inside of the embryo, and then they become the inner cell mass of stem cells.

Previously, it was believed that the mammalian embryo starts its development with identical cells, and only as these inside and outside cells form do differences between cells first emerge.

But Professor Magdelena Zernicka-Goetz claims to having found evidence suggesting that differences between the embryonic cells are already apparent at the 4-cell-stage, before the cells become partitioned between the inside or outside of the embryo.

The lead researcher said that those differences depend on the orientation and order of the very first cleavage divisions of the embryo.

"Our findings were surprising since they showed that cells of the mammalian embryo first start to differ from each other much earlier in development than previously supposed but also they give us a real clue on how to manipulate embryonic cells so that they will develop with the properties of the natural stem cells of the embryo," said Professor Zernicka-Goetz.

The study also found that the level of a methylated form of histone H3, one of the basic proteins around which DNA is packaged and affects the gene expression when modified, determined the cell fate and transcription activity.

They found that the higher the levels of modified form of histone, the more predisposed the mammalian embryonic cells were to develop the qualities of inner embryonic cells, a population that have stem-cell-like properties.

Their findings show that manipulating epigenetic information in this protein in early mouse embryos can influence cell fate determination.