Two research facilities have been given the green light to create part human, part animal embryos.
Embryonic stem cells (ES cells) are stem cells derived from the inner cell mass of an early stage embryo known as a blastocyst. Human embryos reach the blastocyst stage 4-5 days post fertilization, at which time they consist of 50-150 cells. Embryonic stem cells are pluripotent, meaning they are able to grow (i.e. differentiate) into all derivatives of the three primary germ layers: ectoderm, endoderm and mesoderm.
British authorities in the UK have given scientists the green light to create human-animal embryos for research (use of animal eggs to create human stem cells, a ruling that will boost the supply of stem cells for research).
The decision means that researchers will be able to refine their techniques for producing human stem cells by practicing first on animal eggs, of which there is a steady supply. Similar work involving human-animal stem cells is also under way in China and the United States.
Hybrids are made using an animal egg mixed with human genes. However according to this article:
The experiments involves transferring nuclei containing DNA from human cells, such as skin cells, into animal eggs that have had almost all of their genetic information removed.
The resulting cytoplasmic embryos are more than 99% human, with a small animal component, making up around 0.1%.
The embryo would be grown in the lab for a few days, then harvested for stem cells – immature cells that can become many types of tissue.
=> More info at : UK OKs human-animal embryo research