Scientists have announced “the creation, for the very first time, of chimera embryos containing human and porcine cells”. The authors of the study are American, Juan Belmonte from Salk Institute, and Spanish (University of Mercia). The long-term aim of such research is “to produce a human pancreas, liver or heart from pigs” to off-set the shortage of organs for transplant. But this is a “disturbing experiment” because it “renders porous/permeable the boundaries between human beings and all other animal species, thereby raising serious ethical questions”.
The study, which was published in the Cell journal, describes the procedures carried out: human induced pluripotent stem cells (iPS) were injected into porcine embryos. These human cells were not rejected since they were injected into the animal embryo at a very early stage, namely the 5th or 6th day of its development. The chimeric embryos were then transferred to the uterus of surrogate sows. They were developed with a significant number of human cells. The experiment was not continued up to the birth of the piglets as scientists decided to stop it after 4 weeks of development.
Eventually, to develop a specific human organ in these embryos, scientists are considering the use of CRISPR Cas9, a genome handling tool, to neutralise a gene essential for the development of that organ in the porcine embryo. The human cells injected would therefore take its place.
These experiments are controversial and the research projects, “transgressive”. Professor John De Vos from the Department of Cell and Tissue Engineering at Saint Eloi Hospital in Montpellier, believes that “several limits should never be crossed”:
- “The first concerns the migration of human cells into animal brains because this could bestow the animals’ brain with human functions”. Several options are being considered to stop this: “A gene essential for the development of the central nervous system could be deactivated in the iPS cells injected into the animal embryos”, or “these pluripotent cells could already be guided to develop only gastrointestinal or cardiovascular cells as opposed to neurones”. Professor De Vos also suggests “establishing the percentage of human contribution to the animal brain which should never be exceeded”.
- The second “red line that should never be crossed” is “the production of human gametes by reproductive organs of the human-animal chimera”.
- “Finally, animals presenting external human signs, such as a porcine embryo with extremities similar to human hands or feet, should be sacrificed before birth”.
 These iPS stem cells are taken from reprogrammed adult skin cells. They are capable of differentiating into all cell types (liver, pancreas, heart, etc.).