By Nicolás Jouve, Emeritus Professor of Genetics. President of CiViCa, member of the Bioethics Committee of Spain. (Cover image: A monkey-human blastocyst, an early stage of embryonic development Weizhi Ji, Kunming University of Science and Technology).
A work has just been published in the journal Cell  in which the group led by the Spanish researcher Juan Carlos Izpisua Belmonte at the Salk Institute for Biological Studies, in California, collaborates on obtaining chimera man-macaque embryos. For this, the technology of cell reprogramming is followed, by which, from human stem cells, induced pluripotent cells, iPSCs, are obtained by the technique developed by Shinja Yamanaka in 2006 (2012 Nobel Prize in Medicine). IPSCs due to their pluripotent condition are capable of giving rise to very different types of cellular specialties. The human cells thus obtained are then transplanted into embryos of the macaque Macaca fascicularis in the blastocyst stage. Transplantation of these human cells into the macaque embryo produces what is known as «chimera embryos.» That is, embryos in whose development tissues and organs with cells of two species appear. In the case published today in Cell, of man and macaque.
The same researchers have been working on this type of chimeras for a long time, having previously created chimeras with pig and human cells. The idea is to develop humanized animals, that is, with some human organ or tissue that could be used in xenotransplantation, or in tests on the effects of certain drugs . In previous experiments, human cells maintained their functional activity at first, but did not give rise to organs useful for xenotransplantation.
Human embryonic development is much slower than that of the pig, so in the end there is a lack of coordination in the cell growth of both species and a disabling chimerism in the resulting organs. Aware of the difficulties not only biological but also ethical, Belmonte and his collaborators propose this type of experiment solely on an experimental basis, The chimeric embryos of macaque and man have been monitored in the laboratory to follow their development for 19 days before being destroyed.
Human cells survived and integrated with better relative efficiency than previous experiments in porcine embryos. Izpisua Belmonte says that this type of work could pave the way to address the severe shortage of transplantable organs, as well as help understand more about early human development, disease progression and aging. Izpisua Belmonte has pointed out that «this advance reinforces an increasingly inescapable fact: biological categories are not fixed, but fluid, which poses important ethical and legal challenges.» The big question that arises is about the moral status of these embryos.
With reference to this research, the transhumanist philosopher Julian Sabulescu from the University of Oxford, has pointed out that this research opens the Pandora’s box of creating chimeras as a source of organs for human beings, one of the long-term goals of this research, but the key ethical question is: what is the moral status of these new creatures? In my opinion, if the embryos are from animals, the development derived from them would be an animal, with some human biological components. Second, these chimeras most likely will not give rise to useful organs that can serve to meet the demand for organ transplants.
The differences of biological systems are not a simple mixture of elements of two species, but the product of years of divergent evolution, which will have resulted in a functional adaptation of human organs valid in the human biological system and that of the animal in the species in question. It is difficult to pretend that the simple mixture functionally exceeds an organ that is the result of natural selection of thousands of years of independent natural selection in each of the species. Nor should we be disturbed by the crazy idea of creating animals with mental capacities equivalent to humans, since the mind is not a simple emanation of the brain or the nervous system.
 Tan, T., Izpisua Belmonte, J.C. and others. Cell 184: 1–12 (Apr 15, 2021)  Wu, J ,, Izpisua Belmonte, J.C. and others. Interspecies Chimerism with Mammalian Pluripotent Stem Cells. Cell. 168 (3): 473-486 (Jan 26, 2017)