In addition to producing human-animal organs, these experiences seek to study the first stages of embryonic development and even be used to deepen the knowledge of some diseases and their treatment.

An article in the journal Cell (April 15, 2021), was the first work describing the chimera production human-ape (read HERE). The authors said that Monkey (Macaca fascicularis) embryos were injected with a particular type of human stem cells, expanded stem cells (hEPSCs), which were developed in 2017 (see more HERE) and have enhanced chimeric potential, i.e. greater capacity to develop in the host animal embryo. In the work we are discussing, hEPSCs have been shown to survive, proliferate, and generate various cell lines in the monkey embryo in vitro. Until now, Izpisua’s group and other groups had demonstrated the production of chimeric human-animal embryos, such as mice, pigs or cows (read HERE), but despite the efforts of different research groups, they have not been able to produce chimeras in that a sufficient number of human cells are generated to think that these can be the basis for creating tissues and organs in animals with a human component. This, apparently, could be due to the fact that the distance of the evolutionary line of the animals used is very far from the human evolutionary line. To overcome this difficulty, the authors use monkeys here, whose evolutionary line is much closer to humans.

chimera experiments ethical approach

Referring especially to the results of this chimera experimentation, the authors verify the development of the chimeras produced up to the gastrula phase. At this point in time, they eliminate the zona pellucida in the chimera produced, and from it, they obtain human-animal cells, which, when transferred to an external disk, continue to develop. By this means they obtain 132 cell lines of which 3 survived 19 days.

Chimera experimentation ethical considerations

That these experiences raise ethical problems is undoubted (read more HERE), since the same authors, in the discussion of their article, begin by stating that they have consulted with institutions and experts in bioethics at the international level, who apparently have approved their work. In relation to this, it would be necessary to know, first of all, which bioethical experts have been consulted, since as will be commented later, in our opinion, these experiences have an evident utilitarian nature, therefore, if the experts navigate this way, it is natural that they do not encounter ethical difficulties. But apart from this, in our opinion, the ethical difficulties that this work presents are clear.

  • In the first place, and essentially, due to the ethical difficulties that are linked to the production of human-animal chimeras, which we have dealt with previously, and which can be summarized by saying that it is difficult to determine what degree of humanization the chimeric embryo produced reaches.
  • As a function of this, it may not be ethical to generate and manipulate these embryos, much less destroy them.

As in previous reports, we commented (read HERE), in the chimeric tissues and organs produced there may be the colonization of human cells of different degrees, with the possibility that said colonization extends beyond the organ that is to be produced and may even reach the brain, which It can certainly raise ethical problems that are difficult to solve. In some of the previous works, the authors try to solve this problem by producing transgenic animals in which the gene-generating gene to be produced would have been deleted, for which, in their opinion, this organ would only be colonized with human cells, without that there was the option of colonizing other organs, including the brain, but this is far from being reliably proven. In any case, in the article that is being discussed, the monkeys used had not been genetically modified, so the colonization of human cells from different organs remains feasible. On the other hand, in addition to producing human-animal organs, another purpose of these experiences, according to their authors, is to be able to study the first stages of embryonic development, and even be used to deepen the greater knowledge of some diseases and their treatment. To achieve this, it would possibly be enough to use embryos of monkeys, whose use does not present any ethical difficulties, but if with these experiences they want to approach the human, creating the chimeras that are being discussed, these ethical difficulties do not seem to be

In previous Ispizua’s works (read HERE), an additional bioethical difficulty was raised, since they used human embryos, which seems to have been solved here since the hEPSCs used to come from adult humans. In other words, it seems to us that these experiences, since they cannot determine the degree of human colonization of the tissues and organs produced, are not ethically acceptable, so it seems reasonable to apply a principle of prudence before proceeding with these investigations.

We do not agree with the authors’ ethical justification 

Despite all the above, the authors largely justify the ethics of their work by stating that it can lead to the creation of quasi-human organs, which can be used in transplants. Given that at the present time there is an evident lack of human organs for transplantation, the possibility that is illuminated here of creating quasi-human organs in animals could be biotically justified. However, this bioethical foundation is based on clearly utilitarian criteria, with which we do not agree, since our line of thought is based on the principles of personalist bioethics, which should preside over the experiences that are carried out with human-animal chimeras.

Justo Aznar and  Julio Tudela

Bioethics Observatory – Institute of Life Sciences

Catholic University of Valencia


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