Abstract
Research on human-animal chimeras have elicited alarms and prompted debates. Those involving the generation of chimeric brains, in which human brain cells become anatomically and functionally intertwined with their animal counterparts in varying ratios, either via xenografts or embryonic co-development, have been considered the most problematic. The moral issues stem from a potential for “humanization” of the animal brain, as well as speculative changes to the host animals’ consciousness or sentience, with consequential alteration in the animal hosts’ moral status. However, critical background knowledge appears to be missing to resolve these debates. Firstly, there is no consensus on animal sentience vis-à-vis that of humans, and no established methodology that would allow a wholesome and objective assessment of changes in animal sentience resulting from the introduction of human brain cells. Knowledge in interspecies comparative neuropsychology that could allow effective demarcation of a state of “humanization” is also lacking. Secondly, moral status as a philosophical construct has no scientific and objective points of reference. Either changes in sentience or humanization effects would remain unclear unless there are some neuroscientific research grounding. For a bioethical stance based on moral status of human-animal brain chimera to make meaningful contributions to regulatory policies, it might first need to be adequately informed by, and with its arguments constructed, in a manner that are factually in line with the science. In may be prudent for approved research projects involving the generation of human-animal brain chimera to have a mandatory component of assessing plausible changes in sentience.
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Notes
There are papers which used the term “hybrid”, but which might be confused with that describing progenies of interspecies sexual reproduction.
The idea is along the line of an earlier development of the humanized Severe Combined Immunodeficiency (SCID)-hu mouse, in which the mouse immune system is effectively replaced with human immune cell progenitors. Obviously there are profound difficulties associated with humanizing of the mouse brain compared to the immune system, and the work was not eventually done. Henry T, Greely was consulted by Weissman to ponder on the ethical permissibility of the proposed mouse model, and a five-person working group chaired by Greely eventually deemed Weissmann’s “…proposed experiments could be performed ethically, subject to some guidelines” (Greely 2007).
According to the guidelines, research involving the transplantation of human stem cells, their derivatives, or other human cells into postnatal animal hosts is classified under Category 1 A - to be exempted from a specialized scientific and ethics oversight process after being assessed by the appropriate existing mandates and committees for laboratory research. Chimera research in which human pluripotent stem cells or their derivatives with broad potential are introduced into a non-human embryo or foetus in utero, or in vitro followed by transfer into a non-human uterus is classified under Category 2, which is permissible only after review and approval through a specialized scientific and ethics review process. Scientific justification is needed for the use of non-human primates (with the expected exclusion of great and lesser ape species).
The subcommittee has also stated that it “…did not want merely imaginable scenarios surrounding stem cell-based animal research to dictate what the professional standards ought to be for research conduct and oversight, especially before there was good scientific evidence to support imagined concerns”.
The Academy of Medical Sciences (UK), for example, listed in their report on “Animal containing human material” (https://acmedsci.ac.uk/policy/policy-projects/animals-containing-human-material) (Box 3.6), six such aspects. These include episodic memory (particularly the ‘subjective component’ of episodic memory), planning, numerosity, language, theory of mind and social cognition.
It is conceivable that human developmental and social upbringing contribute significantly towards the establishment and manifestation of “humanness”, which would not be available to the chimera.
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Tang, B.L. Debates on humanization of human-animal brain chimeras – are we putting the cart before the horses?. Med Health Care and Philos (2024). https://doi.org/10.1007/s11019-024-10209-8
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DOI: https://doi.org/10.1007/s11019-024-10209-8