Biomedical researchers have created and used animal models containing human cells for decades to gain valuable insights into human biology and disease development. For example, human tumor cells are routinely grown in mice to study cancer disease processes and to evaluate potential treatment strategies. To advance regenerative medicine, it is common practice to validate the potency of pluripotent human cells – which can become any tissue in the body – through introducing them into rodents.
With recent advances in stem cell and gene editing technologies, an increasing number of researchers are interested in growing human tissues and organs in animals by introducing pluripotent human cells into early animal embryos. Formation of these types of human-animal organism, referred to as “chimeras”, holds tremendous potential for disease modeling, drug testing, and perhaps eventual organ transplant. However, uncertainty about the effects of human cells on off-target organs and tissues in the chimeric animals, particularly in the nervous system, raises ethical and animal welfare concerns.