4 A chimeric pig with mostly human T-cells was used to model a human immune system, and provided insight into the mechanisms of the virus as well as ways to combat it. A Stanford team is working closely with human-cow chimeras to study how HIV attacks the human immune system. Chimeras with human tissues offer a fairly accurate substitute for a real human body. Scientists can also use chimeras to study the progress and mechanisms of diseases in live tissues and organs. With the current state of technology, research using human-nonhuman chimeras provides the most accurate method of observation aside from directly studying human embryos themselves. Scientists have proposed experiments like le Douarin’s to study the many mysteries of human development. This study has already “shed light on the formation of the spinal cord and the integration of the central and peripheral nervous systems in the early stages of development.” 3 These results can suggest new procedures in regenerative medicine for patients with spinal cord injuries. Since chimeras develop from a class of stem cells called nuclear transplant stem cells (ntES), they can be used to study the “molecular mechanisms governing fundamental biological phenomena, such as pluripotency, reprogramming, differentiation, and imprinting.” 2 A French team led by Nicole le Douarin at the College de France’s Institut d’Embryologie has replaced cells in the vertebrae of a developing cow fetus with human pluripotent stem cells to observe their differentiation and study the development of the spinal cord. By creating animals with human cells, scientists can monitor and track cell differentiation, tissue development, and organ formation without using human infants as subjects. ![]() Sheng’s research inspired several subsequent studies that demonstrate the enormous potential of chimeras. Sheng and his team removed the genetic material from some of the cells in a rabbit embryo and inserted human DNA, creating a human-rabbit chimera. Such research into chimeras elicited little public attention and outcry until August 2003, when Hui Zhen Sheng at the Shanghai Second Medical University created the first human-nonhuman chimera. In 1989, scientists at the University of California, Davis breached this barrier and created the first artificial chimera, a sheep-goat hybrid dubbed the “geep.” Interspecific chimeras, however, rarely exist in nature due to the unlikelihood of specific conditions required. Chimerism within a species occurs naturally in nearly all animals, including humans. It develops from two fertilized eggs that come into contact and combine to form a single embryo, instead of staying separate and developing into fraternal twins. Technically, a chimera consists of two genomes in a single body, producing two types of cells that work in conjunction to create a viable organism. The debate over chimera research represents the changing perception of humanity and our place in the world.Ĭhimeras differ from the general perception of the genetically modified organism. At closer examination, however, all of these objections stem from an unwillingness to surrender society’s rigid view of human identity and uniqueness. While scientific and public health concerns exist, the majority of the opposition focus on the violation of current ethical and moral codes that arises from creating and using chimeras. Despite its potential, however, chimera research faces significant opposition from a wide spectrum of the population. Useful applications of this technology range from the study of disease, to more accurate testing of drugs and medication, to the possibility of transplants using chimeric organs. Their utility in science lies in their unique biological integration of both human and animal cells, which can give rise to human tissues and even organs within the body of an animal. ![]() These chimeric animals hold enormous potential for the field of medicine as well as basic research into human physiology. Scientists now have created human-nonhuman chimeras-organisms that have both human cells and animals cells in their bodies. Recently, however, advances in genetic engineering and microsurgery have brought these creatures into existence. For most of history, part-human part-animal beings have always resided in the realm of fiction and folklore.
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