Chimerism in humans research paper

These chimeric animals hold enormous potential for the field of medicine as well as basic research into human physiology. 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. 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.

Despite its potential, however, chimera research faces significant opposition from a wide spectrum of the population. 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. The debate over chimera research represents the changing perception of humanity and our place in the world.


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Chimeras differ from the general perception of the genetically modified organism. 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. 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.

Chimerism within a species occurs naturally in nearly all animals, including humans. Interspecific chimeras, however, rarely exist in nature due to the unlikelihood of specific conditions required. Such research into chimeras elicited little public attention and outcry until August , when Hui Zhen Sheng at the Shanghai Second Medical University created the first human-nonhuman chimera. 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.


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By creating animals with human cells, scientists can monitor and track cell differentiation, tissue development, and organ formation without using human infants as subjects. 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 can also use chimeras to study the progress and mechanisms of diseases in live tissues and organs. Chimeras with human tissues offer a fairly accurate substitute for a real human body. A Stanford team is working closely with human-cow chimeras to study how HIV attacks the human immune system.

In addition to helping the study of disease, chimeras also offer a safer and more ethical method of testing for new drugs.

Researchers reprogrammed human cells before injecting them in the monkey embryo

FDA regulations require new drugs to undergo extensive pre-clinical studies to examine their absorption, distribution, metabolism, and toxicity before clinical tests on human subjects. Testing on animals such as the common lab mouse provides inaccurate results due to the large differences in physiology and biochemistry. Greater accuracy requires testing on animals similar to humans, such as primates, which raises ethical and financial challenges. Human-animal chimeras provide a loophole through this dilemma by creating a lower animal that has human cells in the organs and system targeted by the drug.

This method of testing not only reduces the risk to clinical trial subjects, but also helps make new medications available to patients sooner by speeding up the testing procedure. Another application of chimeras lies in the field of xenotransplantation, or transplantation using animal organs.

Ethical concerns

The world currently faces a shortage of organs for patients who need transplants. Many die while waiting, while many more venture abroad to seek alternative sources of organs in the less developed countries. The extensive use of chimeras can potentially remedy the situation. These organs can then be transplanted to the patient without risk of rejection or autoimmune complications.

Current research focuses heavily on pigs and sheep due to similarities in the size of organs and has yielded encouraging results. Jeffrey Platt at the Mayo Clinic Transplantation Biology Program has produced pigs that produce human hemoglobin and have lungs that can pump human blood, which is a step towards the possibility of complete lung transplants.

Scientists warn against creating human-monkey HYBRIDS as calls rise to create the chimeras

If successful, this procedure can provide a safe and cost-effective solution to one of the most debilitating shortcomings of the medical system. Despite its promises, chimera research poses scientific, religious, and ethical problems. The scientific concerns mostly involve the public health consequences of the spreading of disease between species. Many of the deadliest diseases, such as AIDS, have only recently spread from animals to humans. The recent scares of avian and swine influenzas have demonstrated that diseases usually spread across species through constant, close contact.

In this regard, a chimera provides the perfect vessel for diseases to overcome the species barrier, because cells from two different species are integrated in the same body. Cases of porcine virus infecting human cells in human-pig chimeras have already been observed. Similar threats, however, exist for nearly all topics of research involving infectious biological agents.

Human-Animal Chimeras on U.S. Research Farms

The set of safety procedures developed for them can be applied with equal effectiveness to experiments involving chimeras. To further mitigate the threat to public health, the Centers for Disease Control and Prevention has pushed for a rule to require potential human-nonhuman chimeras to be handled in Biosafety Level 4 facilities, the highest security level reserved for biological agents such as the smallpox virus and the Spanish influenza virus, both diseases with proven deadliness.

Chimera research also faces staunch opposition from many religious groups.

Pigs are deemed unclean, and the eating of pork is forbidden. Creating a part-pig, part-human creature, therefore, is obviously sacrilegious. You are not your own. Besides the religious opposition, ethical challenges also exist. One argument asserts that the artificial creation of chimeras constitutes an unnatural breach of the species barrier.

Bernard Dixon, a HealthWatch Award recipient. Many oppose chimeric research based on this idea of backwards evolution and breaching the evolution lineage. Another argument focuses on the moral confusion caused by the creation of human-nonhuman chimeras. Human-nonhuman chimeras sit astride this clear dichotomy in the basis of moral status: they are created for a clear purpose, and they are also partially human.

While few believe that the possession of human cells alone warrants human species membership and consideration, many believe that certain chimeras share enough in common with humans to deserve human treatment. This approach, however, still involves a separation between humans and nonhumans. With our current understanding of human consciousness and identity, however, society can only draw an arbitrary line that determines the fate of possibly human organisms.

Some bioethicists also argue that even if a chimera receives human treatment and consideration, being a chimera still jeopardizes its personhood. Personhood deals with the concept that our cognitive ability grants us a certain level of self-consciousness and autonomy. The problem arises when chimeric animals may have enough cognitive ability to be granted partial personhood. Experiments conducted by Irwin Weissman at Stanford University have produced mice with nearly all human neurons in their brains.

An argument similar to personhood deals with the idea of human dignity, a term often mentioned in debates against practices such as cloning, torture, and abortion. Despite its common usage, the concept is surprisingly vague. The term suggests an innate right to respect an ethical treatment that is exclusive to humans and different from animal dignity. A human-animal chimera can potentially have a high enough biological or mental resemblance to humans to have full dignity.


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  5. There also exists the horrifying prospect that a human can have chimeric parents. An experiment in Brazil, conducted by Irina Kerkis, produced chimeric mice with human gametes. Her experiment, however, opens the possibility of a human having mice parents, for if male and female chimeric mice mated, the zygote produced would be entirely human. Possibilities like this cause many to consider any type of chimeric research an affront to human dignity.

    These problems all arise from the distinct separation between animals and humans in our current system of ethics. A few fundamental, and outdated, ideas underlie its inability to cope with this promising field of research.

    Human-Animal Chimeras in Biomedical Research - ScienceDirect

    The first flaw regarding bioethics comes from an understandable perception that species boundaries are rigidly defined and unchanging, especially with regards to higher primates and human beings. The arguments based on the unnaturalness of breaching species barriers all stem from this idea. In reality, the definition of a species is rather arbitrary in itself.

    The usual list of criteria includes physiological differences, reproductive incompatibility, and genomic differences. Thousands of people die every year for lack of transplantable human organs. In the past, human-animal chimeras have been beyond reach.