18 April 2018

Animal Lego

Why do biologists create chimeras

Tatiana Pichugina, RIA Novosti

For scientists, it will not be difficult to cross two or more living creatures, as well as individuals of different species. There are already chimeras of amphibians, rodents, primates, humans. RIA Novosti understands why researchers create biological monsters and what ethical issues arise in this regard.

Embracing twin embryos

Chimeras of ancient mythology are, as a rule, deities: half-human, half-animal. Such is the Minotaur, which the Greeks depicted as a man with the head of a bull, or a Faun – a man with goat legs. The Egyptians imagined the god Anubis as a man with the head of a jackal. Modern science is able to identify real, living chimeras among people, whose body contains cells of different individuals.

The birth of a chimera is caused by accidental failures in the earliest period of embryo development, for example, when two eggs are fertilized by two sperm cells and merge into one embryo. As a result, the baby receives the cells of another person – his unborn twin. This is called tetragametic chimerism, emphasizing that four different germ cells participated in the formation of the organism: two paternal and two maternal.

It is possible to identify a human chimera only with the help of a series of genetic tests in which DNA from different tissues is compared - blood, hair, saliva, genitals. It is no coincidence that most cases of natural chimerism among people were recorded by forensic criminologists.

Outwardly, it is impossible to recognize human chimeras, with the exception of interracial descendants whose skin color resembles a mosaic, or true hermaphrodites - half–men, half-women. So, in the state of Texas in the USA, a child was born, half of which belonged to a mulatto girl, half to a black boy.

Sometimes chimerism is recognized when looking for the causes of infertility. It turns out that the second genitals develop inside a person. They are surgically removed, and the IVF procedure solves the problem of conceiving a child. 

Often pregnant women exchange cells with their own fetus through the umbilical cord. Doctors call this microchimerism and believe that this condition passes after a few years. However, genetic tests have shown that in some cases, the mother's body retains the cells of the child for life.

Stem Cell Chimera

Successful organ transplantation is nothing but the creation of a chimera, because a person then lives with someone else's organ and someone else's DNA. Organ transplantation is sometimes the only way to save a life, and people agree to it, even if the consequences are unpredictable. 

To reduce the risk of organ rejection, scientists suggest growing it from the patient's own cells, only in another organism, for example, in a pig, whose metabolism is largely similar to that of a human. This means that it is necessary to create a chimera, that is, to introduce human cells into the body of another being.

The discovery of pluripotent stem cells has greatly simplified the technology. These cells are formed at an early stage of embryo development. They can remain in an unformed state for a long time, but they are also able to form any organ or tissue in an adult body. They are implanted into the embryo of an animal, which is then planted in a surrogate female, and they are waiting for the birth of a chimera creature. In addition to the fact that the use of pluripotent cells from a human embryo causes serious ethical difficulties, their number is in principle limited. However, in transplantology, this problem is solved by reprogramming mature adult cells back into pluripotent ones.

From left to right: chimera of rat and mouse, rat, mouse.
© Photo by Tomoyuki Yamaguchi

Chimera in a Petri dish

In the mid-1980s, British scientists created a chimera from a goat and a sheep. The resulting creature was covered partly with goat hair, partly with sheep. This was followed by experiments on crossing two newts with different pigmentation, chicken and partridge, rats and mice. In 2012 , in the laboratory of geneticist Shukhrat Mitalipov from Oregon University of Science and Health (USA) created three rhesus monkeys containing cells of six parents.

In 2013, scientists from Israel received mouse embryos with human cells. In 2017, a pig and a human were crossed. Although all these experiments were stopped at the early stages of embryo development, they caused a heated discussion in scientific circles about the ethics of such research.

Will such a hybrid develop human behavioral traits? Will he have consciousness, as we have, if, for example, we introduce stem cells into an animal to grow brain tissue? Can a child be born from human germ cells produced by a chimera? All these questions remain unanswered for now. Just in case, some governments have restricted and even banned experiments with chimeras – until the situation is clarified.

Roku and Hex are the world's first chimera monkeys from six mothers.
© Photo: OHSU Photos.

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