2024 Author: Adelina Croftoon | [email protected]. Last modified: 2023-12-17 02:07
On August 4, 2016, the US National Institutes of Health (NIH) announced that they are going to lift the moratorium on the creation of chimeras. These are ethically controversial experiments in which human stem cells are injected into animal embryos - as a result, organisms are formed that combine animal and human traits. Scientists call them chimeras.
Shot from the film "Chimera"
In ancient Greece, chimeras were called mythological monsters with the head and neck of a lion, the body of a goat and the tail of a snake. The same chimeras are organisms with genetically dissimilar material. They could serve as convenient biological models for the study of various diseases - for example, cancer or neurodegenerative syndromes, could become a source of organs for transplantation.
However, once experimental biology gets close to science fiction, the public fears that this could lead to unintended consequences.
When creating chimeras, stem cells are used that have the property of pluripotency. In other words, they are able to transform into all cells of a human embryo. The cells are introduced into the tissues of the embryo of model organisms (mice, rats, monkeys, pigs and other animals) at very early stages, after which the embryo is allowed to develop further.
In September 2015, the NIH expressed concern that if stem cells were injected into the brains of mice, the result could be rodents with altered cognitive abilities - that is, animals with "superintelligence". Therefore, the NIH, which awards grants for biomedical research, decided to suspend funding for experiments with chimeras until their experts investigate the ethical issue.
Nevertheless, some research groups in the United States were already busy creating chimeras. MIT Technology Review reports that in 2015, there were about 20 attempts to produce pig-man and sheep-man chimeras. Unfortunately, no scientific work has yet been published, and there have been no reports of the successful production of animals with human tissues.
From left to right: normal mouse, mouse with rat cages, rat with mouse cages, normal rat
Experiments with chimeric organisms combine both genetic engineering and stem cell biology. It is not enough just to introduce pluripotent cells into the embryo of an animal, since in this case an organism with catastrophic developmental disorders can turn out. Scientists usually turn off genes in embryos so that they cannot form specific tissues. In this case, the stem cells take on the task of forming the missing organ, which is no different from a human, making it suitable for transplantation.
According to the testimony of cardiologist Daniel Garry, the first tests of this method were carried out in his laboratory. The researchers designed pigs that lacked some skeletal muscle and blood vessels. Such animals would not be viable, but scientists added stem cells from another pig embryo to the embryos.
The results impressed the US military so much that they gave Harry a $ 1.4 million grant to grow human hearts in pigs. The scientist was going to continue his research despite the NIH moratorium, and was one of 11 authors who published a letter criticizing the decision of the biomedical center.
Scientists said the NIH moratorium posed a threat to the development of stem cell biology, developmental biology and regenerative medicine, and questioned whether stem cells could produce a highly intelligent, humanized animal. In particular, they pointed out that xenotransplantation experiments in which nerve cells from humans are implanted into the brains of mice did not lead to overly intelligent rodents.
Human stem cells (red) in mouse embryos at the blastocyst stage
As a precaution, some researchers working on chimera do not allow their creations to be born. Embryologists study the embryos in order to obtain information about the contribution of human stem cells to the development of the fetus.
Nevertheless, despite the fact that some laboratories are playing it safe, chimeric animals already exist - for example, mice, endowed with the human immune system. Such animals are created through the introduction of liver and thymus cells from aborted human embryos into the body of already born rodents.
Of greatest interest to scientists is the creation of chimeras at the blastocyst stage, when the fetus is a ball consisting of several tens of cells. This method is called embryo complementation.
In 2010, researchers in Japan succeeded in creating mice whose pancreas was composed entirely of rat cells. Hiromitsu Nakauchi, the lead author of the work, later decided to create a "human pig", for which he had to move to the United States, since scientific committees in Japan do not approve of such experiments.
The scientist is now working at Stanford University on a grant from the California Institute of Regenerative Medicine. Most of the pluripotent cells injected into embryos in his laboratory are made from his own blood, he said, as bureaucratic barriers prevent recruiting volunteers from outside.
Most people, hearing the word "chimera", imagine monsters created by mad scientists. Scientists have to prove that human cells can actually multiply and form complete and healthy organs in animals. Mice and rats are quite close genetically, so the creation of chimeras in this case is not a problem. In the case of humans and pigs, whose common ancestor lived 90 million years ago, things may be different.
Scientists are already testing the complementation of pig embryos with human stem cells, but research only began after the approval of three bioethics commissions. Stanford University, which is conducting the research, has limited embryo development time to 28 days (piglets are born at 114 days). Nevertheless, the fetus will be sufficiently developed to determine how well the organ buds are forming.
Last week, the NIH proposed replacing the moratorium with additional expertise to be conducted by a committee of ethicists and animal welfare experts. They will take into account factors such as the type of human cells, where they are located in the embryo, and possible changes in the behavior and appearance of the animal. The experts' findings will help the NIH decide whether the reviewed project is worth funding.
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