Scientists working in China and the U.S. have successfully grown chimeric embryos that were created by injecting human cells into the embryos of monkeys for a considerable length of time amid ethical concerns.
A human-animal chimera is a lifeform that is part-human and part-animal. Theoretically, it can develop into an animal with both animal and human cells when implanted into the uterus of an animal.
In this study, researchers grew human-monkey chimeras in lab dishes for up to about 20 days.
It is hoped that this research will potentially help to understand developmental biology and evolution better. The attempt is to create models that will make the study of human biology and disease easier and more accurate.
“An important goal of experimental biology is the development of model systems that allow for the study of human diseases under in vivo conditions,” said Juan Carlos Izpisua Belmonte, senior author and a professor at the Salk Institute for Biological Sciences.
The research appeared in the journal Cell.
Generating the embryos
Chimeras have been around for decades. Scientists have created them since the 1970s for the study of early developmental processes.
In this study, the researchers injected 25 human cells into each monkey embryo six days following their creation. These cells, known as extended pluripotent stem cells, can potentially add to embryonic and extra-embryonic tissues. They could develop into tissues and organs that can be used for transplantation.
Headed by Kunming University of Science and Technology’s Professor Weizhi Ji, the team spotted human cells in 132 embryos after 24 hours. Human and monkey cells divided and grew mutually in lab dishes.
The scientists found that 103 embryos were still undergoing development after 10 days. However, survival soon started to drop, and only three chimeric embryos made it to Day 19 after fertilization.
“Generation of a chimera between human and non-human primate, a species more closely related to humans along the evolutionary timeline than all previously used species, will allow us to gain better insights into whether there are evolutionarily imposed barriers to chimera generation and if there are any means by which we can overcome them,” Izpisua Belmonte stated.
The researchers also discovered communication pathways that were either new or strengthened in the chimeric cells. Izpisua Belmonte explained that understanding the pathways that play a role in chimeric cell communication will potentially make possible the improvement of this communication and increased efficiency of chimerism in host species that are not closely related to humans in terms of evolution.
According to Nature, members of the team in this study were the first to demonstrate (in 2019) that it was possible to grow monkey embryos outside the body for as long as 20 days following fertilization.
Ethical issues
This research has, however, sparked debate on ethical grounds. A Preview in Cell, the journal in which the study was published, took note of possible issues in this regard.
Some scientists have challenged the need for experiments such as the ones favored by the team in the new study. According to them, animals that come out of such work are probably not going to be used in the same way as rodents and mice. Stricter ethical rules would likely apply.
For instance, Nature reports Alfonso Martinez Arias, a biologist at Barcelona’s Pompeu Fabra University, saying that pigs and some other livestock animals are better alternatives without the risk of ethical issues.
Izpisua Belmonte indicated, however, that the researchers carried out proper ethical consultations and reviews before starting the work. He stated that it was a responsibility for the scientists to carry out their investigations in a thoughtful manner.
References
Chimeric contribution of human extended pluripotent stem cells to monkey embryos ex vivo
