The cultivation of human organs with animals has made progress

rodent chimosa experiments, such as this mouse embryo containing rat heart cells.because of a long-term shortage of human organs, researchers are turning their attention to farm animals. Several biotech companies are genetically engineering pigs to make their organs more compatible with the human body. But some scientists are looking for a completely different solution -- to grow complete human organs in pigs, sheep or other animals before harvesting them for transplantation.
idea is biologically daunting and ethically worrying. But some research groups are working on a key hurdle -- allowing stem cells from one species to thrive in the embryos of another.
, a U.S. organization reported in a preprint that it had grown chimpanzee stem cells in monkey embryos. Japan's latest relaxation of regulations encourages researchers to conduct experiments to improve the survival of human cells in rodent and pig embryos.
Hyun, a bioethicist at Case Western Reserve University in Cleveland, Ohio, said the study is being carried out responsidally. Efforts such as the new chimpanzee-monkey mosaic represent "a small step forward, with scientists collecting data while working". "I think it's a smart thing to do," he said.
, the researchers envision reprogramming a person's cells to be in their original developmental state, which could form most tissues and inject these induced plucer (IPS) cells into the embryos of another species. The embryo will be implanted into the womb of a surrogate animal and allowed to develop to its normal size as an organ donor.
IPS cells can come from human bodies awaiting transplantation. Another potentially faster and cheaper way is for human organs to grow early from the cells of other donors and match key immune signaling proteins to prevent rejection.
so far, the idea has only been modeled on rodents. In 2010, Hiromitsu Nakauchi, a stem cell biologist at the University of Tokyo, and his team reported growing rat pancreas in mice that could not form their own pancreas. In 2017, Nakauchi and colleagues cured diabetes in mice by transplanting insulin-producing mouse pancreatic tissue grown in rats.
, however, success in rodents is not reflected in larger and more evolutionaryly related animals.
In 2017, cell biologist Jun Wu and his colleagues at the Juan Carlos Izpisua Belmonte Laboratory at the Salk Institute of Biology in San Diego, California, reported that they injected human IPS cells into pig embryos and implanted them into sows, resulting in about half of the fetuses developing poorly and growing slowly. Those normal-sized fetuses carry only a small number of human cells after one month of pregnancy.
, who now works at the University of Texas Southwestern Medical Center in Dallas, has explored how human stem cells interact with non-human primates, rats, mice, sheep and cows in a laboratory dish. He found what he called "a very exciting phenomenon: competition between cells of different species".
human cells tend to die when they compete with distant animal cells, and the team is now trying to understand the mechanism. "I think we're almost there." Wu said.
competition is not the only problem. Primate IPS cells are also more advanced than the "primitive" rodent stem cells used in earlier successful chilay experiments. As a result, they are less likely to survive in chimed embryos, said Nakauchi, who also has a lab at Stanford University in Palo Alto, California.
to help primates' IPS cells thrive, his team at Stanford University and his team gave them a gene to prevent their death. In a recently reported experiment, they tested the performance of the modified cells in a primate embryo with close kinship.
to avoid ethical concerns, the team decided not to use human IPS cells.
study, Nakauchi's team modified IPS cells from close relatives of chimpanzees and implanted them in the embryos of rhesus monkeys. Scientists found that IPS cells with survival-promoting genes were more likely to survive two days after implanting a five-day-old monkey embryo than unretouched chimpanzee IPS cells.
Nakauchi says it's hard to keep a monkey embryo alive in a petri dish for more than a week, but his team plans to implant the chimline into the rhesus monkey's uterus "in the near future."
Nakauchi also submitted a proposal to a Japanese government committee to implant survival-promoting genes into human stem cells and inject them into mouse, rat and pig embryos - not non-human primate embryos. The researchers hope that, like early rodent experiments, human cells will begin to form the missing pancreas.
his team will implant embryos into surrogate animals and take them out by the end of the month for research. The proposals are a preliminary test of Japan's new regulations. In March, Japan lifted a total ban on the cultivation of human and animal chimlines or implantation of human animals into the womb for more than 14 days.
other teams are working on different "formulations" for chimed stem cells. In January, a team at Yale University and the Axis Research Foundation in Hamden, Connecticut, described the process of using chemicals to culture IPS cells in monkeys. In April, Yale Stem Cell Biologist Alejandro De Los Angeles reported that the technology has led to similar changes in gene expression in human IPS cells. He is considering testing how these cells survive in mice or other non-human embryos.
but these jobs face obstacles in the United States. （
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