This means that human-monkey chimeras might be able to support more intimate and extensive mixtures of the two types of cell, with less predictability about where they might end up growing. Several embryos still had around 4-7% of human cells by day 15. But the researchers say that, in general, the human cells seemed to integrate better with the monkey cells than they had in pig embryos. Of the 132 chimeric embryos the researchers made and cultured in a dish, most died before day 17 after fertilisation. “If we can learn about the crosstalk between the cells,” says Wu, “we might be able to improve on the pig work.” ![]() Because our evolutionary relationship to monkeys (the researchers used macaques) is closer – we diverged around 20-30 million years ago – the researchers figured the two cell types might get along better, and hoped to discover more about what promotes their compatibility. That’s why he and his colleagues, collaborating with Weizhi Ji’s group at the Kunming University of Science and Technology in Yunnan, China, have now made human-monkey chimeras. Izpisua Belmonte suspects it is because we’re too distantly related to pigs: our branch of the evolutionary tree diverged from theirs 90m years ago. The Chimera of Arezzo, a 5th century BC bronze sculpture, depicts the chimera of Greek mythology, part lion, part goat and part snake. “Why is it so difficult for the human cells to survive?” ![]() “It was a disappointing outcome,” says Jun Wu, who was a postdoctoral researcher in the Salk team and is now at the University of Texas Southwestern Medical Center in Dallas. In 2017 a team led by Juan Carlos Izpisua Belmonte at the Salk Institute in La Jolla, California, showed that human stem cells added to pig embryos could survive for up to four weeks.īut in contrast to rat-mouse chimeras, the human cells grew in only small and dwindling proportions: not enough to generate human tissues and organs. But rodents obviously can’t be the hosts for full-grown human organs for that you need a larger animal. The technique has been shown to work for growing rat organs, such as pancreases, in mice, and vice versa. The embryo will then use the guest stem cells to make it instead. They created a “niche” for the foreign cells by genetically modifying the animal embryos so that they can’t grow the target organ – a liver, say – on their own. Several years ago, Japanese biologists Hiromitsu Nakauchi and Toshihiro Kobayashi pioneered a trick for getting cells injected into an embryo from another species to develop into just the organ you want. In particular, the shortage of human organs such as kidneys for transplantation – which leads to many potentially preventable deaths from organ failure – could be solved by growing “human” organs in animals. Such experiments might evoke grotesque echoes of HG Wells’ maverick mad scientist Doctor Moreau, who “humanised” animals by surgery. Some of these cells can survive for days. Human embryonic stem cells have previously been inserted into sheep foetuses and mouse, rabbit and pig embryos. The unease provoked by such biological patchworks is considerably stronger when the chimeras contain human cells. ![]() ![]() In some circumstances, the foreign cells can keep growing and thriving in their new host. Chimeras are generally made by transferring stem cells, capable of growing into a range of tissue types, from one species into the embryo of another. A chimera with a mixture of goat and sheep tissues, called a geep and having a goat’s head but a sheep’s woolly body, was reported in 1984. We’ve long known that these mosaic-style animals are possible. Living entities containing the cells or tissues of more than one species are technically called chimeras – the name of a legendary monster that Homer described in The Iliad as “lion-fronted and snake behind, a goat in the middle”. What strange hybrids are we creating, and why? So the recent report by a team in the US and China of embryos that contain a mixture of human and monkey cells mines an ancient seam of anxiety.
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