Human stem cell—shown in green—integrating into a mouse embryo / Salk Institute for Biological Studies

Scientists have stumbled upon a new type of human cell, capable of becoming any organ or tissue, that may help unravel the secrets of early embryo development. They may even open a new door into growing human organs in large animals for research and medicinal purposes. The study has been published in Nature

The cell is known as a pluripotent cell. These are shape-shifting cells that are capable of giving rise to all other types of cells in the body, an ability that makes them precious tools in medicine.

A developmental team at the Salk Institute for Biological Studies prepared to transplant human pluripotent cells into mice embryos by first growing human cells in a dish of different combinations of nutrients and chemicals. One of the combinations turned out to be very fruitful; the cells grew and multiplied better than before. These cells hadn't been observed previously: they had different patterns of metabolism and gene expression to other known pluripotent cells.

The new type of pluripotent cell has traits that make it excellent for use in the laboratory. This cell clones itself very efficiently, has a stable passage in culture, and can be easily genetically modified. This is exciting for pluripotent cell research; testing cells is easier if you can grow them in a lab instead of refining samples taken from humans.

The team's first attempts to graft these new pluripotent cells into mouse embryos were unsuccessful. It turns out that the pluripotent stem cells were a bit picky about the location where they were injected into the mouse embryo and would only integrate when injected into the perfect spot. Any random place just wouldn't do. For this reason, they have been called 'region-selective pluripotent stem cells' (rsPSCs).

Integrating human cells into a mouse embryo is all very well, but it's nothing compared to the team's plans for the future. Eventually, the scientists want to grow fully-formed human organs in larger mammals, but this is a bit more complicated.

Replacing animal organs with human organs is done using knockout gene replacement. For example, you take a pig embryo and insert some artificial DNA. This DNA has been specially formulated to replace or disrupt a specific gene in the embryo (knocking it out). For example, to prevent a pig from growing a pig pancreas. Scientists then insert a human pancreas stem cell that will grow in place of a pig pancreas. The resulting, fully-grown animal would be a human/pig chimera; a pig with a human pancreas.

There are a lot of unknown variables and limitations to creating a chimera. For example, the animal's immune system might reject the human organ and attack it, or the developmental rates of the human organ and the animal might be different.

Despite these difficulties, these chimeras have a lot of potential for the future of medicine: Human organs for transplants are often in short supply and growing them in animals would solve this problem.

"Of course, the ethical implications behind creating a human-animal chimera for the purpose of obtaining human tissues and organs to save lives of millions need to be carefully evaluated," says Juan Carlos Izpisua Belmonte from the Salk Institute to The Scientist. Before we start to grow human organs in animals for harvesting, there will likely be a lot of debate about how humanity can morally justify farming organ donors from animals.

[Via Salk Institute press release, Nature]

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