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The shortage of human donor organs has resulted in emphasis on the controversial scientific study of stem cells, leading to the astounding creation of the first living chimera — an embryo with cells from two different species. 

Successful experimentation in chimerism began when scientists discovered how to grow rat tissue inside mice. They proceeded with deleting certain cells in mice responsible for organ growth, replacing them with the appropriate rat cells, and watching the animals survive and flourish into adulthood. This advanced manipulation was accomplished with the use of CRISPR-Cas9, a specific genome editing device.

Scientists took the trial one step further and attempted to introduce cells from rats into pig organs — a manipulation that failed, but not surprisingly due to the extreme differentiation between pig and mouse physiology.

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Pigs and humans have historically been regarded for the similarity between their organs, leading to the theoretical scientific efficacy for growing human cells inside of pigs. Trial and error using three different types of human cells resulted in the concluded necessity for human induced pluripotent stem cells, referred to as iPSCs, as opposed to the more generalized PSC cells utilized in the mouse and rat experiment.

This particular type of human cell injected into pig embryos and subsequently inserted into sows resulted in the growth of adult pigs — containing viable human cells. 

While the percentage of human cells produced was relatively low compared to the mouse and rat experiment, the mechanism behind this success holds undeniable promise for the future of stem cell development.

Of the 186 embryos created, each had an estimated 1 in 100,000 human cells reported expert and author Jun Wu out of the Salk Institute. The full study is published in the journal Cell

The successful creation of this first human-animal hybrid is one step in the right direction towards the development of human organs in a laboratory setting.