Hybrid human–chicken embryos illuminate key developmental milestone

By Sara Reardon

Before a cluster of cells can develop into an embryo, it must first decide which end is up. But that process had never been observed in humans — until now.

For the first time, researchers have watched human ‘organizer’ cells direct the formation of an embryo’s top, bottom, front and back. They did so by developing a technique that sidesteps restrictions on research with human embryos by grafting human cells onto chicken embryos. The method, published on 23 May in Nature1, could supplant the use of human embryos in some laboratory experiments.

Organizer cells were discovered in 1924, during a series of experiments in Germany on salamanders2. A pair of developmental biologists transplanted cells from the back of one salamander embryo onto the front of another, where the cells grew into a second, conjoined salamander. This suggested that certain cells on an embryo’s back could organize their neighbours into the complex array of structures that make up an animal.

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