All complex animals have a head and a tail or, to put it more correctly, an anterior [top] end and a posterior [bottom]. This is set up at a very early stage in development, when the animal is just a little ball of cells. Three different views of a mouse embryo at this stage – made with a high-powered laser microscope – show individual cells stained with a dye that fluoresces blue under laser light. One of the key players in distinguishing where the head forms is a small group of cells called the anterior visceral endoderm (AVE), highlighted here in green. It's important that the AVE stays at the head end, so to ensure this happens, new cells (dyed red) are made mainly in the middle of the embryo. This keeps the AVE in the right place and out of the way of any confusing ‘tail’ signals.
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