These images illustrate changes in ectodermal fate composition as a result of controlling the duration of endogenous Wnt secretion and signaling. Inhibition of Wnt prior to, or at the time of, BMP4 treatment instruct colonies (left column) to form radial patterns of neural (cyan), surface ectoderm (magenta), and placodal (red) fates, while permitting secondary Wnt secretion instructs colonies (right column) to form a ring of neural crest (yellow) at the expense of placodal fates. (Credit: Warmflash Lab/Rice) In a new system, all of the major cell types of ectoderm form in a culture dish in a pattern similar to that seen in embryos. During embryonic development, the entire nervous system, the skin, and the sensory organs emerge from a single sheet of cells called the ectoderm. While there have been extensive studies of how this sheet forms all these derivatives, it hasn’t been possible to study the process in humans, until now. The technique, based on controlling the geometry of stem cell colonies with microscale patterns, has helped them make the most comprehensive analysis yet of signaling pathways that drive patterning of human ectoderm. “There are very few possible signals th...