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Skin development forms the epidermis, underlying supporting tissues and appendages such as hair follicles or feathers, depending on the lineage. Interactions between an epithelium and underlying mesenchyme help specify where appendages arise and how they grow.

Patterning is not just a list of expressed genes: changes in cell movement, tissue forces and local signaling can affect spacing and shape. A feather placode provides a localized precursor in which molecular identity and physical cell aggregation can be studied together.