Figure 3. Models of hair follicle and feather patterning.

Top: In hair follicle patterning a Turing RD system generates a molecular pre-pattern, consisting of high regions of WNT signalling, which defines sites of future placode formation in the epidermis. FGF20 generated by the placode attracts underlying dermal fibroblasts which then aggregate to form the dermal condensate. The placode and dermal condensate then undergo several subsequent stages of morphogenesis to produce mature hair follicles. Bottom: Feather patterning begins at the midline with the generation of a single row of feathers. As development continues new rows are added sequentially in a wave spreading from the midline to generate a hexagonal pattern in flighted birds. Both MC models, involving mechanical instability, and Turing-like systems incorporating FGF20-mediated chemotaxis, have been proposed to describe the process. Refer to the main text for more details. Images created with BioRender.com.