Novel tissue mechanics-guided cellular flows drive the formation of feather follicles.

Complex tissue architecture is achieved through multiple rounds of morphological transitions. Here, we analyzed cellular flows and tissue mechanics during avian skin development by employing chicken and transgenic quail skin explant models. We demonstrate how novel cellular flows initiate chemo-mechanical circuits that guide epithelial protrusion, folding, invagination, and spatial cell fate specification. During initial feather bud formation, stiff dermal condensates protrude vertically from the locally softened epithelial sheet. As the bud elongates, it stretches the epithelial cells at the base, thus mechanically activating YAP, which causes the epithelial sheet to fold downward and form a stiff cylindrical wall that invaginates into the skin. This stiff epithelial tongue is essential for the compaction and formation of the tightly packed dermal papillae. These topological transformational events are mechanically interconnected, and the completion of one circuit initiates the next. In contrast, during scale development, the rigid epithelial sheet restricts dermal cell flows, preventing further topological transformation. Based on these findings, we developed a topological transformation model describing how this process enabled the evolution of feather follicles from scales.