Regulation and guidance of feather epithelial stem cells

The feather is the dominant organ of Aves, and feathers molt regularly, representing a highly robust growth and regenerative ability. How do feather follicles manage this daunting task? Are there stem cells in feather follicles, which are responsible for their growth and regeneration? The formation of highly organized structures depends on regulated growth and differentiation. Given the existence of vast variation of different sizes and shapes, the feather offers an interesting model to study the organization principles. How do the feathers achieve their final forms? What are those guiding principles behind this morphological diversity?; Here we identify a novel "collar bulge" within follicles that contains skin stem cells. We show feather epithelial stem cells are configured as a ring and reside in the proximal follicle. The ring is horizontally placed in radially symmetric feathers, but tilted toward the anterior (rachis) side in bilaterally symmetric feathers. At the ramogenic plane of flight feathers, there are asymmetric molecular gradients that affect branching morphogenesis. RCAS mediated misexpression of Wnt3a, beta-catenin or their antagonists convert bilateral to radial symmetry. Local perturbations with growth factor coated beads show Wnt3a can re-orient barbs and create a duplicated rachis. We propose a model that links stem cell topology, molecular gradients, and biological forms. Thus complex feather forms can be built from epidermal stem cells through simple topological arrangements of cellular activities. During feather growth, new cells are added at the proximal end of the follicle, which then undergoes branching morphogenesis and terminal differentiation along the proximal-distal axis. Here we show that the balance of FGF signaling and its antagonist is important for patterning these events. Perturbation of the balance leads to dramatic phenotypic change in the follicle. Overexpression of FGF10 promotes proximal follicle structures, and inhibits feather branch/differentiation. Overexpression of Sprouty4 promotes distal follicle structures at the expense of proximal structures; consequently, the follicle lost the regeneration ability. These results suggest that FGF signaling is important in patterning feather growth and differentiation.; We then further showed that the regeneration process does not require the collar bulge. The epithelial stem cells can actually be regenerated from competent epithelia if appropriate inductive mesenchyme is provided. Therefore the stem cells represent a status, not necessarily a fixed group of cells, and are actively maintained in the micro-niche.; Taken together, these results stand for a major advance on the study of feather follicles, and skin biology in general. Knowledge acquired here through cell activity and molecular regulation analysis might be helpful in other occasions, e.g. developmental biology, cancer, stem cell and regenerative medicine.