Wedge cells during regeneration of juvenile and adult feathers and their role in carving out the branching pattern of barbs

The present ultrastructural study on regenerating feathers emphasizes the role of supportive cells in determining the branching pattern of barbs. Supportive cells are localized among developing barb and barbule cells, in marginal plates, and underneath the feather sheath, and their differentiative fate, in general, is a form of lipid degeneration. The latter process determines the carving out of barb branching in both downfeathers and pennaceous feathers. In the latter feathers, some supportive cells (barb vane cells and cylindrical cells of marginal plates) degenerate within each barb ridge leaving separate barbules. Other supportive cells, here termed wedge cells, form columns of cornified material that merge into elongated corneous scaffolds localized among barbs and the rachis. This previously undescribed form of cornification of supportive cells derives from the aggregation of periderm and dense granules present in wedge cells. The latter cells give origin to a corneous material different from feather keratin that may initially sustain the early and soft barbules. After barbules are cornified the supportive cells scaffolds are eventually sloughed as the sheath breaks allowing the new feather to open up and form a planar vane. The corneous material of wedge cells may also contribute to molding of the overlapped nodes of barbule cells that form lateral spines or hooklets in mature barbules. Eventually, the disappearance of wedge cell scaffolding determines the regular spacing of barbs attached to the rachis in order to form a close vane.