Cell interactions in barb ridges of developing chick downfeather and the origin of feather branching

Abstract

The epidermis of developing feathers comprises outer and inner periderm, subperiderm and germinal layer. The inner periderm gives origin to sheath and barb ridge vane cells that contain alpha‐keratin and form a belt around feather filaments. This produces mechanical resistance, forcing the inner epidermal cell layers to fold inward and form barb ridges. Mesenchymal‐epidermal contacts occur from the mesenchymal core to external regions of barb ridges and involve thin cytoplasmic processes (filopodia) from mesenchymal cells. Subperiderm cells produce feather keratin that remains in derived barb and barbule cells displaced into two barbule plates and a ramus. Barb medullary cells undergo lipid degeneration while barbule cells accumulate feather keratin and cornify. Supporting cells surround barb and barbule cells and later degenerate leaving separate barbules, while marginal plate cells disappear leaving separate barbs. Caspase‐3, a marker for apoptosis, is absent in cells of barb ridges. Degenerating cells in developing feathers are eliminated by necrosis following vascular retraction. Caspase‐14 is present in cornifying sheath cells as well as in the corneous layer of the follicular epidermis. The sheath detaches from barbs through the formation of a sloughing layer. The differentiation of supporting cells among barb/barbule cells allows the origin and evolution of the branching pattern of feathers.