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The vertebrate body is covered by either scales, feathers or fur to provide warmth and protection. Comparing and contrasting the formation of these different integument appendages may provide insights into their common embryonic origin as well as evolutionary divergence. The reptile integument is mainly made of scales [1]. In birds, there are two major integument appendages: scales on the foot and feathers on most of the rest of the body [2••]. Scales provide protection and prevent water loss. The major innovation of the avian integument was the evolution of feathers, which provide novel functions such as insulation, display (communication), and flight.

Chickens have three major types of scales, which are morphologically similar to reptile scales (Figure 1a, b [1, 3]). Reticulate scales are found on the foot pad: they are radially symmetric and express|-keratin only. Scutate scales are large and rectangular and are the …