Histidine-rich protein B of embryonic feathers is present in the transient embryonic layers of scutate scales.

Based on its amino acid composition and N-terminal sequence, a polypeptide (HRP-B) has been identified as a member of the avian histidine-rich protein (HRP) family. An antiserum against HRP-B has been used to localize this polypeptide in developing feathers and scales of chick embryos. HRP-B was first detectable in the barb ridge cells of feathers at 13 days of incubation and progressively appeared in the distal/proximal and peripheral/central gradients observed previously for the feather-type beta keratins in developing feathers. The HRP-B polypeptide was detected only in the embryonic layers of scutate scales. It first appeared at 16 days of incubation and was not found in the differentiated beta strata of these scales. At no time during the development of reticulate scales or apteric skin regions did the epidermal cells or cells of the embryonic layers express HRP-B. The transient expression of HRP-B by the embryonic layers of the scutate scale epidermis is discussed in light of the feather-forming potential of the presumptive epidermis of the scutate scale-forming region.