Ultrastructural localization of tritiated histidine in down feathers of the chick

During the differentiation of cells in developing down feathers of the chick embryo, keratin and associated proteins are synthesized. Previous studies indicated that a histidine-rich protein with a different amino acid composition but similar molecular weight and localization of feather keratin is produced in forming feathers. The precise localization of the histidine-rich protein, either in feather barb and barbule cells and/or in supportive cells (sheath, barb vane ridge and cylindrical cells) is not known. The present ultrastructural autoradiographic study on developing feathers in the chick embryo shows the subcellular localization of histidine-labeled molecules, presumably representing histidine-rich proteins. Two hours after injection of tritiated histidine in chick embryos, the labeling is mainly present in the cytoplasm or is associated with forming keratin filaments of barb and barbule cells. Neither keratin filaments nor dense granules of barbule cells are specifically or prevalently labeled with tritiated histidine. No labeling is seen in periderm granules or in keratinaceous dark granules of sheath and barb vane ridge cells localized among barbule cells. The present study indicates that histidine-rich material rapidly associates with newly synthesized filaments of keratin. This observation suggests that histidine-labeled material contributes to the formation of long keratin filaments with axial orientation that are utilized for the elongation of barb and barbule cells.