Keratinization of sheath and calamus cells in developing and regenerating feathers

The present ultrastructural and immunocytochemical study on developing and regenerating feathers in the chick, zebrafinch and quail describes the formation of alpha-keratin cells in the sheath and their transition into beta-keratin cells in the calamus. In the first stages of feather formation cells are produced in the lower part of the follicle, migrate upward and form the elongating sheath of the feather filaments that grows outside the follicle. Sheath cells initially (anagen) contain bundles of alpha-keratin filaments. By the end of anagen at the base of the feather inside the follicle cells in continuation with the sheath produce homogenous bundles with the aspect of beta-keratin filaments. These bundles are immunoreactive for beta-keratin antibodies suggesting that beta-keratin is added to the initial framework of alpha-keratin. The packing of dense and hard corneous material in these cells eventually forms the calamus. The latter corneous tissue replaces at the base of feathers the softer sheath present in more apical regions of the feather filaments. The increase of feather (beta)-keratin in cells of the calamus determines the formation of long bundles of corneous material. The modality of cornification in feathers is compared to the process of hard cornification of reptilian scales and claws and mammalian hairs and nails.