Ultrastructural immunolocalization of involucrin in the medulla and inner root sheath of the human hair

The participation of involucrin in the cornification of the human hair has been studied by light and electron microscopy immunohistochemistry. Immunoreactivity for involucrin is absent in keratinized cuticle and cortical cells although some immunolabeling is observed in the corneous membrane of internal cortical cells surrounding the hair medulla. Conversely, immunolabeling for involucrin is present in the cytoplasm of keratinizing cells of the medulla and inner root sheath. During the maturation and final cornification of medullary and inner root sheath cells the immunolabeling for involucrin tends to concentrate in the peripheral cytoplasm and along the cornified cell plasma membrane in both medullary and inner root sheath cells, a pattern similar to that known for corneocytes of the epidermis. This observation suggests that in the hair involucrin mainly participates in the formation of the corneous material of the medulla and inner root sheath in conjunction with trichohyalin, probably by the formation of isopeptide-bonds. Therefore, together with trichohyalin, the cross-linking due to involucrin is also responsible for the mechanical resistance of the corneous trabeculae present among the empty spaces of the medulla of the human hair.