The ultrastructural immunolocalization of loricrin in the hairy epidermis of the platypus (Ornithorhynchus anatinus, Monotremata) indicates it contributes to the formation of the cell corneous envelope

The fine localization of loricrin, a major protein of the cell corneous envelope, is known for the epidermis of few species of placental mammals. The localization of this protein has been studied by immunocytochemistry in the hairy epidermis (orthokeratotic) of the platypus, a representative of monotremes, and compared with the localization of the protein in the epidermis of placental mammals. In the hairy epidermis small keratohyaline granules of 0.1-0.3 μm are present in transitional cells of the stratum granulosum. An anti-loricrin antibody labels the pale component of keratohyaline granules, the corneous mass of transitional corneocytes, and mainly the corneous cell envelope of corneocytes in the stratum corneum. The last pattern resembles that of the hairy epidermis of placental mammals and differs from the diffuse distribution of loricrin previously described in the parakeratotic epidermis of the platypus. The study confirms that the final pattern of distribution of loricrin in the hairy epidermis of the platypus is as previously observed in corneocytes of placental mammals.