Immunohistochemical study of α-keratin, loricrin, filaggrin-like protein, and transglutaminase-1 expression in orthokeratinized and parakeratinized epithelium of the tongue of domestic goose (Anser anser f. domestica) during embryonic development

Cornification is a terminal differentiation process involving the synthesis of structural proteins, including α-keratin, loricrin and filaggrin-like protein, with the enzymatic contribution of transglutaminase 1 (TGM-1). The present study aimed to characterize the expression profiles of these proteins in the ortho- and parakeratinized epithelia of the lingual mucosa in domestic goose embryos from day 9 to day 25 of incubation, using immunohistochemical analysis. The spatial expression of the structural proteins and TGM-1 in both lingual epithelia was analyzed during three developmental stages: embryonic, transformation, and prehatching, revealing a slight temporal shift. During the embryonic stage, the expression of structural proteins and TGM-1 began, indicating early epithelial cytodifferentiation. At the transformation stage, differentiation of the epithelia into three layers and periderm formation with a marked increase in expression of the studied proteins occurred. For the first time, differences in the expression patterns of structural proteins and TGM-1 were observed in the periderm. By the prehatching stage, a cornified layer was present in both epithelia. Notably, only α-keratin expression in both epithelia and filaggrin-like protein and loricrin expression in cornified layer of the orthokeratinized epithelium resembled the adult pattern. These findings indicate that cornification is initiated during the transformation stage, approximately two-thirds of the embryonic period. Before hatching, epithelia have a cornified layer. However, the expression patterns of structural proteins and TGM-1 differ from those observed in adults, suggesting that cornification continues after hatching.