Dynamic Expression of α‐Internexin in the Chicken Pineal Gland: A Study of Developmental, Structural, and Functional Roles

The pineal gland is a photoneuroendocrine organ that regulates circadian rhythms, primarily through rhythmic melatonin secretion. In nonmammalian vertebrates such as birds, pinealocytes retain photosensory and endocrine functions. α-Internexin, a neuronal intermediate filament protein, has been implicated in neurodevelopment and cytoskeletal stability. Its expression in the retinal photoreceptors and pinealocytes has been reported in several vertebrate models; however, its expression pattern in the chicken pineal gland remains unclear. In this study, we used immunohistochemistry and western blot analyses to investigate the developmental expression pattern of α-internexin in the chicken (chkINA) pineal gland from embryonic Day 15 (E15) to post-hatching Day 90 (P90). We also compared its expression with that of two functional markers, visinin and tryptophan hydroxylase 1 (TPH1), to clarify the potential role of chkINA during pinealocyte differentiation. Our findings revealed that chkINA was abundantly expressed in the pineal gland as early as E15, and remained stably expressed throughout development and maturation. Despite the dynamic changes in the expression of visinin and TPH1, chkINA levels remained consistent. These results suggest that chkINA may serve as a critical structural factor that supports pinealocyte maturation during functional transition from photoreceptive to endocrine states.