α-Internexin expression in photoreceptor-like cells of the developing chicken pineal gland

Abstract BackgroundIt has been reported that some pinealocytes have similar functions to retina photoreceptors in many non-mammalian vertebrates. Two types of pinealocytes, rudimentary-receptor pinealocytes and secretory pinealocytes, have been identified in the pineal gland of juvenile gull, but whether they exist in the chicken pineal gland was unknown. Hence, the purpose of this study was to investigate chicken α-internexin (chkINA) expression patterns in photoreceptor-like cells of the developing chicken pineal gland. Method In this study, we observed distribution patterns of chkINA in pineal gland at post-hatching day (P) 1, P7, P14, P21, P35, and at young adulthood chicks by using Immunocytologic analysis, and Western blot analysis. The ultrastructure changes were also observed by electron microscope. ResultsWestern blot and immunohistochemistry showed that chkINA expression was high in early developmental stages, but decreased across development. Visinin-immunoreactive cells were also detected during early development, reaching their greatest expression at post-hatching day 7, then gradually reducing over development. Further, confocal imaging revealed that chkINA and visinin were colocalized in photoreceptor-like cells of the chicken pineal gland. Ultrastructural observations revealed rudimentary-receptor pinealocyte morphology with cytoskeletal intermediate filaments in the embryonic chicken pineal gland. Therefore, we suggest that the existence of rudimentary-receptor pinealocytes might be highly relevant for photoreceptor-like cells. Conclusions We conclude that chkINA may be a useful photoreceptor-like cell marker for studying the chicken pineal gland. In particular, it could be a major cytoskeletal intermediate filament in rudimentary-receptor pinealocytes, which are homologous to photoreceptor cells in the chicken pineal gland. From an evolutionary viewpoint, we suggest that chkINA may be located not only in photoreceptor-like cells, but also in rudimentary-receptor pinealocytes.