Squamous epithelium formation in the respiratory intestine of the bronze Corydoras Corydoras aeneus (Callichthyidae Teleostei)

Accessory respiratory organs in fish exhibit great diversity but share the presence of numerous capillaries covered by a simple squamous epithelium. The adoption of the intestine for respiratory function needs certain special modifications. In this study, we explored immunohistochemical and metabolic fingerprint features that could underlay this adaptation in bronze corydoras Corydoras aeneus. Immunohistochemical localization of the cytoplasmic domain of epidermal growth factor receptor (EGFR) in the respiratory part of intestine demonstrated a strong positive immunoreaction in epithelial cells and connective tissue. Fourier Transfer Infrared (FTIR) spectroscopy coupled with chemometrics discriminated between anterior and posterior region of intestine in terms of secondary structure of proteins and the abundance of p-cresol and other phenolics. The latter were reduced in the posterior part of intestine, indicating the cessation of digestive function in this region. It has been suggested that aquatic hypoxia via endocrine cells (hypoxia-sensitive) activate EGFR, which induce proliferation of squamous epithelial cells, thereby enabling gas diffusion in the posterior part of intestine. It seems that hypoxia and normoxia are opposed conditions adjusting the production of squamous epithelial cells in this intestine. The physiological role of EGFR in the respiratory intestine of bronze corydoras is of interest not only from an evolutionary aspect but also in terms of a potential model for observations process proliferation squamous epithelial cells. Future investigations on the molecular responses to different water oxygen levels in air-breathing bronze corydoras fish are required to clarify the mechanism responsible for squamous cell proliferation.