The presence of thyroid transcription factor 1 (TTF1) in the respiratory stomach of Ancistrus multispinis (Regan, 1912) and the respiratory intestine of the bronze corydoras Corydoras aeneus (Gill).

The emergence of the capacity for squamous epithelial cell proliferation appears to have been a "critical innovation" enabling the gradual evolution of vertebrate lungs. Among air-breathing fish, species classified as gut-breathing fish (GBF) provide a particularly valuable model for investigating early stages in the development of air-breathing structures. In this study, two GBF species were examined: Ancistrus multispinis, which uses the stomach, and the bronze corydoras Corydoras aeneus, which uses the posterior part of the intestine, as accessory respiratory organs. Analyses were performed using antibodies against thyroid transcription factor 1 (TTF1), a protein expressed in the alveolar epithelium and essential for maintaining the normal function of cells of the terminal respiratory unit. Western blot analysis revealed immunoreactive bands corresponding to TTF1 in the respiratory intestine of C. aeneus and in the stomach of A. multispinis, with both monomeric and dimeric forms detected in both species. Immunohistochemical analysis further demonstrated positive immunoreactivity for TTF1 in epithelial cells, connective tissue, and the muscular layer of the intestine of C. aeneus, whereas in the stomach of A. multispinis positive staining was confined to epithelial and connective tissue cells. Spectral analysis precisely localized anti-TTF1 immunoreactivity to the cytoplasmic region of stomach cells in A. multispinis. In both species, cells exhibiting stronger TTF1 immunoreactivity appear to represent progenitor-like cells, analogous to alveolar type II cells in mammalian lungs, which under appropriate conditions, including hypoxia, initiate the formation of squamous epithelium (alveolar type I cells).