Ultrastructural observations on the lung of Pelophylax kl. esculentus tadpoles during development

The anatomical organization of the lung in Pelophylax kl. esculentus tadpoles has been described in early and middle-staged tadpoles (Gosner stages 32 and 39). The lung's ontogenic development has been studied using light microscopy and both scanning and transmission electron microscopy. In the early-staged tadpoles, the lung is well vascularized, but it is provided with only two internal septa types and devoid of ciliated epithelium and lamellar bodies. The complete differentiation process is putatively reached in the middle-staged tadpoles (GS 39), and numerous septa of first, second, and third-order deeply protruding into the lung lumen could be recognized. The histological and ultrastructural features at this stage correspond to that described in the adults' lungs, but mucous secreting goblet cells could not be detected. Pneumocytes, with numerous apical microvilli, surround the growing network of capillaries and show a cytoplasm rich in dense bodies and few mature lamellar bodies. On the epithelial surface, a thin layer of mucous covering the underlying epithelium could also be seen. Neuroepithelial bodies, supposed to be involved in chemoreception, are organized in clusters on the first order septa and surrounded by cytoplasmatic processes originating by neighboring cells. The lung arrangement of P. kl. esculentus is compared with that of other Anurans in order to further elucidate the putative role of the lung in gas exchanges. The present studies reduce the information gap existing in the literature regarding lung morphology and development in amphibian larval stages, also contributing to the discussion on the onset of airbreathing respiration in tadpoles.