Fine structural and enzyme histochemical observations on the respiratory epithelium of the caecilian lungs and gills. a contribution to the understanding of the evolution of the vertebrate respiratory epithelium.

Abstract

The alveolar epithelium of larval and adult caecilians [Chthonerpeton indistinctum, Icthysphis paucesulcus] is composed of 1 cell type only, the cytoplasm of which is divided into 2: an organelle-rich part containing the nucleus and a flattened peripheral part which covers the blood capillaries. Apically, the cells bear variously shaped microvilli, which are sparse on the flattened areas of the cells. Among the cytoplasmic structures electron-dense bodies and lamellated bodies are prominent. Morphological observations suggest that in the adult the lamellated bodies are derived from the dense ones, which presumably represent lysosomes. Acid phosphatase and unspecific esterase are present in the entire alveolar epithelium. The lamellar material is extruded by exocytosis. A close structural relation between lamellated bodies and multivesicular bodies, as found in many mammals, does not exist. In the larvae there exists an extensive lateral labyrinth between neighboring alveolar cells and a considerable variability of the lamellated bodies. The latter frequently arise within or near fields of glycogen or presumably also within membrane systems. The thickness of the air-blood barrier measures, in the thinnest parts, .apprx. 1 .mu.m. In the subepithelial connective tissue, beside the spacious blood and lymphatic capillaries, smooth muscle cells, fibrocytes, collagen and elastic fibers, mast cells and nerve fibers (with granule-containing varicosities) are found. Within the low septa, which divide the inner surface of the wall of the lung into flat polyangular chambers (alveoli), pieces of hyaline cartilage occur in the proximal parts of the lung. Macrophages rarely occur in the caecilian lung. They contain acid phosphatase, .beta.-glucosaminidase and unspecific esterase and are located in the connective tissue and within the alveolar epithelium near epithelial cells exhibiting structural alterations. The epithelium covering the gills of young larvae is composed of 2 layers of squamous cells. The cells of the surface layer contain large secretory granules.