[Ultrastructural studies of the alveolar epithelium of reptiles living in different habitats].

The alveolar epithelium of various reptiles living in different habitats has been investigated with the light and electron microscopes. In all species pneumocytes of type I and II can be distinguished. The comparatively uniform pneumocytes I cover the blood capillaries and usually exhibit a smooth surface. Occasionally they bear in their marginal parts, especially when bordering a pneumocyte II, some microvilli. Their main characteristics are: pinocytotic vesicles and apical bundles of microfilaments. In the aquatic Trionyx particularly thin areas resembling precursor structures of a fenestration have been detected. In this species the blood-air barrier is unusually thin (280...350 nm). The thickness of this barrier otherwise varies considerably: in Agama, Chamaeleo, Hemidactylus and Pseudemys it measures normally less than 400 nm. In Tamnophis, Caiman and various turtles it measures from 400 to over 500 nm. The pneumocytes II exhibit a remarkable morphological variability which concerns a) the cellular dimensions, b) the microvilli, c) diameter, number and structure of the lamellar bodies, d) the number of mucous granules, e) the lateral and basal labyrinth. The lamellar bodies are especially numerous in the terrestrial species Hemidactylus and Testudo. Most aquatic turtles contain few pneumocytes II with few lamellar bodies. In the squamata the blood capillaries bulge into the alveolar lumen, in turtles and tortoises they are deeply embedded into the connective tissue.