ABSTRACT
The tracheal epithelium of lizards and snakes differs with respect to the equivalent epithelium of tortoises. While the first two orders have a prismatic pseudostratified epithelium with ciliated, basal and secretory cells or flat pseudostratified epithelium made up of basal and secretory cells, the epithelium in tortoises is the same in both the cartilaginous and membranous tracheal areas, with a uniform distribution of ciliated cells and goblet cells. The morphology of the cells that compose the bronchial-type epithelium in Chelonia, as well as in Ophidia and Squamata, is very similar to that of the trachea. In the extrapulmonary airways of lizards and snakes, secretory cells do not show the same morphology as the mucous cells of turtles and are classified as a serous type. In the trachea, solitary neuroendocrine cells can be observed by silver and immunohistochemical techniques and electron microscopy in tortoises, lizards and snakes. They have also been observed in Chelonia, where they present serotonin. Under the electron microscope, neuroendocrine cells of Chelonia, Lacertilia and Ophidia show granules of small diameter (90–120 nm approx.). Neuroendocrine cells, alone or forming groups, the latter denominated neuroepithelial bodies, are found in the lung trabecular In most species these cells show immunoreactivity to serotonin and calcitonin. Other regulatory peptides including calcitonin gene-related peptide, Leu-enkephalin, endothelin-1 and -2 are found depending on the species studied. Ultrastructurally both solitary neuroendocrine cells and those that form 186neuroepithelial bodies show low electron density and present dense-core granules in the basal region. The diameter of the granules, which have been described ultrastructurally in a variety of reptiles, varies between 120 and 140 nm. The neuroepithelial bodies are preferentially located in the primary trabeculae where clusters, formed by a varying number of granule-containing cells, are surrounded by flattened secretory or ciliated cells. Abundant peptidergic innervation is observed in the lung, along with vascularization and ganglionic cells. In conclusion, the neuroendocrine system in the respiratory system of reptiles shows a high degree of morphological organization, although more studies in more species are needed in order to demonstrate the differences that exist between the reptile families.
