COMPARATIVE HISTOLOGY OF PULMONARY NEUROENDOCRINE CELL SYSTEM IN MAMMALIAN LUNGS

It is increasingly recognized that neuroepithelial endocrine cells and organoid clusters of these cells termed neuroepithelial bodies are wi dely distributed in the respiratory tract of vertebrates. This review focuses on the neuroepithelial endocrine system in the airway epitheli um of mammals according to observations made in a wide range of specie s, as it appears in light and electron microscopy by means of various visualization techniques under normal and experimental conditions. Bec ause there are similarities but also marked differences between specie s, wherever possible studies in human airways are emphasized. Conventi onal histochemical and ultrastructural studies including microspectrof luorimetry have demonstrated the amine-handling properties and the pre sence of neurosecretory granules in neuroendocrine cells of the epithe lium of all mammals studied so far. The neuroepithelial bodies are inn ervated by morphological afferent- and efferentlike synaptic specializ ations. These, together with the presence of reciprocal synapses, emph asize that these cells may be involved in dual functions of chemorecep tion and secretion. Light and electron microscopic immunohistochemistr y has demonstrated that neuroepithelial endocrine cells may contain va rious biogenic agents, suggesting that the neuroendocrine system is hi ghly heterogeneous. Neural elements may interact in a complex manner, and the activation of certain neural pathways may control the release of biogenic substances to influence physiological airway functions. Th ey may be particularly relevant in pulmonary diseases. Consequently, t he function of this system is complex and it is highly probable that m any of its aspects are still not elucidated. (C) 1997 Wiley-Liss, Inc.