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ABSTRACT 11h311650110 "Neuro-endocrine Cells of the Lung - Morphology av~d Development" E. Cutz, M.D., Dept. Pathology, The Hospital for Sick Children, The Research Institute, and University of Toronto, Toronto, Ontario, Canada. The detailed morphology of pulmonary neuro-endocrine (NE cells) has been defined only during the last decade. This presentation will review the main morphologic features of NE cells in lungs of humans and other mammals and discuss the development and differentiation of these cells. The histochemical techniques and methods used for identification of NE cells in the lung are similar to those used in studies on APUD endocrine cells found in other tissues. These methods demonstrate in an indirect (or direct) way specific cytoplasmic products (amine or peptide) in cells. At light microscopy level, NE cells exhibit cytoplasmic argyrophilia, fluorogenic amine content by formaldehyde induced fluorescent technique, mask metachromasia and stainability with lead haematoxylin. Gredt variation in the results and staining reactions has been observed for different species. By electron microscopy NE cells contain characteristic neurosecretory granules (dense core vesicles, DCV). Using these techniques, NE cells have been idintified in bronchial epithelium of human lung as well as in various animal species. NE cells are distributed as single cells throughout the tracheo-bronchial tree, and in distinctive groups referred to as neuroepithelial bodies (NEB), confined to intrapulmonary airways. Although the overall morphologic and histochemical features ©f both single and group NE cells are similar, there are certain differences. The single cells are pyramidal or flask-shaped, with a base resting on the basement membrane and a thin apical process of sohne cells reaching to the lumen. In addition, cells with long, lateral cytoplasmic processes have also been identified. Based on the size and morphology of cytoplasmic DCV, three types of NE cells have been identified in human fetal lungs and in lungs of some mammalian species. The pulmonary NEB are composed of a group of closely-packed cells forming intraepithelial organoid structures. There are interspecies variations in the size, appearar.ce, localization and distribution of NEB. Distinctive feature of NEB is the presence of numerous unmyelinat.d nerve endings and positive acetylcholinesterase reactivity (in some species only). Ultrastructurally the cells within NEB show DCV's of similar morphology, although 3 n some species (rabbits) two granule types have been described. Scanning electron mic ro.copy shows that apical portions of NEB's may be either exposed to the lumen (such as in fetal rabbits) or is partially or totally covered by nonciliated epithelial cells. The discussion on the developmental aspects will focus mainly on the differen- tiation of NE cells in human fetal lung. Some features of the cytodifferentiation in rabbit fetal lung will also be reviewed.