PRODUCTION OF REGULATORY FACTORS IN THE RESPIRATORY SYSTEM OF VERTEBRATES

Among the different cell types present in the respiratory tract of the vertebrates, some (epithelial, endothelial, neural) specialise in the production of regulatory factors. Endocrine cells occur either single , spread throughout the epithelial lining, or in innervated groups, ca lled 'neuroepithelial bodies' (NEBs). In mammals, these endocrine cell s may be involved in lung maturation during perinatal life and in chem oreception. A neuroendocrine diffuse system is present in the respirat ory organs of ail classes of vertebrates. In amphibians and reptiles, single endocrine cells as well as NEBs are located in the apices of th e lung septa. The respiratory tract shows nerve fibres immunoreactive to several neuropeptides. Since some neurons and fibres contain NO syn thase a broad evolutionary presence of NO-releasing neurons, probably involved in the control of relaxation, is suggested.