The Pulmonary NEB ME Is a Complex Intraepithelial Unit.

Abstract

Among the intrapulmonary myelinated vagal sensory airway receptors, pulmonary neuroepithelial bodies (NEBs) definitely reveal the most complex organisation. This updated review aims at delivering the broad and thorough knowledge of the system that is essential for understanding its physiological relevance.Complexly organised pulmonary NEBs are an integral part of the intrapulmonary airway epithelium of all air-breathing vertebrates. For decades, a quest has been going on to unravel the functional significance of these intriguing structures that appear to be modified in the course of many pulmonary diseases.The pulmonary NEB microenvironment (ME) is composed of organoid clusters of pulmonary neuroendocrine cells (PNECs) that are able to store and release neurotransmitters and are closely contacted by extensive (mainly afferent) nerve terminals, emphasising a potential receptor/effector role and probable signalling to the central nervous system. PNECs are largely shielded from the airway lumen by a special type of Clara cells, the Clara-like cells, with potential stem cell characteristics.Since pulmonary NEBs are widely dispersed in the airway epithelium, and represent less than one percent of the cells in the epithelial lining, investigating the NEB ME largely depends on its unequivocal microscopic identification. Nowadays, multidisciplinary approaches allow to combine functional morphological investigations in cryosections and cleared whole lungs, live cell imaging in lung vibratome slices, and selective gene expression analysis after laser microdissection of genetically tagged NEBs.So far, functional studies of the pulmonary NEB ME revealed that PNECs can be activated by various mechanical and chemical stimuli, resulting in a calciummediated release of neurotransmitters. A number of publications in the past decades have exposed NEBs as potential hypoxia sensors.In the past few years, combination of in vivo experiments and gene expression analysis unveiled the pulmonary NEB ME as a quiescent stem cell niche in healthy postnatal mouse lungs. The stem cell population was shown to be activated by transient mild inflammation, and silencing appears to involve bone morphogenetic protein signalling that may be mediated by vagal afferents.Today, it is clear that only an integrated approach that takes all current information into account will be able to explain the full role of the pulmonary NEB ME in health and disease.