Structure and function of the adventitial and mucosal nerve plexuses of the bronchial tree in the developing lung

1. The aim of the present study was first to determine when airway smooth muscle first appears in the airways of the developing foetal lung and when its contractility is mature and, second, when the airway smooth muscle becomes innervated, both structurally and functionally. 2. Narrowing and relaxation of the airways of the intact bronchial tree from the lungs of foetal pigs at varying stages of gestation was recorded by real time video microscopy. Nerves and smooth muscle were stained immunohistochemically and their spatial relationships were visualized by confocal microscopy. 3. From early in gestation (pseudoglandular stage/branching morphogenesis), airways were covered with a single layer of smooth muscle cells orientated cylindrically around the airway wall. Thick nerve trunks containing loosely packed nerves, with ganglia forming at their junctions ensheathed the airways to the growing tips, with a network of fine varicosed nerves lying on the smooth muscle cells. Some of these nerves were functional and cholinergic, as electrical field stimulation caused substantial narrowing that was blocked by atropine and by tetrodotoxin. By mid-term, an extensive plexus of nerves, well-defined ganglia and varicosed nerves to the smooth muscle had developed. Some nerves extended through the airway smooth muscle to form the elements of a mucosal nerve plexus in conjunction with developing vascular tissue forming a bronchial circulation. In 3 week postnatal pigs the distal and terminal airways were densely supplied with varicosed nerves to the smooth muscle. The ganglia were more centrally located. 4. An abundant network of calcitonin gene-related peptide (CGRP) nerves with prominent nerve endings lay just below the surface of the epithelium. The nerve bundles to the terminal arterioles in the mucosa also contained CGRP-positive fibres as well as sympathetic nerves (neuropeptide Y- and tyrosine hydroxylase-positive). It is hypothesized that these epithelial and arteriolar CGRP nerves form the local axon reflex purported to cause neurogenic inflammation. 5. The spontaneously active tone exerted by the airway smooth muscle from early in gestation is hypothesized to provide the force across the airway wall and adjacent parenchyma that is the stimulus for lung growth in foetal life.