Development of the innervation and airway smooth muscle in human fetal lung

Human and porcine fetal airways have been shown to contract spontaneously from the first trimester, the latter also contracting in response to neural stimulation. Our object was to map immunohistochemically the innervation and its relationship to the airway smooth muscle (ASM) in the human fetal lung from early gestation to the postnatal period. Whole mounts of the bronchial tree were stained with antibodies to the pan-neuronal marker protein gene product 9.5, the Schwann cell marker S-100, and the ASM contractile protein alpha-actin, and imaged using confocal microscopy. By the end of the embryonic period (53 d gestation), the branching epithelial tubules in the primordial lung were covered with ASM to the base of the terminal sacs. An extensive plexus of nerve trunks containing nerve bundles, forming ganglia, and Schwann cells ensheathed the ASM. By 16 wk (canalicular stage), maturation of the innervation was advanced with two major nerve trunks running the length of the bronchial tree, giving rise to varicosed fibers lying on the ASM. An extensive nerve plexus in the mucosa was also present. The distal airways of infants who had died of Sudden Infant Death Syndrome were also covered with smooth muscle and were well innervated. Thus, an essentially complete coat of ASM and an abundant neural plexus ensheathing the airways are an integral part of the branching epithelial tubules from early in lung development.