The aminergic innervation of the human bronchus: a light and electron microscopic study

Portions of bronchial wall, free of neoplasm, were obtained from human lung resected for nearby tumour. The bronchial tissue was freeze dried, fixed in formaldehyde vapour and sections examined by fluorescence microscopy. Wispy green fibres with the appearance of amine-containing nerves were seen associated with the gland acini. Fluorescent cells were also found at this location and infrequently in the epithelium. Fluorescence was blocked by pretreatment of the sections with sodium borohydride; it faded in ultraviolet light and was enhanced by incubation of the tissue with L-dopa. All these features are characteristic of specific formaldehyde-induced fluorescence of biogenic amines. Fluorescent fibres were not identified either in the smooth muscle of the airway or associated with the epithelium, although background fluorescence may have masked them at these sites. Observations by electron microscopy of human bronchial wall showed that, close to gland acini, there were nerves containing large dense-cored vesicles suggesting that they were sympathetic. After incubation of the tissue with 5-hydroxydopamine, nerves containing labelled vesicles were seen as close as 1 micron to the gland acini, though none were seen to run between the acinar cells. It is argued that these were sympathetic nerves and were close enough to the gland acini to influence their secretions. Nerves containing vesicles labelled with 5-hydroxydopamine were also seen in close association with smooth muscle cells indicating a direct sympathetic innervation of the bronchial smooth muscle. These observations are contrary to recent suggestions that the sympathetic nervous system acts only indirectly in the lungs.