Neuroepithelial bodies in the Fawn Hooded rat lung: morphological and neuroanatomical evidence for a sensory innervation.

The lungs of young Fawn Hooded (FH) rats contain neuroepithelial bodies (NEB) with a very dense innervation. They are covered by a unicellular layer of flattened Clara cells and their apices protrude slightly into the airway lumen. The NEB corpuscle cells are loaded with dense core vesicles and are potentially capable of secretion by basal exocytosis. Since they are exposed to the airspace by narrow pores between the covering Clara cells, they may be influenced by the composition of the inhaled air. Facilitated uptake of the secreted substances into the bloodstream is indicated by the presence of extensively fenestrated capillaries in the corium beneath the NEB. The NEB are richly innervated by nerve fibres which loop through the corpuscle and form 'en passant' nerve endings. Most of these are packed with mitochondria and are morphologically afferent nerve endings. A much rarer type of nerve ending contains cholinergic-type vesicles and is morphologically efferent. Since both types are often observed in cytoplasmic continuity, the secretory activity of the NEB may be locally modulated by axon reflexes. The nerve endings often display synaptic junctions with the NEB corpuscle cells, which are invariably oriented towards the central nervous system. The sensory nature of the NEB innervation was confirmed by the results of neuroanatomical experiments. Infranodose vagotomy led to degeneration and disappearance of ipsilateral NEB intracorpuscular nerve endings, while supranodose vagotomy had no significant effect. In contrast to these findings, the end bulbs of preganglionic nerve fibres synapsing with intrapulmonary ganglion cells degenerated and disappeared after both procedures of vagotomy. Therefore, the nerve endings observed in pulmonary NEB must be derived mainly from vagal nodose (i.e. sensory) ganglion cells. It is concluded that although their precise functional significance remains obscure, pulmonary NEB in the Fawn Hooded rat appear to be well adapted to act both as endocrine glands and receptor organs.