Morphological and cytochemical characterization of neuroepithelial bodies in fetal rabbit lung. I. Studies of isolated neuroepithelial bodies

Functional morphology of pulmonary neuroepithelial bodies: extremely complex airway receptors

Innervated groups of neuroendocrine cells, called neuroepithelial bodies (NEBs), are diffusely spread in the epithelium of intrapulmonary airways in many species. Our present understanding of the morphology of NEBs in mammalian lungs is comprehensive, but none of the proposed functional hypotheses have been proven conclusively. In recent reviews on airway innervation, NEBs have been added to the list of presumed physiological lung receptors. Microscopic data on the innervation of NEBs, however, have given rise to conflicting interpretations. Using neuronal tracing, denervation, and immunostaining, we recently demonstrated that the innervation of NEBs is much more complex than the almost unique vagal nodose sensory innervation suggested by other authors. The aim of the present work is to summarize our present understanding about the origin and chemical coding of the profuse nerve terminals that selectively contact pulmonary NEBs. A thorough knowledge of the complex interactions between the neuroendocrine cells and at least five different nerve fiber populations is essential for defining the position(s) of NEBs among the many pulmonary receptors characterized by lung physiologists.

Comparative histology of pulmonary neuroendocrine cell system in mammalian lungs

It is increasingly recognized that neuroepithelial endocrine cells and organoid clusters of these cells termed neuroepithelial bodies are widely distributed in the respiratory tract of vertebrates. This review focuses on the neuroepithelial endocrine system in the airway epithelium of mammals according to observations made in a wide range of species, as it appears in light and electron microscopy by means of various visualization techniques under normal and experimental conditions. Because there are similarities but also marked differences between species, wherever possible studies in human airways are emphasized. Conventional histochemical and ultrastructural studies including microspectrofluorimetry have demonstrated the amine-handling properties and the presence of neurosecretory granules in neuroendocrine cells of the epithelium of all mammals studies so far. The neuroepithelial bodies are innervated by morphological afferent- and efferentlike synaptic specializations. These, together with the presence of reciprocal synapses, emphasize that these cells may be involved in dual functions of chemoreception and secretion. Light and electron microscopic immunohistochemistry has demonstrated that neuroepithelial endocrine cells may contain various biogenic agents, suggesting that the neuroendocrine system is highly heterogeneous. Neural elements may interact in a complex manner, and the activation of certain neural pathways may control the release of biogenic substances to influence physiological airway functions. They may be particularly relevant in pulmonary diseases. Consequently, the function of this system is complex and it is highly probable that many of its aspects are still not elucidated.

NCAM expression in the pulmonary neural and diffuse neuroendocrine cell system

Neural cell adhesion molecule (NCAM)--a membrane protein involved in cell-cell adhesion within the central and peripheral nervous systems--was demonstrated to be a sensitive and specific marker for neuroepithelial bodies (NEB) and neural tissue elements in the cat lung. Using the streptavidin-biotin immunoperoxidase method, NCAM reactive sites were investigated with monoclonal and polyclonal antibodies on serial section of Bouin fixed, paraffin embedded lung tissue. Moreover, NCAM expression was compared with that of neuron-specific enolase (NSE) on adjacent sections. The most obvious NCAM staining was obtained with the monoclonal antibody. From newborn to adult life, cell surface labeling was identified on NEB cells. In mature cat lung, they were no longer positive. Solitary neuroendocrine cells (NEC) were always negative. In contrast to the transient postnatal immunoreactivity of NEB cells, nerve fibers and ganglion cells were stained throughout all life stages and studied. The distribution of NCAM in NEB, nerve fibers and ganglion cells was similar to that NSE, except in the adult lung. This study reveals that during lung growth shared NCAM antigens exist between the pulmonary nervous and endocrine system, whereas in mature lung NCAM proteins are confined to neural tissue elements. The difference in NCAM expression between NEB and NEC might suggest an involvement of NCAM in the formation of contacts between NEB cells and nerves.

A scanning and transmission electron microscopic study of the development of the surface structure of neuroepithelial bodies in the mouse lung

Neuroepithelial bodies (NEBs) are groups of neuroepithelial (NE) cells that are localized on mounds on the bronchiolar epithelium of the lung. The present study examined NEBs in mice ranging in age from 2 days before birth to 80 days after birth. The position and surface architecture of NEBs was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In foetal mice, 2 days before birth, NEBs were distinguished from the rest of the bronchiolar epithelium by a slight elevation of non-ciliated Clara-like cells arranged in a cobblestone-like pattern. The exposed surface of the NEB was identified by small protrusions with regular microvilli intermittently located at the base of deep clefts between the Clara-like cells. The surface of the Clara-like cells had fewer and smaller microvilli and could be easily distinguished from the apical surface of the NEB. Before birth, the surface of all of the apical cells was covered by regularly placed microvilli, however after birth some of the more prominently positional apical cells revealed a bare patch at the centre of the portion of apical cell exposed to the lumen of the lung. As the mice aged there was an increase in the number of apical cell protrusions observed with centrally positioned bare patches. These two morphologically distinct surfaces of apical cells may have separate specialized functions. The exposed surfaces of apical cells were often observed in pairs and this feature has been observed in various sensory organs providing support for chemoreceptive function. However small bright spheres resembling vesicles were frequently observed on the lumenal surface of apical cells of the centrally placed bare patch. Transmission electron microscopy confirmed the presence of vesicles on the surface of apical cells and due to their location these vesicles were thought to contain a substance secreted into the lumen of the lung by apical cells. The significance of the bare region on the apical cells is not clear in terms of the proposed chemoreceptive function usually attributed to NEBs. It may be possible that the morphological changes observed in apical cells after birth are more appropriate for secretion of a substance into the lumen of the lung than for chemoreception. This is supported by the observation in the present study of vesicles lying on the lumenal surface of the bare region of the apical cell, however the mechanism for secretion of whole vesicles is not clear and requires further investigation.

Ultrastructure and innervation of neuroepithelial bodies in the lungs of newborn cats

Neuroepithelial bodies (NEB) occur throughout the airway mucosa and alveolar parenchyma of kitten lungs. In the bronchi, they are often situated on top of a cartilage plate. They form compact corpuscles containing 10-20 corpuscular cells and appear covered with a layer of flattened Clara cells. Kitten NEB are occasionally observed to display mitosis of the corpuscular epithelial cells. A prominent blood capillary lies at their basal pole. The corpuscular cells contain numerous dense core vesicles (DCV), whose number and diameter remain unchanged with age. Kitten NEB are innervated by nerve fibres that "loop" through the corpuscle and form morphologically afferent as well as efferent nerve endings. The nerve endings display afferent synaptic junctions with the corpuscular cells and sometimes run in clusters, so that they contact each other. Many nerve endings undergo spontaneous degeneration. We conclude that kitten NEB are well adapted to function as chemoreceptors and as endocrine or paracrine organs. Their chemoreceptor activity could be modulated by axon reflexes since their afferent nerve endings are often continuous with the efferent ones, as well as by interneural modulation since nerve endings often form clusters. In addition, kitten NEB innervation appears to involute rapidly soon after birth. This may indicate that their chemoreceptor function is only of primary importance during gestation and at birth. However, the secretory function of kitten NEB, as evidenced by the unchanged numbers and dimensions of their DCV, seems to remain steady throughout life.

Pulmonary neuroendocrine cells in pediatric lung disease: alterations in airway structure in infants with bronchopulmonary dysplasia

Despite four decades of investigation, the function of pulmonary neuroendocrine cells (NEC) remains unclear. Since NEC secretory products may influence airway growth or differentiation or alter airway smooth muscle tone, increased numbers of NEC seen in bronchopulmonary dysplasia (BPD) may be partially responsible for the genesis of the structural and pathophysiological alterations seen in this disease state. Changes in airway structure were studied in six infants dying with BPD and six conceptional age-matched control infants dying of noncardiopulmonary disease. Changes in bombesin-, calcitonin-, and serotonin-immunoreactive NEC were quantified in lung specimens from three infants who died at 2 months of age with severe BPD and three conceptional age-matched controls. There were no differences in either bronchiolar or bronchial airway epithelial areas, but significant increases in bronchiolar (1.8-fold) (P < 0.001) and especially bronchial smooth muscle (2.5-fold) (P < 0.001) were documented in infants with BPD. Few bombesin-, calcitonin-, and serotonin-immunoreactive cells were identified in cartilaginous airways; however, there was a clear increase in the total number of bronchiolar immunoreactive cells in infants with severe BPD (28.5 +/- 11.2 cells/mm airway epithelium) compared to control infants (4.5 +/- 4.9) (P < 0.05). Our results confirm that airway wall composition does change in BPD, but there is either no or an inverse correlation between NEC number and airway epithelial and smooth muscle areas and cell numbers. The role of NEC secretory products in airway smooth muscle growth and function requires further investigation.

Evidence for the coexistence of serotonin and calcitonin gene-related peptide at the subcellular level in neuroepithelial bodies in the lung of a marsupial, Isoodon macrourus

The coexistence of serotonin and calcitonin gene-related peptide (CGRP) in neuroepithelial bodies of the bandicoot, Isoodon macrourus, has been examined using immunocytochemistry at the light- and electron-microscope levels. The avidin-biotin technique of antigen localisation was used initially to identify serotonin-like and CGRP-like immunoreactivity (-LI). Serotonin-LI and CGRP-LI were found in neuroepithelial cells in the lungs of 30-day-old bandicoots. CGRP-LI could also be demonstrated in nerve fibres associated with some neuroepithelial bodies. The protein A-gold technique of antigen localisation was used to label neuroepithelial cells and nerve fibres at the subcellular level. Serotonin-LI and CGRP-LI were observed in the same dense-cored vesicles of most neuroepithelial cells; however, some neuroepithelial cells were shown to possess serotonin-LI without CGRP-LI. Nerve fibres immediately adjacent to neuroepithelial bodies exhibited mainly CGRP-LI. These results show that serotonin-LI and CGRP-LI are present in neuroepithelial cells of the bandicoot in the same secretory vesicles. This pattern of co-localisation may reflect co-ordinated or synergistic actions of these two neuroactive substances.

Immunocytochemical studies of subtypes of pulmonary endocrine cells in diethylnitrosamine-treated rabbits

New Zealand White rabbits were injected subcutaneously with 20 mg/kg body weight of diethylnitrosamine (DEN), twice per week, starting when they were 1 week old. The animals were sacrificed 6 to 12 months after the first injection and lung tissues were processed for light microscopy. Using serotonin (5HT) and neuron specific enolase (NSE) as markers for the endocrine cells, tissue sections were stained immunocytochemically by the avidin-biotin complex method. Numerous neuroepithelial bodies (NEBs) positive for 5HT, but negative for NSE, were seen in the alveolar duct regions of DEN-treated rabbits. On the other hand, an increased number of solitary endocrine cells immunoreactive for NSE was found in bronchial or bronchiolar epithelia. The results indicate that DEN induced increases in two distinct types of endocrine cells: the component cells of NEBs are positive for 5HT and solitary cells are positive for NSE.

Calcitonin gene-related peptide, enkephalin and serotonin coexist in neuroepithelial bodies of the respiratory tract of the red-eared turtle, Pseudemys scripta elegans. An immunocytochemical study

The lungs of the red-eared turtle, Pseudemys scripta elegans, have been investigated by light-microscopical immunocytochemistry for serotonin (5-HT) and the neuropeptides calcitonin gene-related peptide (CGRP), enkephalin (ENK), bombesin, somatostatin, calcitonin and cholecystokinin. CGRP- or 5-HT-like immunoreactivity (LI) was demonstrated in neuroepithelial bodies (NEBs) and solitary neuroepithelial endocrine cells lying between the ciliated epithelial cells of the intrapulmonary bronchi, and of the primary, secondary and tertiary trabeculae. Furthermore, ENK-LI NEBs were found between the trabecular ciliated epithelial cells. No reaction was observed with antisera against bombesin, somatostatin, calcitonin, and cholecystokinin. Using consecutive sections, CGRP-LI appears to be colocalized with 5-HT-LI in NEB cells. In addition, it was demonstrated that all the ENK-LI NEBs also contain CGRP-LI, whereas only a part of the CGRP-LI NEBs reveal ENK-LI. To our knowledge this is the first demonstration of CGRP- and/or ENK-LI in pulmonary NEBs of a lower vertebrate.

Neuroendocrine (NE) cells in rat neonatal lungs. A histochemical and immunocytochemical study

A study has been carried out to determine the presence of NE cells in the newborn rat lung. The localization of these cell was achieved by an argyrophil method. Both single NE and NEB, were found. Immunoperoxidase techniques were performed to determine NSE, serotonin and calcitonin production in these NE formations.

Ultrastructural architecture of pulmonary small-granule cell clusters in adult Syrian golden hamster

Throughout the epithelial lining of the respiratory system is a class of cells with characteristics similar to Amine Precursor Uptake and Decarboxylation (APUD) polypeptide hormone-producing cells. In the intrapulmonary airways, these small-granule cells (SGCs) occur either singly or in organized clusters. Although no specific peptide has yet been identified, subclasses have been postulated based on granule geometry or light microscopic staining. The present study characterizes the architectonic and cellular organization of clustered SGCs in the adult Syrian golden hamster. Two morphologically distinct cells can be defined in such clusters, "light" and "dark." Thid distinction was based primarily on differences in the electron density of the cytoplasmic matrix rather than on the remarkable variations in cellular organelles or dense-core secretory vesicles. Both cell types were normal as judged by uniform spherical nuclei, chromatin organization, and distribution of cellular organelles. The "dark" cells, however, presented the profile of a cell actively involved in synthesis with a markedly dilated perinuclear cisterna and endoplasmic reticulum. Additionally, the "dark" cells contained membrane-delimited structures containing concentric membranous whorls, clear vacuoles, and lipofuscin granules. Occasionally, cells were observed to contain features of both cell types, suggesting that they may represent a continuum of common cell lineage. Accordingly, in the absence of additional morphologic or biochemical data, the "light" and "dark" cells most probably correspond to different stages of functional activity or age-related changes of a single type of cell. Unmyelinated nerve endings were occasionally interposed between cells, but synaptic specializations were not observed. Beneath the clusters, nerve fibers were also present, but they were never observed to penetrate the basal lamina or contact any of the SGCs. Of equal occurrence were elements of the vascular system and smooth muscle, suggesting that some SGCs in the adult hamster may function in a paracrine or endocrine manner. Such knowledge is essential to any study attempting to delineate the functional role or roles of these enigmatic organoids.

Microspectrofluorimetric analysis of the formaldehyde-induced fluorophores of 5-hydroxytryptamine and dopamine in intrapulmonary neuroepithelial bodies after administration of L-5-hydroxytryptophan and L-dopa

To demonstrate the intracellular store of 5-hydroxytryptamine and dopamine in pulmonary neuroepithelial bodies of the neonatal rabbit after treatment with the corresponding amino-acid precursors L-5-hydroxytryptophan or L-3,4-dihydroxyphenylalanine, formaldehyde-induced fluorescence in combination with microspectrofluorimetric analysis has been used. Emission spectra and excitation spectra in an extended wavelength range from 240 to 460 nm, the displacement of excitation peaks after exposure to hydrochloric acid vapour, and calculation of peak ratio values 410/260, 380/320, 320/260 for phenylethylamine fluorophores and 385/315 for indolylethylamine fluorophores were performed. Thus, the presence of 5-hydroxytryptamine without occurrence of 5-hydroxytryptophan was demonstrated in pulmonary neuroepithelial bodies after administration of the corresponding biological precursor, while dopamine combined with 5-hydroxytryptamine were clearly revealed after administration of L-3,4-dihydroxyphenylalanine. The rate of photodecomposition always corroborated these findings.

Synaptophysin expressed in the bronchopulmonary tract: neuroendocrine cells, neuroepithelial bodies, and neuroendocrine neoplasms

Synaptophysin is an integral membrane glycoprotein with an Mr of 38,000 that occurs in the small, clear vesicles present in neuronal cells and tumors as well as in pancreatic islet cells and various neuroendocrine (NE) carcinomas. We found that synaptophysin is also expressed in normal NE cells of the lungs of newborn rabbits and mice as well as of human fetuses. In bronchial ganglion cells and in nerves, synaptophysin is coexpressed with neurofilament proteins (NFPs), whereas in solitary NE cells and in at least some of the neuroepithelial bodies (NEBs) of the bronchial mucosal lining, synaptophysin coexists with cytokeratins. We also studied a series of NE neoplasms of the lung covering the entire spectrum of differentiation (i.e., from carcinoids to small-cell NE carcinomas), and found that synpatophysin was present in the majority of them. In these tumors, synaptophysin was invariably coexpressed with cytokeratin filaments and desmoplakin, as well as, occasionally, with NFP. Synaptophysin was identified throughout, the whole range of these NE neoplasms, i.e., from benign to low-grade to aggressive and rapidly metastasizing carcinomas; its presence was unaffected by the highly variable expression of serotonin and/or neuropeptides in these neoplasms, and was unrelated to the presence or absence of associated endocrine syndromes. Our findings indicate that synaptophysin occurs in the neural as well as in the epithelial components of the dispered NE system of the lung as well as in the majority of NE neoplasms of this organ, and that the expression of this protein is therefore independent of the cytoskeletal characteristics and other differentiation features of both normal and transformed NE cells of the lung. We emphasize the value of synaptophysin as an immunocytochemical marker of NE differentiation.

Effect of various vagotomy procedures on the reaction to hypoxia of rabbit neuroepithelial bodies: modulation by intrapulmonary axon reflexes?

Recent neuroanatomical investigations revealed the intrapulmonary neuroepithelial bodies (NEB) to be innervated to a large extent by sensory nerve fibers, displaying peripheral nerve endings of afferent as well as efferent morphology and having their cell bodies in the nodose ganglion of the vagus nerve. Earlier studies also revealed that upon exposure to acute hypoxia NEB exhibit a distinct secretory response, including as well a decrease in the cytoplasmic fluorescence as an increased basal exocytosis and indicating the secretion of serotonin. In the present study, we have tried to establish whether or not this secretory behavior is neurally controlled by combining an exposure to hypoxia with various vagotomy procedures. After long-term (3 days) infranodose vagotomy, the ipsilateral NEB nerve endings have degenerated. The secretory response to hypoxia is modified: the cytoplasmic fluorescence intensifies, while the basal exocytosis remains unchanged. After short-term (1 hour) infranodose as well as long-term (3 days) supranodose vagotomy, the NEB nerve endings are still intact, though no longer connected to the central nervous system. In these circumstances, the hypoxic NEB secretory behavior is indistinguishable from that of intact NEB. From these experimental findings we conclude that the hypoxic NEB secretory response is neurally controlled, since it no longer occurs when the normal innervation has degenerated. This modulation is however not by CNS motor nerve impulses, but probably by intrapulmonary axon reflexes in sensory nerve fibers.