Ontogeny of endocrine cells in the respiratory system of Syrian golden hamsters. II. Intrapulmonary airways and alveoli

The building blocks of mammalian lung development

Progress has recently been made in identifying progenitor cell populations in the embryonic lung. Some progenitor cell types have been definitively identified by lineage-tracing studies. However, others are not as well characterized and their existence is inferred on the basis of lung morphology, or mutant phenotypes. Here, I focus on lung development after the specification of the initial lung primordium. The evidence for various lung embryonic progenitor cell types is discussed and future experiments are suggested. The regulation of progenitor proliferation in the embryonic lung, and its coordinate control with morphogenesis, is also discussed. In addition, the relationship between embryonic and adult lung progenitors is considered.

Targeted disruption of NeuroD, a proneural basic helix-loop-helix factor, impairs distal lung formation and neuroendocrine morphology in the neonatal lung

Despite the importance of airspace integrity in vertebrate gas exchange, the molecular pathways that instruct distal lung formation are poorly understood. Recently, we found that fibrillin-1 deficiency in mice impairs alveolar formation and recapitulates the pulmonary features of human Marfan syndrome. To further elucidate effectors involved in distal lung formation, we performed expression profiling analysis comparing the fibrillin-1-deficient and wild-type developing lung. NeuroD, a basic helix-loop-helix transcription factor, fulfilled the expression criteria for a candidate mediator of distal lung development. We investigated its role in murine lung development using genetically targeted NeuroD-deficient mice. We found that NeuroD deficiency results in both impaired alveolar septation and altered morphology of the pulmonary neuroendocrine cells. NeuroD-deficient mice had enlarged alveoli associated with reduced epithelial proliferation in the airway and airspace compartments during development. Additionally, the neuroendocrine compartment in these mice manifested an increased number of neuroepithelial bodies but a reduced number of solitary pulmonary neuroendocrine cells in the neonatal lung. Overexpression of NeuroD in a murine lung epithelial cell line conferred a neuroendocrine phenotype characterized by the induction of neuroendocrine markers as well as increased proliferation. These results support an unanticipated role for NeuroD in the regulation of pulmonary neuroendocrine and alveolar morphogenesis and suggest an intimate connection between the neuroendocrine compartment and distal lung development.

Lung development and repair: contribution of the ciliated lineage

The identity of the endogenous epithelial cells in the adult lung that are responsible for normal turnover and repair after injury is still controversial. In part, this is due to a paucity of highly specific genetic lineage tools to follow efficiently the fate of the major epithelial cell populations: the basal, secretory, ciliated, neuroendocrine, and alveolar cells. As part of a program to address this problem we have used a 1-kb FOXJ1 promoter to drive CreER in the ciliated cells of the embryonic and adult lung. Analysis of FOXJ1-GFP transgenic lungs shows that labeled cells appear in a proximal-distal pattern during embryogenesis and that the promoter drives expression in all ciliated cells. Using FOXJ1CreER adult mice, we have followed the fate of ciliated cells after epithelial injury by naphthalene or sulfur dioxide. From quantitative analysis and confocal microscopy we conclude that ciliated cells transiently change their morphology in response to lung injury but do not proliferate or transdifferentiate as part of the repair process.

The diffuse endocrine system: from embryogenesis to carcinogenesis

In the present review we will summarise the current knowledge about the cells comprising the Diffuse Endocrine System (DES) in mammalian organs. We will describe the morphological, histochemical and functional traits of these cells in three major systems gastrointestinal, respiratory and prostatic. We will also focus on some aspects of their ontogeny and differentiation, as well as to their relevance in carcinogenesis, especially in neuroendocrine tumors. The first chapter describes the characteristics of DES cells and some of their specific biological and biochemical traits. The second chapter deals with DES in the gastrointestinal organs, with special reference to the new data on the differentiation mechanisms that leads to the appearance of endocrine cells from an undifferentiated stem cell. The third chapter is devoted to DES of the respiratory system and some aspects of its biological role, both, during development and adulthood. Neuroendocrine hyperplasia and neuroendocrine lung tumors are also addressed. Finally, the last chapter deals with the prostatic DES, discussing its probable functional role and its relevance in hormone-resistant prostatic carcinomas.

Hyperplasia of alveolar neuroendocrine cells in rat lung carcinogenesis by silica with selective expression of proadrenomedullin-derived peptides and amidating enzymes

Pulmonary neuroendocrine (NE) cells are found as clusters called neuroepithelial bodies (NEBs) or as single cells scattered in the respiratory epithelium. They express a variety of bioactive peptides, and they are thought to be the origin of NE lung tumors. Proadrenomedullin N-terminal 20 peptide (PAMP) is a peptide derived from the same precursor as adrenomedullin (AM). AM and PAMP are C-terminally amidated during their processing by a well-characterized amidating enzyme, peptidylglycine alpha-amidating monooxygenase (PAM). We explored AM, PAMP, and PAM expression as markers for NE hyperplasia in three rodent species (Fischer 344 rats, Syrian golden hamsters, and A/J mice) after a single intratracheal instillation of crystalline silica (quartz), which was previously found to induce different reactions in the three species. Rats developed a marked silicosis, with alveolar and bronchiolar hyperplasia and formation of peripheral lung epithelial tumors. Mice developed a moderate degree of silicosis, but not epithelial hyperplasia or tumors. Hamsters showed dust-storage lesions, but not silicosis or tumors. NE cells were immunolabeled for calcitonin gene-related peptide (CGRP), AM, PAMP, and PAM in serial sections of each lung. The numbers of positive NEBs per lung area and positive cells per NEB were quantified. A marked hyperplastic reaction in the NEBs of silica treated rats occurred only in alveolar NEBs, but not in bronchiolar NEBs. From Month 11 onwards, there were marked differences in the number of alveolar NEBs per section and in the number of cells per alveolar NEB immunoreactive for CGRP. No hyperplastic NE cell reaction was observed in silica-treated mice and hamsters. Significant PAMP and PAM expression was seen only in rat hyperplastic alveolar and in bronchiolar NEBs from Month 11 onwards. In E18, rat fetal lung NEBs were found to be strongly positive for PAMP and PAM.

Basic helix-loop-helix transcription factors regulate the neuroendocrine differentiation of fetal mouse pulmonary epithelium

To clarify the mechanisms that regulate neuroendocrine differentiation of fetal lung epithelia, we have studied the expression of the mammalian homologs of achaete-scute complex (Mash1) (Ascl1 - Mouse Genome Informatics); hairy and enhancer of split1 (Hes1); and the expression of Notch/Notch-ligand system in the fetal and adult mouse lungs, and in the lungs of Mash1- or Hes1-deficient mice. Immunohistochemical studies revealed that Mash1-positive cells seemed to belong to pulmonary neuroendocrine cells (PNEC) and their precursors. In mice deficient for Mash1, no PNEC were detected. Hes1-positive cells belong to non-neuroendocrine cells. In the mice deficient in Hes1, in which Mash1 mRNA was upregulated, PNEC appeared precociously, and the number of PNEC was markedly increased. NeuroD (Neurod1 - Mouse Genome Informatics) expression in the lung was detected in the adult, and was enhanced in the fetal lungs of Hes1-null mice. Expression of Notch1, Notch2, Notch3 and Notch4 mRNAs in the mouse lung increased with age, and Notch1 mRNA was expressed in a Hes1-dependent manner. Notch1, Notch2 and Notch3 were immunohistochemically detected in non-neuroendocrine cells. Moreover, analyses of the lungs from the gene-targeted mice suggested that expression of Delta-like 1 (Dll1 - Mouse Genome Informatics) mRNA depends on Mash1. Thus, the neuroendocrine differentiation depends on basic helix-loop-helix factors, and Notch/Notch-ligand pathways may be involved in determining the cell differentiation fate in fetal airway epithelium.

Differentiation and proliferation of pulmonary neuroendocrine cells

In this review article the morphological profiles of pulmonary neuroendocrine cells (PNEC) in experimental animals and humans are described. Although the mechanisms of differentiation and proliferation of neuroendocrine cells in the airway epithelium remain to be solved, several experimental studies using explant culture and cell culture systems of fetal animal lungs have been performed to clarify fundamental phenomena associated with neuroendocrine differentiation and proliferation. Experimental animal studies using chronic hypoxia, toxic substances and carcinogens have succeeded in inducing alterations in PNEC systems, and these studies have elucidated the reactions of PNEC in cell injury and inflammation, and functional aspects of PNEC in disease conditions. Human pulmonary neuroendocrine tumors include various histological subtypes, and show divergent morphological and biological varieties. Molecular abnormalities of small cell carcinoma, the most aggressive subtype of pulmonary neuroendocrine tumors, have been extensively studied, but the mechanism of neuroendocrine differentiation of this tumor is still largely unknown. PNEC share common phenotypes with neuronal cells, and developmental studies have begun contributed evidence that similar transcriptional networks, including active and repressive basic helix-loop-helix (bHLH) factors, function in the differentiation of both PNEC and neuronal cells. Such a bHLH network may also play a central role in determining cell differentiation in lung carcinomas. Further studies of the neuronal bHLH network, its regulatory system and related signal transduction pathways, will be required for understanding the mechanisms of neuroendocrine differentiation and proliferation in normal and pathological lung conditions.

The pulmonary neuroendocrine system: the past decade

The pulmonary neuroendocrine system consists of specialized airway endocrine epithelial cells, associated with nerve fibres. The epithelial cells, the pulmonary neuroendocrine cells (PNEC), can be solitary or clustered to form neuroepithelial bodies (NEB). During the last thirty years, the pulmonary neuroendocrine system has been intensively investigated and much knowledge of its function has been obtained. This text reviews work which dates from the last ten years. In this period, the picture of the pulmonary neuroendocrine system we previously had, has not fundamentally changed. The pulmonary neuroendocrine system is still regarded as an oxygen sensitive chemoreceptor with local and reflex-mediated regulatory functions, and as a regulator of airway growth and development. Continuing research has much more refined this picture. This text reviews several aspects of the pulmonary neuroendocrine system: phylogeny, the amine and peptide content of its epithelial cells, ontogeny and influence on lung development, the influence of hypoxia and nonhypoxic stimuli, immunomodulatory function, innervation and pathology. Among the discoveries of the past decade, three stand out prominently because of their great significance: additional proof that the neural component of the pulmonary neuroendocrine system is sensory, sound experimental evidence that PNEC stimulate airway epithelial cell differentiation and the discovery of a specific membrane oxygen receptor in the PNEC.

Comparison of the ontogeny of protein gene product 9.5, chromogranin A and proliferating cell nuclear antigen in developing human lung

Pulmonary neuroendocrine cell products, especially bombesin-like peptides, are important modulators of fetal lung growth, morphogenesis and maturation. In the present study, we describe the ontogeny of protein gene product 9.5 (PGP 9.5) in 28 midtrimester human fetal lungs, in comparison to chromogranin A (CGA), a marker of differentiated neuroendocrine cells, and proliferating cell nuclear antigen (PCNA), which is expressed by actively dividing cells. PGP 9.5 immunostaining colocalized with CGA in many cells, although the peak abundance of PGP 9.5 preceded that of CGA by 4 to 6 weeks. In addition, a novel staining pattern was noted for PGP 9.5: diffuse cytoplasmic staining of undifferentiated epithelial cells, which was demonstrated by all of the airways before 15 weeks gestation. After gestational week 15, only the smallest airways demonstrated this pattern. PCNA immunostaining demonstrated age-dependent regional variation. All samples had approximately 25% epithelial cells immunopositive for PCNA. Between 11 and 14 weeks gestation over 50% of the mesenchymal cells were PCNA positive. This mesenchymal staining decreased after 14 weeks, and was rare by week 19. There was no definite correlation between the immunostaining for PGP 9.5 and that for PCNA, although PGP 9.5 positive cells were usually PCNA negative. These observations suggest that other growth factors produced by non-neuroendocrine epithelial cells also participate in lung development.

Ontogeny of neuroepithelial bodies: correlations with mitogenesis and innervation

This paper summarizes current knowledge and advances speculation about the formation of the neuroendocrine system of mammalian lungs (comprising uninnervated solitary and clustered small-granule cells and innervated neuroepithelial bodies). It relates the initial appearance of neuroendocrine cells to regulation of mitotic activity in the epithelium during the development of the lung and pays special attention to the later in growth of nerves that converts some of them into neuroepithelial bodies, structures considered ideally adapted to function as chemoreceptors. A few original observations from ongoing immunohistochemical, electron microscopic, and analytical studies have been included here and there to point the discussion. The neuroendocrine cells are derived from undifferentiated precursors present in the endodermal pulmonary epithelium. At an early pseudoglandular stage of lung development these precursors begin to differentiate into neuroendocrine small-granule cells, commencing in the larynx and upper trachea, and expanding centrifugally into pulmonary airways almost as rapidly as these are laid down. Subsequently many of the intrapulmonary small-granule cell clusters become innervated. This event, the delayed appearance of small-granule cells synthesizing other than the dominant peptides and amines (calcitonin gene-related peptide and serotonin in rodents, gastrin-releasing peptide and serotonin in human beings), and other regional adjustments yield the population distribution present in the lungs of adults. Neuroendocrine cell precursors normally differentiate into typical serotonin- or peptide-synthesizing small-granule cells without requiring direct contact by nerves, and dissociated cells from a previously innervated population continue to exhibit physiological characteristics of oxygen sensors despite the loss of contact with nerves. Development of the innervation occurs in stages. Small-granule cell clusters are reached first by ganglion cells derived from pulmonary neuroblasts and later on by processes of extrinsic sensory nerves. The latter not only convey information to the central nervous system but also serve in a variety of ways to extend the neuroepithelial bodies' sphere of influence within the lung itself.

Stem cells of the respiratory epithelium and their in vitro cultivation

Parenchymal (epithelial or mesenchyma) stem cells are rapidly drawing both scientific and clinical attention in solid organs like the liver, skin, intestine and abdominal mesothelium, just as has been the case in the hematopoietic system. For the stem cells of these organs various definitions, markers for identification, methods of isolation and in vitro cultivation, and lineage mechanisms have been proposed and some of them are now proven to be valid and useful. In this article attempts will be made to explore whether there are stem cells in the lower respiratory system (from the trachea to the lung periphery) and what they look like. Because of its anatomical and functional complexity the stem cell concept for the respiratory system has been developing rather slowly. Nevertheless, the data available seem to indicate that in analogy to the above mentioned organs there is only one type of epithelial stem cells throughout all sections of the lower respiratory system during fetal through adult stages. They are multipotent for cell differentiation and able to yield lineage progenitors for ciliated, goblet, basal. Clara neuroendocrine, alveolar type 1 and alveolar type 2 cells.

Protein gene product 9.5-like and calbindin-like immunoreactivity in the nasal respiratory mucosa of perinatal humans

BACKGROUND

Protein gene product 9.5 (PGP) and calbindin-D28k (calbindin) are neuroendocrine markers that have been localized to neuroendocrine cells in the developing tracheobronchial epithelium. Neuroendocrine cells may play some role in the development of the tracheobronchial epithelium. Little is known about the development of the nasal respiratory epithelium (RE).

METHODS

Nasal respiratory mucosa from fetal and newborn humans was examined to determine immunoreactivity for PGP and calbindin.

RESULTS

At all stages studied, cells of different morphologies displayed PGP-like immunoreactivity (-LI) and calbindin-LI. Columnar immunoreactive cells for both markers predominated, but labeled cells of different shapes were also observed. Most labeled columnar cells were in the RE at its border with olfactory epithelium (OE); a few similarly labeled columnar cells also appeared in this OE. In the lamina propria, PGP-LI was also seen in numerous thin branching fibers. Some of these branches penetrated into the epithelium, where fiber varicosities appeared to contact cells, some of which also exhibited PGP-LI.

CONCLUSION

This study demonstrates that during development the human nasal RE contains different cell types, as illustrated by the assortment of epithelial cells displaying PGP-LI and calbindin-LI among unlabeled cells. Because PGP and calbindin immunoreactivities were found within neuroendocrine cells in previous studies, the present results indicate that the developing human nasal RE also may support a number of neuroendocrine cells. Furthermore, at least some of these cells may form synaptic contacts with nerve fibers from outside the epithelium.

Pulmonary Neuroendocrine Cells and Lung Development

Pulmonary neuroendocrine cells produce bioactive peptides such as gastrin-releasing peptide (GRP) at high levels in developing fetal lung. The role of GRP and other peptides in promoting branching morphogenesis, cell proliferation, and cell differentiation during lung organogenesis is reviewed. Possible roles for bioactive peptides derived from these cells in the pathophysiology of perinatal lung disorders are discussed.

Ontogeny of endocrine cells in the respiratory system of Syrian golden hamsters. I. Larynx and trachea

The ontogeny of protein gene product 9.5 (PGP 9.5), serotonin (5-HT), calcitonin gene-related peptide (CGRP), and calcitonin (CT) immunoreactivity was evaluated in small-granule endocrine cells of hamster laryngotracheal epithelium from fetal day 11 to adulthood. Two centrifugal (proximal-to-distal) patterns of differentiation occur. The first pattern begins during fetal life. Endocrine cells, single and clustered in groups (presumptive- or protoneuroepithelial bodies, pNEBs), initially colocalize immunostaining for PGP 9.5, 5-HT, and CGRP in the larynx and proximal 2/3 of the trachea on day 12 and spread to the caudal trachea on day 13. 5-HT disappears fleetingly during the 24 h preceding birth; otherwise immunoreactivity for all three substances persists into adulthood. The clusters of endocrine cells survive beyond birth but are so diluted by expansion of the nonendocrine epithelium as to become inconspicuous. Since innervation was not actually observed, these clusters may persist as pNEBs, without developing connections to afferent or efferent nerve fibers. The second pattern concerns single small-granule cells stainable for CGRP but not for 5-HT. These cells first appear in the larynx and cartilaginous part of the cranial trachea on postnatal day 3, and in the middle and caudal trachea, on day 5. The cells increase in number on day 7. In adults, they predominate among endocrine cells of the cartilaginous region. A subset of these cells begins to co-express CT proximally on postnatal day 10, reaching the caudal end of the trachea by 3 weeks. A few elements of the older 5-HT-positive population may also become immunoreactive for CT in juvenile hamsters.