Signaling molecules and transcription factors involved in the development of the sympathetic nervous system, with special emphasis on the superior cervical ganglion

Expression of group II metabotropic glutamate receptors in rat superior cervical ganglion

BACKGROUND

The superior cervical ganglion (SCG) plays critical roles in the regulation of blood pressure and cardiac output. Metabotropic glutamate receptors (mGluRs) in the SCG are not clearly elucidated yet. Most studies on the expression and functions of mGluRs in the SCG focused on the cultured SCG neurons, and yet little information has been reported in the SCG tissue. Chronic intermittent hypoxia (CIH), one of the major clinical features of obstructive sleep apnea (OSA) patients, is a critical pathological cause of secondary hypertension in OSA patients, but its impact on the level of mGluRs in the SCG is unknown.

OBJECTIVE

To explore the expression and localization of mGluR2/3 and the effect of CIH on mGluR2/3 level in rat SCG tissue.

METHODS

RT-PCR and immunostaining were conducted to examine the mRNA and protein expression of mGluR2/3 in rat SCG. Immunofluorescence staining was conducted to examine the distribution of mGluR2/3. Rats were divided into control and CIH group which the rats were exposed to CIH for 6 weeks. Western blots were performed to examine the level of mGluR2/3 in rat SCG.

RESULTS

mRNAs of mGluR2/3 expressed in rat SCG. mGluR2 distributed in principal neurons and small intensely fluorescent cells but not in satellite glial cells, nerve fibers, and vascular endothelial cells; mGluR3 was detected in nerve fibers rather than in the cells mentioned above. CIH exposure reduced the protein level of mGluR2/3 in rat SCG.

CONCLUSION

mGluR2/3 exists in rat SCG with diverse distribution patterns, and may be involved in CIH-induced hypertension.

Stem Cell Niche in the Mammalian Carotid Body

Accumulating evidence suggests that the mammalian carotid body (CB) constitutes a neurogenic center that contains a functionally active germinal niche. A variety of transcription factors is required for the generation of a precursor cell pool in the developing CB. Most of them are later silenced in their progeny, thus allowing for the maturation of the differentiated neurons. In the adult CB, neurotransmitters and vascular cytokines released by glomus cells upon exposure to chronic hypoxia act as paracrine signals that induce proliferation and differentiation of pluripotent stem cells, neuronal and vascular progenitors. Key proliferation markers such as Ki-67 and BrdU are widely used to evaluate the proliferative status of the CB parenchymal cells in the initial phase of this neurogenesis. During hypoxia sustentacular cells which are dormant cells in normoxic conditions can proliferate and differentiate into new glomus cells. However, more recent data have revealed that the majority of the newly formed glomus cells is derived from the glomus cell lineage itself. The mature glomus cells express numerous trophic and growth factors, and their corresponding receptors, which act on CB cell populations in autocrine or paracrine ways. Some of them initially serve as target-derived survival factors and then as signaling molecules in developing vascular targets. Morphofunctional insights into the cellular interactions in the CB stem cell microenvironment can be helpful in further understanding the therapeutic potential of the CB cell niche.

Development of the sympathetic trunks in human embryos

Although the development of the sympathetic trunks was first described >100 years ago, the topographic aspect of their development has received relatively little attention. We visualised the sympathetic trunks in human embryos of 4.5-10 weeks post-fertilisation, using Amira 3D-reconstruction and Cinema 4D-remodelling software. Scattered, intensely staining neural crest-derived ganglionic cells that soon formed longitudinal columns were first seen laterally to the dorsal aorta in the cervical and upper thoracic regions of Carnegie stage (CS)14 embryos. Nerve fibres extending from the communicating branches with the spinal cord reached the trunks at CS15-16 and became incorporated randomly between ganglionic cells. After CS18, ganglionic cells became organised as irregular agglomerates (ganglia) on a craniocaudally continuous cord of nerve fibres, with dorsally more ganglionic cells and ventrally more fibres. Accordingly, the trunks assumed a "pearls-on-a-string" appearance, but size and distribution of the pearls were markedly heterogeneous. The change in position of the sympathetic trunks from lateral (para-aortic) to dorsolateral (prevertebral or paravertebral) is a criterion to distinguish the "primary" and "secondary" sympathetic trunks. We investigated the position of the trunks at vertebral levels T2, T7, L1 and S1. During CS14, the trunks occupied a para-aortic position, which changed into a prevertebral position in the cervical and upper thoracic regions during CS15, and in the lower thoracic and lumbar regions during CS18 and CS20, respectively. The thoracic sympathetic trunks continued to move further dorsally and attained a paravertebral position at CS23. The sacral trunks retained their para-aortic and prevertebral position, and converged into a single column in front of the coccyx. Based on our present and earlier morphometric measurements and literature data, we argue that differential growth accounts for the regional differences in position of the sympathetic trunks.

5-HT3 Signaling Alters Development of Sacral Neural Crest Derivatives That Innervate the Lower Urinary Tract

The autonomic nervous system derives from the neural crest (NC) and supplies motor innervation to the smooth muscle of visceral organs, including the lower urinary tract (LUT). During fetal development, sacral NC cells colonize the urogenital sinus to form pelvic ganglia (PG) flanking the bladder neck. The coordinated activity of PG neurons is required for normal urination; however, little is known about the development of PG neuronal diversity. To discover candidate genes involved in PG neurogenesis, the transcriptome profiling of sacral NC and developing PG was performed, and we identified the enrichment of the type 3 serotonin receptor (5-HT3, encoded by Htr3a and Htr3b). We determined that Htr3a is one of the first serotonin receptor genes that is up-regulated in sacral NC progenitors and is maintained in differentiating PG neurons. In vitro cultures showed that the disruption of 5-HT3 signaling alters the differentiation outcomes of sacral NC cells, while the stimulation of 5-HT3 in explanted fetal pelvic ganglia severely diminished neurite arbor outgrowth. Overall, this study provides a valuable resource for the analysis of signaling pathways in PG development, identifies 5-HT3 as a novel regulator of NC lineage diversification and neuronal maturation in the peripheral nervous system, and indicates that the perturbation of 5-HT3 signaling in gestation has the potential to alter bladder function later in life.

Comparative morphological and molecular studies on the oxygen-chemoreceptive cells in the carotid body and fish gills

Oxygen-chemoreceptive cells play critical roles for the respiration control. This review summarizes the chemoreceptive cells in the carotid body and fish gills from a morphological and molecular perspective. The cells synthesize and secrete biogenic amines, neuropeptides, and neuroproteins and also express many signaling molecules and transcription factors. In mammals, birds, reptiles, and amphibians, the carotid body primordium is consistently formed in the wall of the third arch artery which gives rise to the common carotid artery and the basal portion of the internal carotid artery. Consequently, the carotid body is located in the carotid bifurcation region, except birds in which the organ is situated at the lateral side of the common carotid artery. The carotid body receives branches of the cranial nerves IX and/or X dependent on the location of the organ. The glomus cell progenitors in mammals and birds are derived from the neighboring ganglion, i.e., the superior cervical sympathetic ganglion and the nodose ganglion, respectively, and immigrate into the carotid body primordium, constituting a solid cell cluster. In other animal species, the glomus cells are dispersed singly or forming small cell groups in intervascular stroma of the carotid body. In fishes, the neuroepithelial cells, corresponding to the glomus cells, are distributed in the gill filaments and lamellae. All oxygen-chemoreceptive cells sensitively respond to acute or chronic hypoxia, exhibiting degranulation, hypertrophy, hyperplasia, and upregulated expression of many genes.

Carotid body type I cells engage flavoprotein and Pin1 for oxygen sensing

Carotid body (CB) type I cells sense the blood Po₂ and generate a nervous signal for stimulating ventilation and circulation when blood oxygen levels decline. Three oxygen-sensing enzyme complexes may be used for this purpose: 1) mitochondrial electron transport chain metabolism, 2) heme oxygenase 2 (HO-2)-generating CO, and/or 3) an NAD(P)H oxidase (NOX). We hypothesize that intracellular redox changes are the link between the sensor and nervous signals. To test this hypothesis type I cell autofluorescence of flavoproteins (Fp) and NAD(P)H within the mouse CB ex vivo was recorded as Fp/(Fp+NAD(P)H) redox ratio. CB type I cell redox ratio transiently declined with the onset of hypoxia. Upon reoxygenation, CB type I cells showed a significantly increased redox ratio. As a control organ, the non-oxygen-sensing sympathetic superior cervical ganglion (SCG) showed a continuously reduced redox ratio upon hypoxia. CN^-, diphenyleneiodonium, or reactive oxygen species influenced chemoreceptor discharge (CND) with subsequent loss of O₂ sensitivity and inhibited hypoxic Fp reduction only in the CB but not in SCG Fp, indicating a specific role of Fp in the oxygen-sensing process. Hypoxia-induced changes in CB type I cell redox ratio affected peptidyl prolyl isomerase Pin1, which is believed to colocalize with the NADPH oxidase subunit p47_phox in the cell membrane to trigger the opening of potassium channels. We postulate that hypoxia-induced changes in the Fp-mediated redox ratio of the CB regulate the Pin1/p47_phox tandem to alter type I cell potassium channels and therewith CND.

Disorders of neural crest derivates in oncoradiological practice

Hundreds of articles discuss the imaging characteristics and molecular background of prominent gastrointestinal (GI) motility disorders and tumors of the peripheral nervous system, but according to our knowledge an article focusing on the classification and developmental background of these heterogeneous diseases is not to be found. Our aim is to give insight on the common features of several diseases and tumors, starting with their common source of origin, the neural crest (NC). The NC is a transient cell population of the embryo, which differentiates into several organs/structures of our body (sympathetic trunk, adrenal medulla). Although the incidence of the individual tumors of NC cells is not high by themselves, the summation of these incidences may be relevant in the daily routine. In the introduction we mention the most prominent developmental routes and molecular pathways of NC cells, which is crucial to understand the pathogenesis and the wide range of involved cell types from the colon to the adrenal gland. We summarized the most important, useful pathological findings and imaging techniques from the X-ray to the positron emission tomography—computed tomography (CT) in order to help the identification of these diseases. This article may help to better understand NC lineage and its unique, diverse role during ontogeny, which may influence the radiologists to change several convictions, or understand better the background and/or connections of a wide range of tumors and syndromes.

HIF-1α is required for development of the sympathetic nervous system

The molecular mechanisms regulating sympathetic innervation of the heart during embryogenesis and its importance for cardiac development and function remain to be fully elucidated. We generated mice in which conditional knockout (CKO) of the Hif1a gene encoding the transcription factor hypoxia-inducible factor 1α (HIF-1α) is mediated by an Islet1-Cre transgene expressed in the cardiac outflow tract, right ventricle and atrium, pharyngeal mesoderm, peripheral neurons, and hindlimbs. These Hif1aCKO mice demonstrate significantly decreased perinatal survival and impaired left ventricular function. The absence of HIF-1α impaired the survival and proliferation of preganglionic and postganglionic neurons of the sympathetic system, respectively. These defects resulted in hypoplasia of the sympathetic ganglion chain and decreased sympathetic innervation of the Hif1aCKO heart, which was associated with decreased cardiac contractility. The number of chromaffin cells in the adrenal medulla was also decreased, indicating a broad dependence on HIF-1α for development of the sympathetic nervous system.

Accessory right spermatic ganglion: possible embryological basis and clinical significance

The spermatic ganglia are collections of sympathetic neuron cell bodies located within the cords of the infrarenal aortic plexus, positioned at the origin of the testicular arteries in males. During routine dissection of the aortic plexus at our institution, one specimen exhibited a second (accessory) testicular artery on the right side that coursed retrocaval. Histology was used to confirm the presence of an accessory right spermatic ganglion at the base of the accessory retrocaval testicular artery. Interestingly, the accessory spermatic ganglion was also supplied by its own right lumbar splanchnic nerve. This is the first case to describe the anatomy of an accessory spermatic ganglion in a specimen that exhibits an accessory testicular artery on the right side. This neurovascular variation is of interest to surgeons who aim to perform nerve-sparing retroperitoneal lymph node dissections for malignancy.

Collaborative ISL1/GATA3 interaction in controlling neuroblastoma oncogenic pathways overlapping with but distinct from MYCN

Background: Transcription factor ISL1 plays a critical role in sympathetic neurogenesis. Expression of ISL1 has been associated with neuroblastoma, a pediatric tumor derived from sympatho-adrenal progenitors, however the role of ISL1 in neuroblastoma remains unexplored. Method: Here, we knocked down ISL1 (KD) in SH-SY5Y neuroblastoma cells and performed RNA-seq and ISL1 ChIP-seq analyses. Results: Analyses of these data revealed that ISL1 acts upstream of multiple oncogenic genes and pathways essential for neuroblastoma proliferation and differentiation, including LMO1 and LIN28B. ISL1 promotes expression of a number of cell cycle associated genes, but represses differentiation associated genes including RA receptors and the downstream target genes EPAS1 and CDKN1A. Consequently, Knockdown of ISL1 inhibits neuroblastoma cell proliferation and migration in vitro and impedes tumor growth in vivo, and enhances neuronal differentiation by RA treatment. Furthermore, genome-wide mapping revealed a substantial co-occupancy of binding regions by ISL1 and GATA3, and ISL1 physically interacts with GATA3, and together they synergistically regulate the aforementioned oncogenic pathways. In addition, analyses of the roles of ISL1 and MYCN in MYCN-amplified and MYCN non-amplified neuroblastoma cells revealed an epistatic relationship between ISL1 and MYCN. ISL1 and MYCN function in parallel to regulate common yet distinct oncogenic pathways in neuroblastoma. Conclusion: Our study has demonstrated that ISL1 plays an essential role in neuroblastoma regulatory networks and may serve as a potential therapeutic target in neuroblastoma.

Progenitor Cell Heterogeneity in the Adult Carotid Body Germinal Niche

Somatic stem cells confer plasticity to adult tissues, permitting their maintenance, repair and adaptation to a changing environment. Adult germinal niches supporting somatic stem cells have been thoroughly characterized throughout the organism, including in central and peripheral nervous systems. Stem cells do not reside alone within their niches, but they are rather accompanied by multiple progenitor cells that not only contribute to the progression of stem cell lineage but also regulate their behavior. Understanding the mechanisms underlying these interactions within the niche is crucial to comprehend associated pathologies and to use stem cells in cell therapy. We have described a stunning germinal niche in the adult peripheral nervous system: the carotid body. This is a chemoreceptor organ with a crucial function during physiological adaptation to hypoxia. We have shown the presence of multipotent stem cells within this niche, escorted by multiple restricted progenitor cell types that contribute to niche physiology and hence organismal adaptation to the lack of oxygen. Herein, we discuss new and existing data about the nature of all these stem and progenitor cell types present in the carotid body germinal niche, discussing their role in physiology and their clinical relevance for the treatment of diverse pathologies.

Descending musculospinal branch of the ascending pharyngeal artery as a feeder of carotid body tumors: Angio-architecture and embryological consideration

INTRODUCTION

Although the ascending pharyngeal artery (APhA) is known as a main feeder of carotid body tumors (CBTs), its detailed architecture and embryological consideration have not been described. The objective of this study was to describe the architecture of a descending feeder of the APhA and to consider its origin embryologically through a review of our CBT embolizations.

METHODS

We retrospectively analyzed data from patients with CBTs who underwent transarterial embolization or angiographic examination-only between July 2010 and February 2017. The arterial supply of the tumors, the number of feeder pedicles, the mean tumor size, embolization materials, complication of embolization, and extent of tumor removal were assessed. The embryological origin of feeding artery was considered based on the literature.

RESULTS

Eighteen patients with 20 CBTs underwent preoperative embolization or angiographic examination. The number of feeder pedicles was significantly related to the size of the CBT (P = 0.0002). The main feeding artery was the descending branch of APhA, which was hypertrophied and tortuous (18/20, 90%). Embryologically, this artery originated from the musculospinal branch and is termed the "descending musculospinal branch".

CONCLUSION

The main feeder of the CBTs was the "descending musculospinal branch" of the APhA and needs special consideration such as dangerous anastomosis for embolization.

Acute oxygen sensing-Role of metabolic specifications in peripheral chemoreceptor cells

Acute oxygen sensing is essential for humans under hypoxic environments or pathologic conditions. This is achieved by the carotid body (CB), the key arterial chemoreceptor, along with other peripheral chemoreceptor organs, such as the adrenal medulla (AM). Although it is widely accepted that inhibition of K⁺ channels in the plasma membrane of CB cells during acute hypoxia results in the activation of cardiorespiratory reflexes, the molecular mechanisms by which the hypoxic signal is detected to modulate ion channel activity are not fully understood. Using conditional knockout mice lacking mitochondrial complex I (MCI) subunit NDUFS2, we have found that MCI generates reactive oxygen species and pyridine nucleotides, which signal K⁺ channels during acute hypoxia. Comparing the transcriptomes from CB and AM, which are O₂-sensitive, with superior cervical ganglion, which is practically O₂-insensitive, we have found that CB and AM contain unique metabolic gene expression profiles. The "signature metabolic profile" and their biophysical characteristics could be essential for acute O₂ sensing by chemoreceptor cells.

Striking parallels between carotid body glomus cell and adrenal chromaffin cell development

Carotid body glomus cells mediate essential reflex responses to arterial blood hypoxia. They are dopaminergic and secrete growth factors that support dopaminergic neurons, making the carotid body a potential source of patient-specific cells for Parkinson's disease therapy. Like adrenal chromaffin cells, which are also hypoxia-sensitive, glomus cells are neural crest-derived and require the transcription factors Ascl1 and Phox2b; otherwise, their development is little understood at the molecular level. Here, analysis in chicken and mouse reveals further striking molecular parallels, though also some differences, between glomus and adrenal chromaffin cell development. Moreover, histology has long suggested that glomus cell precursors are ‘émigrés’ from neighbouring ganglia/nerves, while multipotent nerve-associated glial cells are now known to make a significant contribution to the adrenal chromaffin cell population in the mouse. We present conditional genetic lineage-tracing data from mice supporting the hypothesis that progenitors expressing the glial marker proteolipid protein 1, presumably located in adjacent ganglia/nerves, also contribute to glomus cells. Finally, we resolve a paradox for the ‘émigré’ hypothesis in the chicken - where the nearest ganglion to the carotid body is the nodose, in which the satellite glia are neural crest-derived, but the neurons are almost entirely placode-derived - by fate-mapping putative nodose neuronal 'émigrés' to the neural crest.

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Glomus cell precursors express the neuron-specific marker Elavl3/4 (HuC/D).

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Developing glomus cells express multiple ‘sympathoadrenal' genes.

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Glomus cell development requires Hand2 and Sox4/11, but not Ret or Tfap2b.

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Multipotent progenitors with a glial phenotype contribute to glomus cells.

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Fate-mapping resolves a paradox for the ganglionic 'émigré' hypothesis in birds.

NGF-dependent neurons and neurobiology of emotions and feelings: Lessons from congenital insensitivity to pain with anhidrosis

NGF is a well-studied neurotrophic factor, and TrkA is a receptor tyrosine kinase for NGF. The NGF-TrkA system supports the survival and maintenance of NGF-dependent neurons during development. Congenital insensitivity to pain with anhidrosis (CIPA) is an autosomal recessive genetic disorder due to loss-of-function mutations in the NTRK1 gene encoding TrkA. Individuals with CIPA lack NGF-dependent neurons, including NGF-dependent primary afferents and sympathetic postganglionic neurons, in otherwise intact systems. Thus, the pathophysiology of CIPA can provide intriguing findings to elucidate the unique functions that NGF-dependent neurons serve in humans, which might be difficult to evaluate in animal studies. Preceding studies have shown that the NGF-TrkA system plays critical roles in pain, itching and inflammation. This review focuses on the clinical and neurobiological aspects of CIPA and explains that NGF-dependent neurons in the peripheral nervous system play pivotal roles in interoception and homeostasis of our body, as well as in the stress response. Furthermore, these NGF-dependent neurons are likely requisite for neurobiological processes of 'emotions and feelings' in our species.

Temporal requirements for ISL1 in sympathetic neuron proliferation, differentiation, and diversification

Malformations of the sympathetic nervous system have been associated with cardiovascular instability, gastrointestinal dysfunction, and neuroblastoma. A better understanding of the factors regulating sympathetic nervous system development is critical to the development of potential therapies. Here, we have uncovered a temporal requirement for the LIM homeodomain transcription factor ISL1 during sympathetic nervous system development by the analysis of two mutant mouse lines: an Isl1 hypomorphic line and mice with Isl1 ablated in neural crest lineages. During early development, ISL1 is required for sympathetic neuronal fate determination, differentiation, and repression of glial differentiation, although it is dispensable for initial noradrenergic differentiation. ISL1 also plays an essential role in sympathetic neuron proliferation by controlling cell cycle gene expression. During later development, ISL1 is required for axon growth and sympathetic neuron diversification by maintaining noradrenergic differentiation, but repressing cholinergic differentiation. RNA-seq analyses of sympathetic ganglia from Isl1 mutant and control embryos, together with ISL1 ChIP-seq analysis on sympathetic ganglia, demonstrated that ISL1 regulates directly or indirectly several distinct signaling pathways that orchestrate sympathetic neurogenesis. A number of genes implicated in neuroblastoma pathogenesis are direct downstream targets of ISL1. Our study revealed a temporal requirement for ISL1 in multiple aspects of sympathetic neuron development, and suggested Isl1 as a candidate gene for neuroblastoma.

Hypoxia-regulated catecholamine secretion in chromaffin cells

Adrenal catecholamine (CAT) secretion is a general physiological response of animals to environmental stressors such as hypoxia. This represents an important adaptive mechanism to maintain homeostasis and protect vital organs such as the brain. In adult mammals, CAT secretory responses are triggered by activation of the sympathetic nervous system that supplies cholinergic innervation of adrenomedullary chromaffin cells (AMC) via the splanchnic nerve. In the neonate, the splanchnic innervation of AMC is immature or absent, yet hypoxia stimulates a non-neurogenic CAT secretion that is critical for adaptation to extra-uterine life. This non-neurogenic, hypoxia-sensing mechanism in AMC is gradually lost or suppressed postnatally along a time course that parallels the development of splanchnic innervation. Moreover, denervation of adult AMC results in a gradual return of the direct hypoxia-sensing mechanism. The signaling pathways by which neonatal AMC sense acute hypoxia leading to non-neurogenic CAT secretion and the mechanisms that underlie the re-acquisition of hypoxia-sensing properties by denervated adult AMC, are beginning to be understood. This review will focus on current views concerning the mechanisms responsible for direct acute hypoxia sensing and CAT secretion in perinatal AMC and how they are regulated by innervation during postnatal development. It will also briefly discuss plasticity mechanisms likely to contribute to CAT secretion during exposures to chronic and intermittent hypoxia.

From proliferation to target innervation: signaling molecules that direct sympathetic nervous system development

The sympathetic division of the autonomic nervous system includes a variety of cells including neurons, endocrine cells and glial cells. A recent study (Furlan et al. 2017) has revised thinking about the developmental origin of these cells. It now appears that sympathetic neurons and chromaffin cells of the adrenal medulla do not have an immediate common ancestor in the form a "sympathoadrenal cell", as has been long believed. Instead, chromaffin cells arise from Schwann cell precursors. This review integrates the new findings with the expanding body of knowledge on the signalling pathways and transcription factors that regulate the origin of cells of the sympathetic division of the autonomic nervous system.

Morphological and molecular evolution of the ultimobranchial gland of nonmammalian vertebrates, with special reference to the chicken C cells

This review summarizes the current understanding of the nonmammalian ultimobranchial gland from morphological and molecular perspectives. Ultimobranchial anlage of all animal species develops from the last pharyngeal pouch. The genes involved in the development of pharyngeal pouches are well conserved across vertebrates. The ultimobranchial anlage of nonmammalian vertebrates and monotremes does not merge with the thyroid, remaining as an independent organ throughout adulthood. Although C cells of all animal species secrete calcitonin, the shape, cellular components and location of the ultimobranchial gland vary from species to species. Avian ultimobranchial gland is unique in several phylogenic aspects; the organ is located between the vagus and recurrent laryngeal nerves at the upper thorax and is densely innervated by branches emanating from them. In chick embryos, TuJ1-, HNK-1-, and PGP 9.5-immunoreactive cells that originate from the distal vagal (nodose) ganglion, colonize the ultimobranchial anlage and differentiate into C cells; neuronal cells give rise to C cells. Like C cells of mammals, the cells of fishes, amphibians, reptiles, and also a subset of C cells of birds, appear to be derived from the endodermal epithelium forming ultimobranchial anlage. Thus, the avian ultimobranchial C cells may have dual origins, neural progenitors and endodermal epithelium. Developmental Dynamics 246:719-739, 2017. © 2017 Wiley Periodicals, Inc.

Oxygen sensing by the carotid body: mechanisms and role in adaptation to hypoxia

Oxygen (O2) is fundamental for cell and whole-body homeostasis. Our understanding of the adaptive processes that take place in response to a lack of O2(hypoxia) has progressed significantly in recent years. The carotid body (CB) is the main arterial chemoreceptor that mediates the acute cardiorespiratory reflexes (hyperventilation and sympathetic activation) triggered by hypoxia. The CB is composed of clusters of cells (glomeruli) in close contact with blood vessels and nerve fibers. Glomus cells, the O2-sensitive elements in the CB, are neuron-like cells that contain O2-sensitive K(+)channels, which are inhibited by hypoxia. This leads to cell depolarization, Ca(2+)entry, and the release of transmitters to activate sensory fibers terminating at the respiratory center. The mechanism whereby O2modulates K(+)channels has remained elusive, although several appealing hypotheses have been postulated. Recent data suggest that mitochondria complex I signaling to membrane K(+)channels plays a fundamental role in acute O2sensing. CB activation during exposure to low Po2is also necessary for acclimatization to chronic hypoxia. CB growth during sustained hypoxia depends on the activation of a resident population of stem cells, which are also activated by transmitters released from the O2-sensitive glomus cells. These advances should foster further studies on the role of CB dysfunction in the pathogenesis of highly prevalent human diseases.

Cellular and molecular events on the development of mammalian thyroid C cells

Thyroid C cells synthesize and secrete calcitonin, a serum calcium-lowering hormone. This review provides our current understanding of mammalian thyroid C cells from the molecular and morphological perspectives. Several transcription factors and signaling molecules involved in the development of C cells have been identified, and genes expressed in the pharyngeal pouch endoderm, neural crest-derived mesenchyme in the pharyngeal arches, and ultimobranchial body play critical roles for the development of C cells. It has been generally accepted, without much-supporting evidence, that mammalian C cells, as well as the avian cells, are derived from the neural crest. However, by fate mapping of neural crest cells in both Wnt1-Cre/R26R and Connexin(Cxn)43-lacZ transgenic mice, we showed that neural crest cells colonize neither the fourth pharyngeal pouch nor the ultimobranchial body. E-cadherin, an epithelial cell marker, is expressed in thyroid C cells and their precursors, the fourth pharyngeal pouch and ultimobranchial body. Furthermore, E-cadherin is colocalized with calcitonin in C cells. Recently, lineage tracing in Sox17-2A-iCre/R26R mice has clarified that the pharyngeal endoderm-derived cells give rise to C cells. Together, these findings indicate that mouse thyroid C cells are endodermal in origin.