Abnormalities of smooth muscle, basal laminae, and nerves in the aganglionic segments of the bowel of lethal spotted mutant mice

News from the endothelin-3/EDNRB signaling pathway: Role during enteric nervous system development and involvement in neural crest-associated disorders

The endothelin system is a vertebrate-specific innovation with important roles in regulating the cardiovascular system and renal and pulmonary processes, as well as the development of the vertebrate-specific neural crest cell population and its derivatives. This system is comprised of three structurally similar 21-amino acid peptides that bind and activate two G-protein coupled receptors. In 1994, knockouts of the Edn3 and Ednrb genes revealed their crucial function during development of the enteric nervous system and melanocytes, two neural-crest derivatives. Since then, human and mouse genetics, combined with cellular and developmental studies, have helped to unravel the role of this signaling pathway during development and adulthood. In this review, we will summarize the known functions of the EDN3/EDNRB pathway during neural crest development, with a specific focus on recent scientific advances, and the enteric nervous system in normal and pathological conditions.

Epigenetics in ENS development and Hirschsprung disease

Hirschsprung disease (HSCR, OMIM 142623) is a neurocristopathy caused by a failure of the enteric nervous system (ENS) progenitors derived from neural crest cells (NCCs), to migrate, proliferate, differentiate or survive to and within the gastrointestinal tract, resulting in aganglionosis in the distal colon. The formation of the ENS is a complex process, which is regulated by a large range of molecules and signalling pathways involving both the NCCs and the intestinal environment. This tightly regulated process needs correct regulation of the expression of ENS specific genes. Alterations in the expression of these genes can have dramatic consequences. Several mechanisms that control the expression of genes have been described, such as DNA modification (epigenetic mechanisms), regulation of transcription (transcription factor, enhancers, repressors and silencers), post-transcriptional regulation (3'UTR and miRNAs) and regulation of translation. In this review, we focus on the epigenetic DNA modifications that have been described so far in the context of the ENS development. Moreover we describe the changes that are found in relation to the onset of HSCR.

Smooth muscle strips for intestinal tissue engineering

Functionally contracting smooth muscle is an essential part of the engineered intestine that has not been replicated in vitro. The purpose of this study is to produce contracting smooth muscle in culture by maintaining the native smooth muscle organization. We employed intact smooth muscle strips and compared them to dissociated smooth muscle cells in culture for 14 days. Cells isolated by enzymatic digestion quickly lost maturity markers for smooth muscle cells and contained few enteric neural and glial cells. Cultured smooth muscle strips exhibited periodic contraction and maintained neural and glial markers. Smooth muscle strips cultured for 14 days also exhibited regular fluctuation of intracellular calcium, whereas cultured smooth muscle cells did not. After implantation in omentum for 14 days on polycaprolactone scaffolds, smooth muscle strip constructs expressed high levels of smooth muscle maturity markers as well as enteric neural and glial cells. Intact smooth muscle strips may be a useful component for engineered intestinal smooth muscle.

Epigenetic regulation in neural crest development

The neural crest is a migratory and multipotent cell population that plays a crucial role in many aspects of embryonic development. In all vertebrate embryos, these cells emerge from the dorsal neural tube then migrate long distances to different regions of the body, where they contribute to formation of many cell types and structures. These include much of the peripheral nervous system, craniofacial skeleton, smooth muscle, and pigmentation of the skin. The best-studied regulatory events guiding neural crest development are mediated by transcription factors and signaling molecules. In recent years, however, growing evidence supports an important role for epigenetic regulation as an additional mechanism for controlling the timing and level of gene expression at different stages of neural crest development. Here, we summarize the process of neural crest formation, with focus on the role of epigenetic regulation in neural crest specification, migration, and differentiation as well as in neural crest related birth defects and diseases.

The enteric nervous system

The enteric nervous system (ENS), the intrinsic innervation of the gastrointestinal tract, consists of numerous types of neurons, and glial cells, that are distributed in two intramuscular plexuses that extend along the entire length of the gut and control co-ordinated smooth muscle contractile activity and other gut functions. All enteric neurons and glia are derived from neural crest cells (NCC). Vagal (hindbrain) level NCC provide the majority of enteric precursors along the entire length of the gut, while a lesser contribution, that is restricted to the hindgut, arises from the sacral region of the neuraxis. After leaving the dorsal neural tube NCC undergo extensive migration, proliferation, survival and differentiation in order to form a functional ENS. This article reviews the molecular mechanisms underlying these key developmental processes and highlights the major groups of molecules that affect enteric NCC proliferation and survival (Ret/Gdnf and EdnrB/Et-3 pathways, Sox10 and Phox2b transcription factors), cell migration (Ret and EdnrB signalling, semaphorin 3A, cell adhesion molecules, Rho GTPases), and the development of enteric neuronal subtypes and morphologies (Mash1, Gdnf/neurturin, BMPs, Hand2, retinoic acid). Finally, looking to the future, we discuss the need to translate the wealth of data gleaned from animal studies to the clinical area and thus better understand, and develop treatments for, congenital human diseases affecting the ENS.

Differential distribution of some extracellular matrix fibers in an experimentally denervated rat megaileum

Absence of enteric neurons is associated with thickening of the intestinal muscularis externa in Chagas' disease. The thickening is due to hyperplasia and hypertrophy of the smooth muscle cells and increased extracellular matrix components. The influence of the nervous system on the structure of the smooth muscle cells and its associated matrix has been poorly investigated. An experimental model of denervation of the ileum in rats was performed by application of the surfactant agent benzalkonium chloride that selectively destroys the myenteric plexus. Three months later, ileal tissue samples were obtained and studied by histochemistry and transmission electron microsocopy. Sham operated rats were used as controls. The diameter of collagen fibrils was evaluated in electron micrographs. The histopathological analysis showed thickening of the muscular layer. The thin and weakly arranged collagen and reticulin fibers surrounding the smooth muscle cells, observed in control cases by Picrosirius polarization (PSP) stain method, corresponded to a population of loosely packed thin collagen fibrils of uniform diameters (mean=29.16 nm) at the ultrastructural level. In contrast, the thick and strongly birefringent fibers around the muscle cells, observed in the treated group, stained by PSP, corresponded to densely packed thicker fibrils with large variation in diameter (mean=39.41 nm). Comparison of the data demonstrated statistically significant difference between the groups suggesting that the replacement of loosely arranged reticulin fibers by fibrous tissue (with typical collagen fiber), may alter the biomechanical function resulting in impairment of muscular contraction.

Are interstitial cells of Cajal plurifunction cells in the gut?

The proposed functions of the interstitial cells of Cajal (ICC) are to 1) pace the slow waves and regulate their propagation, 2) mediate enteric neuronal signals to smooth muscle cells, and 3) act as mechanosensors. In addition, impairments of ICC have been implicated in diverse motility disorders. This review critically examines the available evidence for these roles and offers alternate explanations. This review suggests the following: 1) The ICC may not pace the slow waves or help in their propagation. Instead, they may help in maintaining the gradient of resting membrane potential (RMP) through the thickness of the circular muscle layer, which stabilizes the slow waves and enhances their propagation. The impairment of ICC destabilizes the slow waves, resulting in attenuation of their amplitude and impaired propagation. 2) The one-way communication between the enteric neuronal varicosities and the smooth muscle cells occurs by volume transmission, rather than by wired transmission via the ICC. 3) There are fundamental limitations for the ICC to act as mechanosensors. 4) The ICC impair in numerous motility disorders. However, a cause-and-effect relationship between ICC impairment and motility dysfunction is not established. The ICC impair readily and transform to other cell types in response to alterations in their microenvironment, which have limited effects on motility function. Concurrent investigations of the alterations in slow-wave characteristics, excitation-contraction and excitation-inhibition couplings in smooth muscle cells, neurotransmitter synthesis and release in enteric neurons, and the impairment of the ICC are required to understand the etiologies of clinical motility disorders.

Idiopathic megacolon

This leading article refers to the paper by Meier-Ruge WA, Muller-Lobeck H, Stoss F, Bruder E. The pathogenesis of idiopathic megacolon. Eur J Gastroenterol Hepatol 2006; 18:1209-1215. We apologise to all concerned for the dissociation between the two papers, which was due to an administrative error. The pathogenesis of idiopathic megacolon is still unclear. Besides abnormalities of the enteric nervous system, alterations in the function of intestinal smooth muscle cells and connective tissue elements might play an important role. A permanent extension of the bowel diameter without concrete hints to its aetiology is termed idiopathic megacolon. Evidence exists that idiopathic megacolon comprises a heterogeneous group of conditions characterized by alterations of the enteric nervous system, smooth muscle cells and/or connective tissue. Innovative molecular techniques are needed to get further insights into the pathogenesis of these intestinal motility disorders.

Vesicle-associated protein-A is differentially expressed during intestinal smooth muscle cell differentiation

Gastrointestinal (GI) smooth muscle diseases represent a major health concern affecting in excess of 2 million people each year. Little is currently known regarding the molecular mechanisms controlling either normal or pathogenic GI smooth muscle development. In an effort to identify the specific gene products responsible for modulating GI smooth muscle cell (SMC) differentiation, we performed differential display on distinct intestinal SMC (ISMC) phenotypes. This analysis identified over 40 unique transcripts that appeared to be differentially expressed in distinct SMC phenotypes. One such transcript that appeared to be preferentially expressed in immature smooth muscle myocytes was identified as vesicle-associated membrane protein, associated protein A (VAP-A). Northern blot analysis confirmed that VAP-A was expressed threefold higher in immature smooth muscle myocytes when compared with both smooth muscle myoblasts and mature smooth muscle myocytes. VAP-A mRNA was differentially expressed during normal rat development and showed peak levels of expression in the intestine during late embryogenesis and early neonatal development. These observations provide the first evidence that VAP-A-mediated membrane trafficking may play an important role in modulating ISMC differentiation.

Development of visceral smooth muscle

The development of the smooth musculature of viscera has attracted the interest of only relatively few investigators, and thus the field appears somewhat underexplored. The major emphasis on histochemical evidence--at the expense of ultrastructural and functional studies--may have limited the progress in this area. Mature tissue is formed through the differentiation of precursors into muscle cells and through the organization of these cells into a complex tissue where distribution and orientation of muscle cells, deployment of abundant extracellular materials and addition of other cellular elements (interstitial cells, fibroblasts, nerves, blood vessels) are characteristic and specific features. The precursor cells are found at sites where a muscle develops, and they derive predominantly from the mesoderm, but also from the neuroectoderm and from the endoderm. The process starts at different times in different organs. The earliest stages of differentiation are characterized by the precursor cells aggregating and becoming elongated; their longitudinal axis lies in a position similar to the one they will have in the mature muscle. Both the cytological and the histochemical differentiation follow distinct patterns in various muscles, with characteristic temporal sequences in the appearance of key features. This process must impart distinct functional properties to a muscle cell at each stage of its development. However, the chronological correspondence between ultrastructural and histochemical development is poorly understood. Histochemical studies have detected gradients of maturation of the muscle cells, for example, across the thickness of the gizzard musculature and along the length of the small intestine; ultrastructural studies have not yet confirmed the existence of these gradients. Muscle growth is accounted for by muscle cell enlargement (without nucleus duplication) and an increase in muscle cell number by mitosis of pre-existing differentiated muscle cells. De-differentiation and division of muscle cells, migration of muscle cells and late development of muscle cell precursors have all also been considered as possible mechanisms for muscle growth. Several authors have described the presence of precursor cells within developing smooth muscles, and they have described late differentiation of some muscle cells or waves of differentiation that would give rise to phenotypic heterogeneity of the mature muscle cell population. In contrast, other studies, mainly by electron microscopy, have suggested that, within large visceral muscles, the muscle cells differentiate synchronously. There are interesting data on the influence of adjacent tissues on the development of a smooth muscle, but the interplay of these and other factors has not been fully investigated. Smooth muscles contract from early in their development, hence mechanical factors are likely to influence development: on the one hand, passive stresses imposed on the muscle by other tissues, such as adjacent muscles or the contents of the viscera and, on the other hand, active forces generated by the muscle itself. The very attraction of visceral smooth muscles in the study of cellular morphogenesis--an attraction that has not yet been highlighted or exploited in scientific studies, either descriptively or experimentally--is that, onto a single type of cell, a large range of factors interact, such as the genetic expression, chemical influences (from other muscles, endocrine glands, nerves, other intramuscular cells) and mechanical factors.

The development of colon innervation in trisomy 16 mice and Hirschsprungs disease

AIM: To study the colon innervation of trisomy 16 mouse, an animal model for Down’s syndrome, and the expression of protein gene product 9.5 (PGP 9.5) in the stenosed segment of colon in Hirschsprungs disease (HD).

METHODS: Trisomy 16 mouse breeding; cytogenetic analysis of trisomy 16 mice; and PGP 9.5 immunohistochemistry of colons of trisomy 16 mice and HD were carried out.

RESULTS: Compared with their normal littermates, the nervous system of colon in trisomy 16 mice was abnormally developed. There existed developmental delay of muscular plexuses of colon, no submucosal plexus was found in the colon, and there was 5 mm aganglionic bowel aparting from the anus in trisomy 16 mice. The mesentery nerve fibers were as well developed as shown in their normal littermates. Abundant proliferation of PGP 9.5 positive nerve fibers was evealed in the stenosed segment of HD colon.

CONCLUSION: Trisomy 16 mice could serve as an animal model for Hirschsprung’s disease for aganglionic bowel in the distal part of colon. Abundant proliferation of PGP 9.5 positive fibers resulted from extrinsic nerve compensation, since no ganglionic cells were observed in the stenosed segment of the colon in HD. HD has a genetic tendency.

Endothelin and the development of the enteric nervous system

1. The enteric nervous system (ENS) is derived from cells that migrate to the bowel from the neural crest. These émigrés must find the gut, reach their correct locations within its wall and finally differentiate as neurons or glia. 2. Because the crest-derived precursor population is multipotent when it colonizes the bowel, the enteric micro-environment plays a prominent role in ENS development. 3. A number of molecules of the enteric micro-environment have been found to promote the development of neurons. 4. However, endothelin (ET)-3 appears to be different from any of these in that its role appears to be to prevent premature neuronal differentiation. 5. By activating ETB receptors, ET-3 inhibits the differentiation of crest-derived cells into neurons and promotes the development of smooth muscle. 6. The effect of ET-3 on smooth muscle down-regulates the secretion of laminin-1, which is a promoter of the formation of neurons. 7. In the absence of ET-3/ETB, crest-derived cells develop as neurons and, thus, cease migrating before they complete the colonization of the bowel. This premature development leaves the terminal colon aganglionic.

Hirschsprung disease and other enteric dysganglionoses

Hirschsprung disease has become a paradigm for multigene disorders because the same basic phenotype is associated with mutations in at least seven distinct genes. As such, the condition poses distinct challenges for clinicians, patients, diagnostic pathologists, and basic scientists, who must cope with the implications of this genetic complexity to comprehend the pathogenesis of the disorder and effectively manage patients. This review focuses on the anatomic pathology, genetics, and pathogenesis of Hirschsprung disease and related conditions. The nature and functions of "Hirschsprung disease genes" are examined in detail and emphasis is placed on the importance of animal models to this field. Where possible, potential uses and limitations of new data concerning molecular genetics and pathogenesis are discussed as they relate to contemporary medical practices.

Inhibition of in vitro enteric neuronal development by endothelin-3: mediation by endothelin B receptors

The terminal colon is aganglionic in mice lacking endothelin-3 or its receptor, endothelin B. To analyze the effects of endothelin-3/endothelin B on the differentiation of enteric neurons, E11-13 mouse gut was dissociated, and positive and negative immunoselection with antibodies to p75(NTR)were used to isolate neural crest- and non-crest-derived cells. mRNA encoding endothelin B was present in both the crest-and non-crest-derived cells, but that encoding preproendothelin-3 was detected only in the non-crest-derived population. The crest- and non-crest-derived cells were exposed in vitro to endothelin-3, IRL 1620 (an endothelin B agonist), and/or BQ 788 (an endothelin B antagonist). Neurons and glia developed only in cultures of crest-derived cells, and did so even when endothelin-3 was absent and BQ 788 was present. Endothelin-3 inhibited neuronal development, an effect that was mimicked by IRL 1620 and blocked by BQ 788. Endothelin-3 failed to stimulate the incorporation of [3H]thymidine or bromodeoxyuridine. Smooth muscle development in non-crest-derived cell cultures was promoted by endothelin-3 and inhibited by BQ 788. In contrast, transcription of laminin alpha1, a smooth muscle-derived promoter of neuronal development, was inhibited by endothelin-3, but promoted by BQ 788. Neurons did not develop in explants of the terminal bowel of E12 ls/ls (endothelin-3-deficient) mice, but could be induced to do so by endothelin-3 if a source of neural precursors was present. We suggest that endothelin-3/endothelin B normally prevents the premature differentiation of crest-derived precursors migrating to and within the fetal bowel, enabling the precursor population to persist long enough to finish colonizing the bowel.

Identification of distinct molecular phenotypes in cultured gastrointestinal smooth muscle cells

BACKGROUND & AIMS

Cultured gastrointestinal smooth muscle cells have been shown to dedifferentiate and reinitiate their myogenic program in vitro. The aim of this study was to determine whether the cellular phenotypes observed in vitro were similar to those previously characterized in vivo.

METHODS

Differential isoactin expression was examined in primary cultures of intestinal smooth muscle cells (ISMCs) by Northern blot and immunohistochemical analysis. Cellular phenotype was determined for cultured ISMCs grown at high density, at low density, in the presence and absence of serum supplementation, and on several distinct substrates including collagen type IV, laminin, fibronectin, and plastic.

RESULTS

The unique patterns of isoactin protein and gene expression observed in cultured ISMCs indicate that distinct cellular phenotypes were present in vitro. The production and maintenance of these distinct smooth muscle cell phenotypes was dependent on cell density, serum supplementation, and substrate used.

CONCLUSIONS

Cultured ISMCs appear to recapitulate a portion of their in vivo myogenic program in vitro, providing a unique opportunity for the molecular mechanisms controlling gastrointestinal smooth muscle myogenesis and pathogenesis to begin to be identified.

Development of LBP110 expression by neural crest-derived enteric precursors: migration and differentiation potential in ls/ls mutant mice

Neural crest-derived cells acquire a 110-kD laminin-binding protein (LBP110) when they colonize the murine bowel. Laminin stimulates LBP110-expressing cells to develop as neurons. We have followed the development of LBP110 by neural crest-derived cells as they enter the gut of control and ls/ls mutant mice. The expression of neurofilament and choline acetyltransferase was used as markers of a neuronal phenotype. Tyrosine hydroxylase was used as a marker for the mash-1-dependent lineage of enteric precursors, while calcitonin gene-related peptide was used as a marker for the mash-1-independent lineage of crest-derived cells. A subset of cells expressing LBP110 was located along the vagi at E10 at cervical and thoracic levels. At E12, cells expressing LBP110 extended from the foregut to the midgut. The expression of neurofilament protein lagged behind that of LBP110 by about 0.5 day and then became coincident with LBP110 immunoreactivity. By E15, cells doubly labeled with antibodies to LBP110 and neurofilament protein were located along the entire extent of the bowel up to but not including the terminal colon. By E16, both the proximal and terminal colon contained cells expressing LBP110 and neurofilaments. The pattern of immunoreactivity could not be distinguished between ls/ls and control animals prior to E16. By E16, when the terminal colon of control animals contained many cells expressing LBP110 and neurofilaments, the terminal colon of ls/ls animals lacked cells expressing these proteins; nevertheless, structures outside of the terminal colon were heavily endowed with cells expressing LBP110 and neurofilaments. These ectopically located cells derived from both mash-1-dependent and -independent lineages of crest-derived precursors.

The relationship between myenteric neuronal denervation, smooth muscle thickening and epithelial cell proliferation in the rat colon

The effects of myenteric neuronal denervation on smooth muscle thickening and epithelial cell proliferation were studied in the descending colon of rats treated by serosal application of 2 mM benzalkonium chloride (BAC) for 30 min. Control animals were treated with saline (0.9% NaCl). The animals were divided into six groups of 13 animals each and killed 10, 45 and 120 days after BAC treatment. A significant reduction in neuron number was observed in the myenteric plexus of animals treated with BAC, as well as smooth muscle thickening and an increase in crypt cell population, crypt cell production per crypt and a decrease in cell cycle time. These findings permit us to conclude that a relationship may exist between the increase of epithelial cell proliferation, smooth muscle thickening and myenteric neuron denervation in the descending colon caused by BAC, the latter probably playing an important role in the integration of the other two.

Distribution of nitric oxide synthase-containing nerves in the aganglionic intestine of mutant rats: a histochemical study

We examined the distribution of nerves containing nitric oxide synthase in the intestine of congenitally aganglionic rats, using a reduced nicotinamide adenine dinucleotide phosphate diaphorase histochemical method for whole-mount and cryostat specimens. A constricted intestinal segment extends from the terminal ileum to the anus in this mutant. No nerve elements with the activity were found in the affected terminal ileum, cecum and proximal colon. Although intrinsic ganglionic neurons were absent along the constricted intestine, nerve fibers with the activity were found in both the submucous and intermuscular layers distal to the proximal colon. These fibers increased in density towards the rectum, forming hypertrophic nerve bundles and unusual fiber networks. However, positive fibers were never seen within the circular and longitudinal musculature of the constricted lesion. Some of these hypertrophic nerve bundles were continuous with ectopic ganglia that were situated in the adventitial connective tissue around the lower rectum and in the submucosa near the anus. The hypertrophic nerve bundles seemed to have an extrinsic origin; some of them may have originated from ectopic ganglia. These results suggest that the defective distribution of nerves containing nitric oxide synthase may be involved in the pathogenesis of congenital colonic aganglionosis.

Distinct subpopulations of enteric neuronal progenitors defined by time of development, sympathoadrenal lineage markers and Mash-1-dependence

Enteric and sympathetic neurons have previously been proposed to be lineally related. We present independent lines of evidence that suggest that enteric neurons arise from at least two lineages, only one of which expresses markers in common with sympathoadrenal cells. In the rat, sympathoadrenal markers are expressed, in the same order as in sympathetic neurons, by a subset of enteric neuronal precursors, which also transiently express tyrosine hydroxylase. If this precursor pool is eliminated in vitro by complement-mediated lysis, enteric neurons continue to develop; however, none of these are serotonergic. In the mouse, the Mash-1−/− mutation, which eliminates sympathetic neurons, also prevents the development of enteric serotonergic neurons. Other enteric neuronal populations, however, including those that contain calcitonin gene related peptide are present. Enteric tyrosine hydroxylase-containing cells co-express Mash-1 and are eliminated by the Mash-1−/− mutation, consistent with the idea that in the mouse, as in the rat, these precursors generate serotonergic neurons. Serotonergic neurons are generated early in development, while calcitonin gene related peptide-containing enteric neurons are generated much later. These data suggest that enteric neurons are derived from at least two progenitor lineages. One transiently expresses sympathoadrenal markers, is Mash-1-dependent, and generates early-born enteric neurons, some of which are serotonergic. The other is Mash-1-independent, does not express sympathoadrenal markers, and generates late-born enteric neurons, some of which contain calcitonin gene related peptide.

Abnormal expression and distribution of nidogen in Hirschsprung's disease

Hirschsprung's disease (congenital colonic aganglionosis) is associated with abnormalities in the distributions and amounts of basement membrane and other extracellular matrix components in the human gut. The authors have investigated the possible significance of nidogen in Hirschsprung's disease, because this glycoprotein is necessary for the formation of ternary complexes with the other basement membrane components, laminin and collagen type IV, and thus may contribute the pathology of the disease. Increased nidogen immunoreactivity in the smooth muscle basement membranes and muscularis mucosae of the distal aganglionic zone in Hirschsprung's bowel was observed, the nidogen immunoreactivity demonstrating that the thickness of the muscularis mucosae was increased in this region. However, steady-state nidogen mRNA levels were found to be significantly lower in both proximal and distal Hirschsprung's bowel (relative to controls). In contrast, no significant differences were observed in the steady-state levels of the mRNAs coding for laminin subunits. These results indicate that although abnormalities in the amount or distribution of nidogen may contribute to the abnormalities seen in the extracellular matrix in Hirschsprung's disease, the levels of expression of the genes coding for either nidogen or laminin are unlikely to be primarily responsible for the lesions.

Intercellular signals downstream of endothelin receptor-B mediate colonization of the large intestine by enteric neuroblasts

Mice homozygous for the piebald lethal (sl) mutation, which have a complete deletion of endothelin receptor-B, fail to form ganglion cells in the distal large intestine and are nearly devoid of cutaneous melanocytes. These phenotypic features stem from incomplete colonization of the hindgut and skin by neural crest-derived neuroblasts and melanoblasts, respectively. We have used expression of a transgene, dopamine-beta-hydroxylase-nlacZ, to study colonization of the enteric nervous system in sl/sl embryos and sl/sl <--> wild-type chimeric mice. Enteric neuroblasts derived from the vagal neural crest colonize the developing foregut, midgut and distal small intestine of sl/sl embryos in a cranial-to-caudal manner indistinguishable from sl/+ or +/+ embryos. However, colonization of the large intestine is retarded and the distal large intestine is never colonized, a developmental defect identical to that observed in lethal spotted (endothelin-3 deficient) embryos. The coat pigmentation and relative distributions of mutant and wild-type ganglion cells in sl/sl <--> wild-type chimeras indicate that the defect associated with endothelin receptor-B gene deletion is not strictly neuroblast autonomous (independent of environmental factors). Instead, intercellular interactions downstream of the endothelin receptor-B mediate complete colonization of the skin and gut by neural crest cells.

Spatial distribution and pattern of extrinsic nerve strands in the aganglionic segment of congenital aganglionosis: stereoscopic analysis in spotting lethal rats

The distribution and pathways of extrinsic enteric nerves in the aganglionic segment in Hirschsprung's disease have not completely delineated. The zinc iodide-osmic acid method was used to investigate nerve fibers of the intestine in aganglionic rats. A light microscope equipped with a polarizing filter system clarified the major pathways and the fine distribution of extrinsic nerves in the aganglionic segment of a long-segment aganglionic rat model. Two sets of extrinsic nerve strands were observed in the distal aganglionic colon; one consisted of a continuous hypertrophied network located between the longitudinal and circular muscle coats, and the other involved branches that coursed along the blood vessels in the submucosal layer and that originated in the nearby mesentery. These two sets of nerve strands were connected by fine nerve fibers. Hypertrophied myenteric nerve strands were also present in normal (ganglionic) colon, fewer strands were found in the proximal aganglionic colon, and none were found in the ileum. The authors speculate that the ascending extrinsic nerve strands disappear from the ileum in cases of long-segment colonic aganglionosis, by the same mechanism that occurs in normally innervated bowel.

Structural abnormalities associated with congenital megacolon in transgenic mice that overexpress the Hoxa-4 gene

Congenital megacolon develops in transgenic mice that overexpress the homeobox-containing gene, Hoxa-4. The current study was done to identify abnormalities of the terminal colon that might account for the phenotype. The terminal bowel of transgenic mice was compared with that of control and lethal spotted (ls/ls) mice, a strain in which megacolon also develops. The terminal colon of the transgenic mice contained fewer ganglia than that of controls, but was hypoganglionic, rather than aganglionic like that of ls/ls mice. The neurons present in the adult transgenic colon were significantly increased in size and a subset of very large neurons (> 40 microns in maximum diameter) were observed. Electron microscopic studies of young adult transgenic mice revealed that the ganglia and nerves of the myenteric plexus had the ultrastructure of extraenteric peripheral nerve rather than that of the enteric nervous system (ENS). The myenteric ganglia in the transgenic animals contained Schwann cells associated with a basal lamina that enveloped axons completely and individually, instead of glia. Although collagen is excluded from the ganglia and thin nerve fibers of the normal ENS, a collagen-containing endoneurium surrounded each of the axon-Schwann cell units of the abnormal nerve fibers of the transgenic colon. Some of the neurons of the transgenic mice were located in these nerve bundles rather than in ganglia. There were also smooth muscle abnormalities in the terminal bowel of the transgenic mice. Wide gaps were present in the longitudinal muscle of the transgenic mice; these gaps contained ganglia that were in contact with the adventitia. These longitudinal smooth muscle cells were more irregular than those of controls and they contained fewer puncta adherens; moreover, a larger proportion of the volume of the cytoplasm of transgenic smooth muscle cells was occupied by organelles. Finally, an extensive thickening and reduplication of the basal lamina surrounding the smooth muscle cells of the muscularis mucosa was observed in the transgenic colon and resembled that found in ls/ls mice. These data suggest that both smooth muscle and the innervation of the terminal bowel of neonatal Hoxa-4 transgenic mice are structurally abnormal. Although some of the abnormalities seen in Hoxa-4 transgenic mice are similar to those which arise in ls/ls mice, the two conditions are not identical. In both animals, the data are consistent with the hypothesis that the defects arise as a result of a defective interaction between the precursors of enteric neurons and smooth muscle.

Altered isoactin gene expression in the affected bowel segments of the lethal spotted mouse

BACKGROUND

Actin is a key contractile protein associated with the normal differentiation and function of gastrointestinal smooth muscle cells. Distinct changes in gastrointestinal smooth muscle cell morphology and function have been reported for the aganglionic rectum and megacolon of the adult lethal spotted mouse. This study examines what effect these changes in smooth muscle cell morphology and function have on the expression of the actin multigene family in both the aganglionic rectum and megacolon of the lethal spotted mouse.

METHODS

Expression of the smooth muscle and cytoplasmic isoactins was examined by Northern blot analysis of the aganglionic rectum and megacolon of the homozygotic lethal spotted mouse and the equivalent bowel segments of control animals.

RESULTS

The megacolon of the lethal spotted mouse showed a significant increase in gamma-smooth muscle isoactin expression. The aganglionic rectum of the lethal spotted mouse displayed a complex pattern of altered isoactin gene expression that included changes in both gamma-smooth muscle and beta-cytoplasmic isoactin expression. Strain-specific differences in the quantitative levels of isoactin gene expression were observed for the various bowel segments examined in this study.

CONCLUSIONS

These results show that the changes in smooth muscle cell morphology and function observed in the lethal spotted mutant mouse are accompanied by significant alterations in isoactin gene expression.

Aggregation chimeras demonstrate that the primary defect responsible for aganglionic megacolon in lethal spotted mice is not neuroblast autonomous

The lethal spotted (ls) mouse has been used as a model for the human disorder Hirschsprung's disease, because as in the latter condition, ls/ls homozygotes are born without ganglion cells in their terminal colons and, without surgical intervention, die early as a consequence of intestinal obstruction. Previous studies have led to the conclusion that hereditary aganglionosis in ls/ls mice occurs because neural crest-derived enteric neuroblasts fail to colonize the distal large intestine during embryogenesis, perhaps due to a primary defect in non-neuroblastic mesenchyme rather than migrating neuroblasts themselves. In this investigation, the latter issue was addressed directly, in vivo, by comparing the distributions of ls/ls and wild-type neurons in aggregation chimeras. Expression of a transgene, D beta H-nlacZ, in enteric neurons derived from the vagal neural crest, was used as a marker for ls/ls enteric neurons in chimeric mice. In these animals, when greater than 20% of the cells were wild-type, the ls/ls phenotype was rescued; such mice were neither spotted nor aganglionic. In addition, these ‘rescued’ mice had mixtures of ls/ls and wild-type neurons throughout their gastrointestinal systems including distal rectum. In contrast, mice with smaller relative numbers of wild-type cells exhibited the classic ls/ls phenotype. The aganglionic terminal bowel of the latter mice contained neither ls/ls nor wild-type neurons. These results confirm that the primary defect in ls/ls embryos is not autonomous to enteric neuroblasts, but instead exists in the non-neuroblastic mesenchyme of the large intestine.

Mesenteric neurons in the adult rat are responsive to ileal treatment with benzalkonium chloride

The application of a 2 mM solution of the cationic surfactant benzalkonium chloride (BAC) to an ileal segment produced a selective and extensive myenteric denervation. The aim of the present study was to investigate whether such a selective unbalance of the enteric nervous system in the adult rat elicited any plastic response within the mesenteric nervous structures contacting it. Acetylcholinesterase (AChE) staining, beta-nicotinamide adenine dinucleotide (NADH) and beta-nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemistry and glyoxylic acid-induced fluorescence were performed on whole-mount preparations of myenteric plexus and mesenteric nerves. In both control and BAC-treated animals nervous elements were detected along the mesenteric nerves. Although rather similar in position, shape and size, these neurons displayed striking differences with regard to their arrangement and density per nerve. In the controls, few small aggregates of neurons could be detected; more commonly, isolated nerve cell bodies were scattered along the nerve trunks. In the BAC-treated animals, numerous spherical or spindle-shaped clusters of neurons were located along the nerves; the mean neuronal density per nerve displayed a two-five-fold increase over the control values. The observed changes within the mesenteric nerves might be involved in an attempt at reinnervation of the BAC-treated intestinal segments from extra-enteric sources.

Neuroplasticity in the smooth muscle of the myenterically and extrinsically denervated rat jejunum

The objective of this study was to examine the effects of two different denervation procedures on the distribution of nerve fibers and neurotransmitter levels in the rat jejunum. Extrinsic nerves were eliminated by crushing the mesenteric pedicle to a segment of jejunum. The myenteric plexus and extrinsic nerves were eliminated by serosal application of the cationic surfactant benzyldimethyltetradecylammonium chloride (BAC). The effects of these two denervation procedures were evaluated at 15 and 45 days. The level of norepinephrine in whole segments of jejunum was initially reduced by more than 76% after both denervation procedures, but by 45 days the level of norepinephrine was the same as in control tissue. Tyrosine hydroxylase (noradrenergic nerve marker) immunostaining was absent at 15 days, but returned by 45 days. However, the pattern of noradrenergic innervating axons was altered in the segment deprived of myenteric neurons. Immunohistochemical studies showed protein gene product 9.5 (PGP 9.5)-immunoreactive fibers in whole-mount preparations of the circular smooth muscle in the absence of the myenteric plexus and extrinsic nerves. At 45 days, the number of nerve fibers in the circular smooth muscle increased. Vasoactive intestinal polypeptide (VIP)-immunoreactive fibers, a subset of the PGP 9.5 nerve fibers, were present in the circular smooth muscle at both time points examined. Choline acetyltransferase (CAT) activity and VIP and leucine enkephalin levels were measured in separated smooth muscle and submucosa-mucosal layers of the denervated jejunum. VIP and leucine-enkephalin levels were no different from control in tissue that was extrinsically denervated alone. However, the levels of these peptides were elevated two-fold in the smooth muscle 15 and 45 days after myenteric and extrinsic denervation. In the submucosa-mucosa, VIP and leucine enkephalin levels also were elevated two-fold at 15 days, but comparable to control at 45 days. CAT activity was equal to control in the smooth muscle but elevated two-fold in the submucosa-mucosa at both times. These results provide evidence for innervation of the circular smooth muscle by the submucosal plexus. Moreover, these nerve fibers originating from the submucosal plexus proliferate in the absence of the myenteric plexus. Furthermore, the myenteric neurons appear to be essential for normal innervation of the smooth muscle by the sympathetic nerve fibers. It is speculated that the sprouting of the submucosal plexus induced by myenteric plexus ablation is mediated by increased production of trophic factors in the hyperplastic smooth muscle.

Contemporary approaches toward understanding the pathogenesis of Hirschsprung disease

Hirschsprung disease (HD), or congenital aganglionosis coli, is a birth defect with heterogeneous causes. In an effort to understand the molecular and cellular bases for this disorder, researchers have investigated enteric neurodevelopment in normal animals and compared these findings with observations of inbred animal strains that develop aganglionosis coli due to mutations at specific genetic loci. Recent technological advances, including use of retroviral and fluorescent lineage makers, immunohistochemical probes, and transgenic mice, have provided insights into the origins, behavior, and properties of enteric neuroblasts. Experiments with mutant murine embryos indicate that aganglionosis coli results from primary failure of neural crest-derived neuroblasts to colonize the distal colon. In at least one model, impaired colonization by neuroblasts may be secondary to environmental defects restricted to colonic mesenchyme. The discovery that human piebald trait, a hereditary disorder with a high incidence of HD, is caused by mutations in a growth factor receptor highlights the importance of regulatory intercellular interactions between nonneuroblastic mesenchyme and neuroblasts during normal development of the enteric nervous system. These observations, coupled with advances in molecular genetics, set the stage for dramatic progress in this field of research in the near future.

A transgenic model for studying development of the enteric nervous system in normal and aganglionic mice

The dopamine beta-hydroxylase promoter has been shown to direct expression of the reporter gene product, beta-galactosidase, to enteric neurons and putative embryonic neuroblasts in transgenic mice (Mercer et al., 1991; Kapur et al., 1991). In this paper, expression of the transgene, D beta H-nlacZ, in the gastrointestinal tract is characterized in more detail in wild-type mice and mice which are also homozygous for the lethal spotted allele (ls). Expression of the transgene in wild-type embryos was first detected in scattered mesenchymal cells in the proximal foregut on embryonic day 9.5, and progressed distally until embryonic day 13.5 when the entire length of the gut was colonized by such cells. Several observations suggest that the mesenchymal cells which express the transgene (MCET) are, in fact, enteric neuroblasts, probably derived from the vagal neural crest. (1) The presence of MCET in progressively more caudal portions of the embryonic gut correlated with the neurogenic potential of isolated gastrointestinal segments grafted under the renal capsule. (2) Mitotic activity of MCET was demonstrated by incorporation of [3H]thymidine in utero. (3) The migratory behavior of MCET and/or their precursors was revealed in anastomotic subcapsular grafts of gut from transgenic and non-transgenic embryos; enteric ganglia of the latter were populated by MCET from the former. (4) Enteric expression of the transgene postnatally was restricted to intrinsic neurons that coexpressed other phenotypic markers of neuronal differentiation. The pattern of transgene expression in ls/ls mice was identical to that seen in ls/+ and +/+ mice until embryonic day 12.5.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of a neurally related laminin binding protein by neural crest-derived cells that colonize the gut: relationship to the formation of enteric ganglia

In order to give rise to the enteric nervous system (ENS), cells migrating from the neural crest must find the bowel and cease migrating at appropriate locations within the gut. Previous studies of the development of the ENS in a mutant mouse have led to the hypothesis that laminin in the enteric mesenchyme may act as a signal to crest-derived cells to cease migrating and extend neurites (or glial processes). Implied in this hypothesis is the idea that crest-derived cells, as a prelude to their participation in ganglion formation, acquire a neurally related laminin receptor, which they do not express at pre-enteric stages of migration. As a partial test of this hypothesis, single and double label immunocytochemistry at light and electron microscopic (EM) levels were used to study the expression of cell surface laminin binding proteins by crest-derived cells in the process of migrating to or within the developing chick gut. Two antibodies (called 3070 and alpha-110) raised against neuronal cell surface laminin binding proteins were employed for this purpose. Laminin binding protein immunoreactivity was found to be expressed within the bowel and ganglion of Remak by a subset of crest-derived cells (identified immunocytochemically with NC-1/HNK-1 antibodies) and by all of those developing as neurons (identified immunocytochemically with antibodies to neurofilament-associated proteins). Laminin binding protein immunoreactivity was also found to be expressed in fixed neural structures elsewhere in the embryos, including cranial and spinal roots, nerves, and ganglia. In contrast, laminin binding protein immunoreactivity was not expressed by migrating crest-derived cells in the vicinity of the vagal or sacral regions of the neuraxis (from which the precursors of the ENS take origin); nor was it expressed by juxta-pharyngeal vagal crest-derived cells migrating to the foregut through the caudal branchial arches or by the caudal stream of sacral crest-derived cells approaching the hindgut. EM immunocytochemistry confirmed that laminin binding protein immunoreactivity in the bowel was located on the surfaces of crest-derived cells, and was exhibited both by those cells that could only be distinguished from their neighbors by their NC-1/HNK-1 immunoreactivity and by cells developing as neurons or glia. EM immunocytochemistry also revealed that the surfaces of crest-derived cells migrating through the enteric mesenchyme were contacted by many small osmiophilic "puffs" of laminin-immunoreactive extracellular material. These puffs coincided in location with membrane sites that expressed the immunoreactivity of the laminin binding protein. These observations are consistent with the hypothesis that laminin plays a role in the formation of enteric ganglia.

The chronological relationship between the thickening of smooth muscle, epithelial cell proliferation and myenteric neural denervation in the rat jejunum

The jejunum of rats was treated by serosal application of a 0.2% solution of benzalkonium chloride (BAC) for 30 min. Control animals were treated with saline (0.9% NaC1). The animals were allocated to eight groups of 10 rats each and sacrificed 15, 30, 45, 60 days after BAC treatment. Segments were removed from the jejunum for neuronal counting, measurement of the smooth muscle area and morphokinetic study of the epithelium. There was a significant reduction in neuron number in the myenteric plexus 30 days after BAC treatment, thickening of smooth muscle 15-60 days after BAC treatment, but no change in epithelial cell proliferation in the jejunum at either time.

Formation and malformation of the enteric nervous system in mice: an organ culture study

Despite some progress in the treatment of congenital malformations of the enteric nervous system, there is no knowledge about the pathogenesis. The study of the normal formation of the enteric nervous system is hampered by the difficulty of manipulating and culturing mammalian embryos and their organs. Three methods to culture bowel explants of murine embryos, (chorioallantoic membrane grafting, organotypic tissue culture, and renal subcapsular space grafting) were compared. The three-dimensional cytoarchitecture of the bowel developed almost normally in the renal subcapsular space cultures. Using this culture system, it was found that neural crest cells colonize the murine bowel in distinct phases. The distal bowel was colonized at the 13th day of development. In a spontaneous mouse mutant model for intestinal aganglionosis, the lethal spotted mouse, the colonization of the distal 2 mm of the bowel did not occur at E13.

Distribution of hyaluronic acid and chondroitin sulfate proteoglycans in the presumptive aganglionic terminal bowel of ls/ls fetal mice: an ultrastructural analysis

The terminal colon of the ls/ls mouse is aganglionic because an intrinsic defect prevents its colonization by cells migrating from the neural crest. Previous studies showed that laminin, type IV collagen, and glycosaminoglycans accumulate in the region of the presumptive aganglionic ls/ls bowel through which crest-derived cells would be expected to migrate. It was suggested that crest-derived cells might fail to enter the abnormal bowel because they receive inappropriate signals from a defective extracellular matrix. This hypothesis was evaluated by analyzing the ultrastructure of the extracellular matrix in mutant and control gut. Tissue was fixed in the presence of ruthenium red before or after selective enzymatic digestion. Heparan sulfate proteoglycan (diameter approximately equal to 15 nm) and chondroitin sulfate proteoglycan (diameter approximately equal to 20-50 nm) granules were found in both control and presumptive aganglionic gut. The heparan sulfate proteoglycan granules were primarily located within formed basal laminae, while chondroitin sulfate proteoglycan granules decorated plasma membranes and 5 nm hyaluronic acid microfibrils that formed a network in the extracellular matrix. At day E11.5, the mutant gut differed from the control in the following: 1) Hyaluronic acid microfibrils were longer and more numerous. 2) There were larger numbers of chondroitin sulfate proteoglycan granules associated with cell membranes and with hyaluronic acid microfibrils. By day E13 the spaces between mesenchymal cells of the outer wall of the control bowel contained a regular lattice of hyaluronic acid microfibrils studded with chondroitin sulfate proteoglycan granules. Instead of this lattice, tangles of excessively long hyaluronic acid microfibrils, coated more heavily than in the control with chondroitin sulfate proteoglycan granules, were found in the presumptive aganglionic gut. These results confirm that the extracellular matrix is abnormal in the presumptive aganglionic bowel of the ls/ls mouse; moreover, they also indicate that the defect involves not one, but several components of the extracellular matrix, as well as their distribution. The defective extracellular matrix is apparent at a time when crest-derived cells would be expected to be migrating in the terminal bowel and is located in their path. The observations thus support the idea that a localized abnormality of the extracellular matrix interferes with the colonization of the terminal bowel by crest-derived cells in the ls/ls mouse.

Mouse mutants provide new insights into the role of extracellular matrix in cell migration and differentiation

Migratory cell populations in the developing embryo disperse, localize and eventually differentiate in environments rich in extracellular matrix material. The extracellular matrix provides both a substratum for migration and a source of differentiative cues for the developing cells. Mutations in mice and other animals that alter embryonic interstitial environments are now providing information about the role of the extracellular matrix in these early developmental processes.

Neural and glial phenotypic expression by neural crest cells in culture: effects of control and presumptive aganglionic bowel from ls/ls mice

The enteric nervous system is formed by cells that migrate to the bowel from the neural crest. Previous experiments have established that avian crest cells in vitro will colonize explants of murine bowel and there give rise to neurons. It has been proposed that phenotypic expression by the crest-derived precursors of enteric neurons and glia is critically influenced by the microenvironment these cells encounter within the gut. To test this hypothesis, quail crest cells were cocultured with explants of control or presumptive aganglionic bowel from the ls/ls mutant mouse, and the effects of the enteric tissue on five phenotypic markers of crest cell development were followed. Aganglionosis develops in the terminal region of the colon of the ls/ls mouse because viable crest-derived neural and glial precursors fail to colonize this tissue. Expression of the phenotypic markers in the cocultures was compared with that in cultures of crest alone, crest plus neural tube, and gut grown alone. The markers examined were melanogenesis and immunostaining with antisera to 5-hydroxytryptamine (5-HT) and tyrosine hydroxylase (TH) and the monoclonal antibodies, NC-1 and GlN1. Explants of control, but not presumptive aganglionic ls/ls gut were found to increase the incidence of the expression of 5-HT and NC-1 immunoreactivities; moreover, especially near the gut, the assumption of a neuronal morphology by 5-HT-, NC-1-, and GlN1-immunoreactive cells was also increased. Coincidence of expression of 5-HT with NC-1 and GlN1 immunoreactivities was observed. The effect of the bowel was selective in that the expression of TH immunoreactivity, which is not a marker of mature enteric neurons, was reduced rather than enhanced. The effect of enteric explants on crest cell development was specific in that it was not mimicked by explants of metanephros, which inhibited expression of 5-HT immunoreactivity and the acquisition of a neuritic form by NC-1-immunoreactive cells. It is concluded that the enteric microenvironment affects the phenotypic expression of subsets of crest cells and that this action of the bowel is manifested in vitro. The inability of presumptive aganglionic gut from ls/ls mice to influence neural phenotypic expression may be due to the failure of this tissue to produce putative factor(s) required for the effect or to the inability of the crest-derived precursor cells to migrate into the abnormal enteric tissue.

Accumulation of components of basal laminae: association with the failure of neural crest cells to colonize the presumptive aganglionic bowel of ls/ls mutant mice

Aganglionosis occurs in the terminal colon of the ls/ls mouse because an intrinsic defect of the presumptive aganglionic tissue prevents the entry and colonization of this portion of the bowel by migrating neural crest cells. The current study was undertaken to determine if abnormalities of the extracellular matrix could be identified in this segment that might account for migratory failure. Since basal laminae of the muscularis mucosa are overproduced in the aganglionic segment of adult ls/ls mice, we examined components of basal laminae in fetal gut from Day E 11 to Day E 16 of gestation. This period spans the time of enteric ganglion formation. Laminin and collagen type IV were studied by immunocytochemistry and proteoglycans by staining glycosaminoglycans with Alcian blue. Abnormalities of each of these components occur during development of the presumptive aganglionic bowel in the ls/ls mouse and could be detected as early as Day E 11. These defects consist mainly of an overabundance of these materials, both in defined basal laminae and throughout the extracellular space of the mesenchyme. Electron microscopic observations in the presumptive aganglionic ls/ls colon revealed a thickening of basal laminae and exceptionally wide intercellular spaces between smooth muscle myoblasts that contained an irregular fibrillar material, consisting of 4.5- to 6.0-nm filaments associated with 14- to 20-nm granules. Fibrillar and flocculant material was continuous with formed basal laminae, and was concentrated in the same areas found to have an overabundance of laminin immunoreactivity. These observations indicate that there is an accumulation of extracellular matrix material, including components of basal laminae, that (i) precedes the formation of enteric ganglia, (ii) is in the path through which enteric neural precursors from the crest would have to migrate, and (iii) is limited to the aganglionic and hypoganglionic ls/ls bowel. These data are consistent with the hypothesis that components of basal laminae contribute to the inability of crest cells to colonize the terminal bowel of ls/ls mice.

Origin and morphology of nerve fibers in the aganglionic colon of the lethal spotted (ls/ls) mutant mouse

The lethal spotted mutant mouse (ls/ls) develops congenital megacolon because of the absence of ganglia in the terminal colon. This aganglionosis results from a failure of neural crest cells to colonize this area during fetal life. We have postulated that the microenvironment of the aganglionic segment of bowel is abnormal. Our hypothesis suggests that this abnormal enteric microenvironment fosters the sprouting of neuritic processes. We further propose that neural and glial precursors cease to migrate once they have extended their definitive processes. As a result, the area distal to the site where neurite extension is favored does not become colonized by neural or glial precursors. A prediction of this hypothesis is that the aganglionic tissue should be innervated by axons from neurons located both in the more proximal ganglionated bowel and in ganglia located outside the gut. Neurons and their processes in control and ls/ls terminal gut were located by the histochemical demonstration of acetylcholinesterase (AChE) activity and their structure was classified as intrinsic (enteric) or extrinsic in type by electron microscopy. In ls/ls mice the submucosal plexus was much more severely affected than the myenteric plexus. No submucosal ganglia were found within 30 mm of the anus. In contrast, myenteric ganglia extended to within 4 mm of the anus on the mesenteric side of the gut and to within 15 mm on the antimesenteric side. Rostral to the areas that were absolutely aganglionic, both plexuses were hypoganglionic, especially the submucosal plexus, which was hypoganglionic throughout the entire colon. Both the aganglionic and caudal hypoganglionic zones of the ls/ls bowel were penetrated by large nerve trunks that had the ultrastructural characteristics of extra-enteric peripheral nerve. Unusual ganglia, outside the enteric musculature in the adventitia of the colon, were connected to these trunks. The location of the cell bodies of origin of the nerve fibers in the terminal colon of control mice and in the aganglionic segment of the bowel in ls/ls mice was determined by following the retrograde transport of tracers injected as close as possible to the anus. An extrinsic innervation originating from the inferior mesenteric ganglion and dorsal root ganglia (L6-S1) was found in both types of animal. In control but not ls/ls mice retrograde labeling was also observed in the sacral parasympathetic nucleus of the spinal cord. In addition, neuritic processes were traced to neurons in myenteric ganglia. In control mice, these labeled neurons were present in ganglia within the injection site as well as in bowel rostral and caudal to it.(ABSTRACT TRUNCATED AT 400 WORDS)

Colonization of the bowel by the precursors of enteric glia: studies of normal and congenitally aganglionic mutant mice

The terminal portion of the ls/ls mouse is congenitally aganglionic because the precursors of enteric neurons fail to enter this region. This animal was studied in order to gain insight into the origin of enteric glia and into the process by which the precursors of these cells colonize the gut. In control (CD-1) mice, immunoreactivity of the glial marker, glial fibrillary acidic protein, appeared for the first time in the fetal bowel at day E16 and, in adults, was much more intense within intraenteric neural elements than in nerves outside the bowel. Glial fibrillary acidic protein developed in tissue cultures of fetal intestine explanted before the protein appeared in situ, and before the bowel became innervated by extrinsic nerves; thus, the precursors of cells able to elaborate glial fibrillary acidic protein must have been present, but unrecognizable, in the original explants. This explant assay demonstrated that these glial precursors were present in all regions of the bowel of control mice, but not in the presumptive aganglionic bowel of ls/ls mice. The nerves (of extrinsic origin) in the aganglionic tissue of ls/ls mice showed a high level of immunoreactive glial fibrillary acidic protein; nevertheless, their ultrastructure was typical of peripheral nerve, not enteric plexus, and they contained Schwann cells, not enteric glia. These observations support the view that enteric glia are derived from the single wave of neural crest colonists that populates the enteric nervous system before the gut receives its extrinsic innervation. These glial precursors, like neuronal precursors, tend to be excluded from the presumptive aganglionic ls/ls bowel. In contrast, Schwann cells grow into the abnormal ls/ls gut with the extrinsic innervation. The enteric microenvironment appears to promote the expression of glial fibrillary acidic protein in both enteric glia and Schwann cells; however, even within the bowel, Schwann cells retain their characteristic morphology. It is thus probable that the normal enteric nervous system contains supporting cells of separate lineages, enteric glia and Schwann cells.