Interstitial cells of Cajal mediate enteric inhibitory neurotransmission in the lower esophageal and pyloric sphincters

Selective knockout of intramuscular interstitial cells reveals their role in the generation of slow waves in mouse stomach

1. Intracellular recording techniques were used to compare the patterns of electrical activity generated in the antral region of the stomachs of wild-type and W/W(V) mutant mice. Immunohistochemical techniques were used to determine the distribution of c-kit-positive interstitial cells of Cajal (ICC) within the same region of the stomach. 2. In wild-type mice interstitial cells were found at the level of the myenteric plexus (ICC(MY)) and distributed within the smooth muscle bundles (ICC(IM)). In these preparations slow waves, which consisted of initial and secondary components, were detected. 3. In W/WV mutant mice ICC(MY) could be identified at the level of the myenteric plexus but ICC(IM) were not detected within smooth muscle bundles. Intracellular recordings revealed that smooth muscle cells generated waves of depolarization; these lacked a secondary component. 4. These results indicate that the secondary regenerative component of a slow wave is generated by ICC(IM). Thus the depolarization arising from the pacemaker cells, ICC(MY), is augmented by ICC(IM), so causing a substantial membrane depolarization in the circular muscle layer. Rather than contributing directly to rhythmical electrical activity, smooth muscle cells appear to depolarize at the command of the two subpopulations of ICC.

Ulcerative proctitis, rectal prolapse, and intestinal pseudo-obstruction in transgenic mice overexpressing hepatocyte growth factor/scatter factor

Hepatocyte growth factor/scatter factor (HGF/SF) can stimulate growth of gastrointestinal epithelial cells in vitro; however, the physiological role of HGF/SF in the digestive tract is poorly understood. To elucidate this in vivo function, mice were analyzed in which an HGF/SF transgene was overexpressed throughout the digestive tract. Nearly a third of all HGF/SF transgenic mice in this study (28 of 87) died by 6 months of age as a result of sporadic intestinal obstruction of unknown etiology. Enteric ganglia were not overtly affected, indicating that the pathogenesis of this intestinal lesion was different from that operating in Hirschsprung's disease. Transgenic mice also exhibited a rectal inflammatory bowel disease (IBD) with a high incidence of anorectal prolapse. Expression of interleukin-2 was decreased in the transgenic colon, indicating that HGF/SF may influence regulation of the local intestinal immune system within the colon. These results suggest that HGF/SF plays an important role in the development of gastrointestinal paresis and chronic intestinal inflammation. HGF/SF transgenic mice may represent a useful model for the study of molecular mechanisms associated with a subset of IBD and intestinal pseudo-obstruction. Moreover, our data identify previously unappreciated side effects that may be encountered when using HGF/SF as a therapeutic agent.

Interstitial cells of Cajal: primary targets of enteric motor innervation

For many years morphologists have noted the close relationship between interstitial cells of Cajal (ICC) and nerve fibers within the tunica muscularis of gastrointestinal (GI) organs. These observations led to speculations about a role for ICC in mediating neural inputs to the GI tract. Immunohistochemical and functional studies demonstrated the presence of receptors for the neurotransmitters utilized by enteric motor neurons, and changes in second messengers in ICC after field stimulation of intrinsic enteric neurons showed that ICC were functionally innervated in GI muscles. Recent double labeling experiments have shown that both excitatory and inhibitory enteric motor neurons are closely associated with ICC in the deep muscular plexus (IC-DMP) of the small intestine and intramuscular ICC (IC-IM) of the proximal and distal GI tract. Enteric motor neurons form synaptic-like structures with IC-IM and IC-DMP. Far fewer close contacts are found between enteric motor neurons and smooth muscle cells. Experiments on W/W(V) mutants that lack IC-IM in the stomach, lower esophageal sphincter, and pylorus have shown that these ICC are critical components of the neuromuscular junction. Cholinergic excitatory and nitrergic inhibitory neurotransmission are severely decreased in tissues lacking IC-IM, yet there is no loss of cholinergic or nitrergic neurons in W/W(V) mutants. These data suggest that either the post-junctional mechanisms responsible for receiving and transducing neurotransmitter signals are specifically expressed by ICC, or that the large extracellular spaces typically between nerve terminals and smooth muscle cells may not allow effective concentrations of neurotransmitters to reach receptors expressed by smooth muscle cells. These findings indicate an important role for certain classes of ICC in enteric neurotransmission and predict that loss of ICC in human motor disturbances may significantly compromise neural regulation of GI motility.

Development of interstitial cells of Cajal in a full-term infant without an enteric nervous system

The relationship between the development of the enteric nervous system and interstitial cells of Cajal (ICC) in the human small intestine was investigated in a full-term infant who presented with intestinal pseudo-obstruction. Immunohistochemistry revealed absence of enteric nerves and ganglia but abundant c-Kit immunoreactivity associated with Auerbach's plexus (ICC-AP). However, c-Kit immunoreactivity associated with the deep muscular plexus (ICC-DMP) and intermuscular ICC was absent. Electron microscopy showed ICC-AP with a normal ultrastructure; ICC-DMP were seen but were severely injured, suggesting degeneration. In vitro recording of intestinal muscle showed slow wave activity as well as response to cholinergic stimulation. Fluoroscopic examination of the small bowel showed a variety of motor patterns, including rhythmic, propagating contractions. In conclusion, total absence of enteric nerves was associated with absence of normal ICC-DMP. However, a normal musculature, including a network of ICC-AP, allowed for generation of rhythmic, propagating contractile activity, suggesting the presence of functional motor activity.

Interstitial cells of Cajal: primary targets of enteric motor innervation

For many years morphologists have noted the close relationship between interstitial cells of Cajal (ICC) and nerve fibers within the tunica muscularis of gastrointestinal (GI) organs. These observations led to speculations about a role for ICC in mediating neural inputs to the GI tract. Immunohistochemical and functional studies demonstrated the presence of receptors for the neurotransmitters utilized by enteric motor neurons, and changes in second messengers in ICC after field stimulation of intrinsic enteric neurons showed that ICC were functionally innervated in GI muscles. Recent double labeling experiments have shown that both excitatory and inhibitory enteric motor neurons are closely associated with ICC in the deep muscular plexus (IC-DMP) of the small intestine and intramuscular ICC (IC-IM) of the proximal and distal GI tract. Enteric motor neurons form synaptic-like structures with IC-IM and IC-DMP. Far fewer close contacts are found between enteric motor neurons and smooth muscle cells. Experiments on W/W(V) mutants that lack IC-IM in the stomach, lower esophageal sphincter, and pylorus have shown that these ICC are critical components of the neuromuscular junction. Cholinergic excitatory and nitrergic inhibitory neurotransmission are severely decreased in tissues lacking IC-IM, yet there is no loss of cholinergic or nitrergic neurons in W/W(V) mutants. These data suggest that either the post-junctional mechanisms responsible for receiving and transducing neurotransmitter signals are specifically expressed by ICC, or that the large extracellular spaces typically between nerve terminals and smooth muscle cells may not allow effective concentrations of neurotransmitters to reach receptors expressed by smooth muscle cells. These findings indicate an important role for certain classes of ICC in enteric neurotransmission and predict that loss of ICC in human motor disturbances may significantly compromise neural regulation of GI motility.

Muscular innervation of the proximal duodenum of the guinea pig

We investigated the muscular structure and innervation of the gastroduodenal junction in the guinea pig. In the gastroduodenal junction, the innermost layer of the circular muscle contained numerous nerve fibers and terminals. Since this nerve network continued onto the deep muscular plexus (DMP) of the duodenum, we surmised that the numerous nerve fibers in the gastroduodenal junction were specialized DMP in the most proximal part of the duodenum. The innermost layer containing many nerve fibers was about 1,000 microm in length and 100 microm in thickness in the proximal duodenum. This layer contained numerous connective tissue fibers composed of collagen and elastic fibers. Five to 30 smooth muscle cells lay in contact with each other and were surrounded by fine connective tissue. The nerve fibers in the proximal duodenum contained nerve terminals immunoreactive for choline acetyltransferase, dynorphin, enkephalin, galanin, gastrin-releasing peptide, nitric oxide synthase, substance P, and vasoactive intestinal polypeptide. Adrenergic fibers which contained tyrosine hydroxylase immunoreactivity were rare in the proximal duodenum. In the innermost layer of the proximal duodenum, there were numerous c-Kit immunopositive cells that were in contact with nerve terminals. This study allowed us to clarify the specific architecture of the most proximal portion of the duodenum. The functional significance of the proximal duodenum in relation to the electrical connection and neural cooperation of the musculature between the antrum and the duodenum is also discussed.

Gastric stasis in neuronal nitric oxide synthase-deficient knockout mice

BACKGROUND & AIMS

Nitric oxide (NO) is a major inhibitory neurotransmitter in the gut. This study aimed to identify the effect of chronic deprivation of NO derived from neuronal (nNOS) or endothelial (eNOS) nitric oxide synthase on gastric emptying.

METHODS

nNOS-deficient (knockout) mice were compared with wild-type mice for gastric size, fluoroscopic appearance after gavage of contrast, and histology of the pyloric sphincter. Wild-type mice treated with the NOS inhibitor N(omega)-nitro L-arginine (L-NA) and eNOS-deficient mice were also compared with wild-type and nNOS-deficient mice for liquid and solid gastric emptying.

RESULTS

nNOS-deficient mice showed gastric dilation. Fluoroscopy showed delayed gastric emptying of radiologic contrast. There was no marked localized hypertrophy or luminal narrowing at the pyloric sphincter by histology of relaxed wild-type, nNOS-deficient, and eNOS-deficient tissues. Gastric emptying of both solids (28% +/- 27%) and liquids (22% +/- 18%) was significantly delayed in nNOS-deficient mice compared with control wild-type mice (82% +/- 22% for solids; 48% +/- 17% for liquids). eNOS-deficient mice showed no significant difference from wild-type mice (74% +/- 28% for solids; 47% +/- 23% for liquids). Wild-type mice treated acutely with L-NA showed delay in emptying of solids (43% +/- 31%) but not liquids (39% +/- 15%).

CONCLUSIONS

Chronic depletion of NO from nNOS, but not eNOS, results in delayed gastric emptying of solids and liquids.

Vagal inhibition in the antral region of guinea pig stomach

The effects of vagal stimulation in the presence of a muscarinic antagonist were examined on three distinct rhythmically active cells located in guinea pig antrum. Vagal stimulation inhibited contractions of the circular muscle layer but did not change their rate of occurrence. With the use of intracellular recording techniques, these stimuli were found to initiate inhibitory junction potentials in the circular layer but produced smaller potential changes in driving and follower cells. Inhibition of the circular muscle layer involved two separate components. The dominant component was independent of changes in membrane potential and was abolished by nitro-L-arginine. After abolishing Ca(2+) entry into smooth muscle cells with a Ca(2+) antagonist, vagal stimulation continued to inhibit the residual contractions associated with each slow wave. When the cyclic changes in intracellular Ca(2+) concentration associated with each slow wave were measured, they were found to be unchanged by vagal stimulation. The observations suggest that vagal inhibition of stomach movements does not alter pacemaker activity in the stomach; rather, it results from a change in the sensitivity of smooth muscle contractile proteins to Ca(2+).

Molecular markers expressed in cultured and freshly isolated interstitial cells of Cajal

Located within the tunica muscularis of the gastrointestinal (GI) tract are networks of cells known as interstitial cells of Cajal (ICC). ICC are critical for important basic functions of GI motility such as generation and propagation of slow-wave pacemaker activity and reception of regulatory inputs from the enteric nervous system. We have developed a novel procedure to identify and isolate individual ICC from freshly dispersed cell preparations of the murine small intestine and gastric fundus and to determine differential transcriptional expression We have compared the expression profiles of pacemaker ICC isolated from the murine small intestine (IC-MY) and ICC involved in neurotransmission from the gastric fundus (IC-IM). We have also compared expression profiles between ICC and smooth muscle cells (SMC) and between freshly isolated ICC and cultured ICC. Cultured ICC express smooth muscle myosin, whereas freshly dispersed ICC do not. All cell types express muscarinic receptor types M(2) and M(3), neurokinin receptors NK(1) and NK(3), and inhibitory receptor VIP-1, whereas only cultured ICC and SMC express VIP-2. Both cultured and freshly dispersed IC-IM and IC-MY express the soluble form of stem cell factor, whereas SMC from the gastric fundus express only the membrane-bound form.

Arginosuccinate synthetase, arginosuccinate lyase and NOS in canine gastrointestinal tract: immunocytochemical studies

Nitric oxide synthase (NOS) requires the substrate L-arginine for NO production to support multiple gastrointestinal functions. We asked, 'Where do enzymes to regenerate L-arginine from L-citrulline exist?'. We examined loci of immunoreactivities in the canine gastrointestinal tract for arginosuccinate synthetase and arginosuccinate lyase, enzymes that resynthesize L-arginine from L-citrulline, in relation to the distribution of nNOS immunoreactivity or NADPH-diaphorase histochemistry. Arginosuccinate synthetase and lyase were present in many neurones and nerve fibres in the myenteric plexus of the lower oesophageal sphincter (LOS), antrum, pylorus, ileum and colon; in the submucosal plexus of ileum and colon; in longitudinal muscle of ileum and colon; and in nerve bundles in circular muscle everywhere. LOS muscle was also immunoreactive for both enzymes. Circular and longitudinal muscle cells of the ileum and colon and cells resembling interstitial cells of Cajal in the deep muscular plexus of the ileum and the submuscular plexus of the colon also appeared immunoreactive. In neurones, arginosuccinate synthetase and nNOS were usually co-localized. NADPH diaphorase activity was present in LOS and likely in pylorus, but not in muscularis externa of ileum or colon. We conclude that resynthesis of L-arginine probably occurs in enteric nerves, interstitial cells of Cajal (ICC) and LOS muscle; also apparently in some cells without NOS to utilize it.

Gastrointestinal stromal tumors may originate from a subset of CD34-positive interstitial cells of Cajal

Most gastrointestinal stromal tumors (GISTs), a subgroup of mesenchymal neoplasms of the gut wall, express both Kit (CD117) and CD34 proteins. It has been suggested that GISTs originate from or differentiate into interstitial cells of Cajal (ICC), after several reports indicated that ICC are likely the only cells in the gut which express both Kit and CD34. ICC are among the few cell types resident in the gut which express Kit, together with mast cells. However, the question whether or not ICC express CD34 is currently disputed. Using single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) on cultured murine intestinal cells, single ICC were selected by morphology and tested for the expression of c-kit and CD34 mRNA. Most ICC were only c-kit-positive, however a subset (7 out of 43) were double positive for both c-kit and CD34. In the human small intestine, sequential immunohistochemical staining for Kit and CD34 proteins on the same 3-microm sections showed that some of the ICC surrounding Auerbach's plexus and ICC within the circular muscle layer of the small intestine were positive for both Kit and CD34. In addition, CD34(+)Kit(-) cells were seen adjacent to ICC. These data from two different techniques indicate that ICC can be double positive for Kit and CD34. Thus, GISTs with the Kit(+)CD34(+) phenotype may arise from a subpopulation of CD34(+) Kit(+) ICC.

Interstitial cells of Cajal mediate cholinergic neurotransmission from enteric motor neurons

Interstitial cells of Cajal (ICC) are interposed between enteric neurons and smooth muscle cells in gastrointestinal muscles. The role of intramuscular ICC (IC-IM) in mediating enteric excitatory neural inputs was studied using gastric fundus muscles of wild-type animals and W/W(v) mutant mice, which lack IC-IM. Excitatory motor neurons, labeled with antibodies to vesicular acetylcholine transporter or substance-P, were closely associated with IC-IM. Immunocytochemistry showed close contacts between enteric neurons and IC-IM. IC-IM also formed gap junctions with smooth muscle cells. Electrical field stimulation yielded fast excitatory junction potentials in the smooth muscle that were blocked by atropine. Neural responses were greatly reduced in muscles of W/W(v) animals. Loss of cholinergic responses in W/W(v) muscles seemed to be caused by the loss of close synaptic contacts between motor neurons and IC-IM, because these muscles were not less responsive to exogenous acetylcholine than were wild-type muscles. W/W(v) muscles also responded to excitatory nerve stimulation when the breakdown of acetylcholine was blocked by neostigmine. The density of cholinergic nerve bundles within the muscles was not significantly different in wild-type and W/W(v) muscles, and similar amounts of (14)[C]choline were released from preloaded wild-type and W/W(v) muscles in response to nerve stimulation. The impact of losing IC-IM on gastric compliance was also evaluated in intact stomachs. Pressure increased as a function of fluid volume and infusion rate in wild-type animals, but W/W(v) animals showed little basal tone and minimal increases in pressure with fluid infusions. These data suggest that IC-IM play a major role in receiving cholinergic excitatory inputs from the enteric nervous system in the murine fundus.

Interstitial cells of Cajal in human gut and gastrointestinal disease

This paper reviews the distribution of interstitial cells of Cajal (ICC) in the human gastrointestinal (GI) tract, based on ultrastructural and immunohistochemical evidence. The distribution and morphology of ICC at each level of the normal GI tracts is addressed from the perspective of their functional significance. Alterations of ICC reported in achalasia of cardia, infantile hypertrophic pyloric stenosis, chronic intestinal pseudoobstruction, Hirschsprung's disease, inflammatory bowel diseases, slow transit constipation, and some other disorders of GI motility as well as in gastrointestinal stromal tumors are reviewed, with emphasis on the place of ICC in the pathophysiology of disease.

Heme oxygenase-2 distribution in anorectum: colocalization with neuronal nitric oxide synthase

Recent investigations have suggested carbon monoxide (CO) as a putative messenger molecule. Although several studies have implicated the heme oxygenase (HO) pathway, responsible for the endogenous production of CO, in the neuromodulatory control of the internal anal sphincter (IAS), its exact role is not known. Nitric oxide, produced by neuronal nitric oxide synthase (nNOS) of myenteric neurons, is an important inhibitory neural messenger molecule mediating nonadrenergic noncholinergic (NANC) relaxation of the IAS. The present studies were undertaken to investigate in detail the presence and coexistence of heme oxygenase-2 (HO-2) with nNOS in the opossum anorectum. In perfusion-fixed, frozen-sectioned tissue, HO-2 immunoreactive (IR) and nNOS IR nerves were identified using immunocytochemistry. Ganglia containing HO-2 IR neuronal cell bodies were present in the myenteric and submucosal plexuses throughout the entire anorectum. Colocalization of HO-2 IR and nNOS IR was nearly 100% in the IAS and decreased proximally from the anal verge. In the rectum, colocalization of HO-2 IR and nNOS IR was approximately 70%. Additional confocal microscopy studies using c-Kit staining demonstrated the localization of HO-2 IR and nNOS IR in interstitial cells of Cajal (ICC) of the anorectum. From the high rate of colocalization of HO-2 IR and nNOS IR in the IAS as well as the localization of HO-2 IR and nNOS IR in ICC in conjunction with earlier studies of the HO pathway, we speculate an interaction between HO and NOS pathways in the NANC inhibitory neurotransmission of the IAS and rectum.

Receptors in interstitial cells of Cajal: identification and possible physiological roles

Interstitial cells of Cajal (ICCs) are specialized cells of the gastrointestinal tract forming distinct populations depending on their location in the gut wall. Morphological observations and functional data have led to the hypothesis of two functions for the ICCs: (1) as pacemakers of the rhythmic activity; (2) as intermediaries in neural inputs to the muscle. The identification of specific receptors on the ICCs has represented an important step in the knowledge of these cells. Immunohistochemical labeling of these receptors provided information on both ICC morphology and contacts (particularly those with nerve endings) and on the functions of these cells. All ICC possess the Kit receptor, which represents the best tool to identify these cells under the light microscope. It has been demonstrated that this receptor is essential for ICC differentiation, and, by using mutant mice lacking the Kit-related gene, it has been possible to discriminate among all the ICC those with a primary role as pacemakers. The ileal ICC, in particular those at the deep muscular plexus, express the tachykinin receptor NK1 and a subtype of somatostatin receptors and contain nitric oxide synthase. All these data support a primary role of these ICC in neural transmission.

Gap junctions in intestinal smooth muscle and interstitial cells of Cajal

This manuscript reviews gap junctions' roles in control of intestinal motility. Gap junctions (GJs) of small intestine (SmIn) are found between circular muscle (CM) cells, between interstitial cells of Cajal (ICC) of deep muscular plexus (DMP) and between them and adjacent outer circular muscle (OCM). GJs between longitudinal muscle (LM) cells or between cells of inner circular muscle (ICM) have not been reported. Occasional GJs have been reported between ICC of the myenteric plexus (MyP) and rarely between these ICC and adjacent LM or CM cells, or between ICC within CM and smooth muscle cells. In the colon (Co) of several species a special network of ICC lines the inner border of CM, the submuscular plexus (SP). GJs are found between ICCs and between them and CM cells. The ICC of MyP of Co are associated with LM and CM; occasional GJs exist between ICC and each muscle layer. Small GJs are missed by electron microscopy or light microscopic Immunocytochemistry. Therefore, GJ coupling may exist without demonstrated GJs. The consequences for the pacemaking functions of ICC networks of varied densities of GJ between ICC and between ICC of MyP or DMP or of SP and CM are considered. Connexins (Cxs) that compose intestinal GJs may affect coupling, but are incompletely known. Understanding of the role of GJs in coordinating intestinal motility requires knowing: (1) what passes through gap junctions to couple ICC to smooth muscle cells; (2) what Cx with what conductances and what modulatory controls connect ICC and smooth muscle cells; (3) whether smooth muscles can generate slow waves independent of ICC networks; and (4) what happens to motility, slow waves, and IJPs when GJs are selectively uncoupled.

Interstitial cells of Cajal in human gut and gastrointestinal disease

This paper reviews the distribution of interstitial cells of Cajal (ICC) in the human gastrointestinal (GI) tract, based on ultrastructural and immunohistochemical evidence. The distribution and morphology of ICC at each level of the normal GI tracts is addressed from the perspective of their functional significance. Alterations of ICC reported in achalasia of cardia, infantile hypertrophic pyloric stenosis, chronic intestinal pseudoobstruction, Hirschsprung's disease, inflammatory bowel diseases, slow transit constipation, and some other disorders of GI motility as well as in gastrointestinal stromal tumors are reviewed, with emphasis on the place of ICC in the pathophysiology of disease.

Gastrointestinal peristalsis: joint action of enteric nerves, smooth muscle, and interstitial cells of Cajal

Peristalsis is a propulsive motor pattern orchestrated by neuronal excitation and inhibition in cooperation with intrinsic muscular control mechanisms, including those residing in interstitial cells of Cajal (ICC). Interstitial cells of Cajal form a network of cells in which electrical slow waves originate and then propagate into the musculature initiating rhythmic contractile activity upon excitaton by enteric nerves. Interstitial cells of Cajal have now been isolated and their intrinsic properties reveal the presence of rhythmic inward currents not found in smooth muscle cells. In tissues where classical slow waves are not present, enteric cholinergic excitation will evoke slow wave-like activity that forces action potentials to occur in a rhythmic manner. Intrinsic and induced slow wave activity directs many of the peristaltic motor patterns in the gut.

One hundred years of interstitial cells of Cajal

This review is a portrayal of the evolution of ideas involving the interstitial cells of Cajal in changing disguises as dull fibroblasts, not very exciting Schwann cells, or perhaps quite important, though primitive neurons. However, today unmasked (we believe), they reveal themselves as myoid cells, a role that, judging by current interest, is far more exciting than former ones. Close to 500 publications from 1860-1999 have contributed to the discussion in one way or the other. This literature contains a wealth of correct observations but obviously also wrong interpretations, which are seen as a result of too blind a belief in specificities of visualization methods, combined with a desire to interpret even the hidden detail. It has been my objective to attempt to trace the origins of viable ideas, and I have therefore focused on relatively few authors. The most recent development from 1980 until today is so well covered by easily accessible reviews that I have resorted to a mere, but hopefully complete, list of them. Modern ICC'ists have so far been caught in the external muscle of the gut and kept their hands off its internal affairs. However, while working my way through the literature it struck me that a number of recent studies may provide the elements of a plausible model for the villous contraction mechanism. In the present context, an important point is that the very first published interstitial "neurons" from Cajal's hand-of the intestinal villus, 1889-may achieve new significance as a possible correlate to the regulatory ICC of the intestinal muscularis. Partly to make this point, I have taken the liberty of giving a short account of recent results from our lab.

Guide to the identification of interstitial cells of Cajal

The interstitial cell of Cajal, abbreviated ICC, is a specific cell type with a characteristic distribution in the smooth muscle wall throughout the alimentary tract in humans and laboratory mammals. The number of publications relating to ICC is rapidly increasing and demonstrate a rich variation in the structure and organization of these cells. This variation is species-, region-, and location-dependent. We have chosen to define a "reference ICC," basically the ICC in the murine small intestine, as a platform for discussion of variability. The growing field of ICC markers for light and electron microscopy is reviewed. Although there is a rapidly increasing number of approaches applicable to bright field and fluorescence microscopy, the location of markers by electron microscopy still suffers from inadequate preservation of ultrastructural detail. Finally, we summarize evidence related to ICC ultrastructure under conditions differing from those of the normal, adult individual (during differentiation, in pathological conditions, transplants, mutants, and in cell culture).

Ultrastructural characterization of the interstitial cells of Cajal

Recent studies on the interstitial cells of Cajal (ICC) have determined ultrastructural criteria for the identification of these previously enigmatic cells. This review deals with the electron microscopic findings obtained by the author's research group in different tissue regions of the gut in mice, rats and guinea-pigs, comparing these with reports from other groups in different species and in humans. ICC are characterized by the following morphological criteria: numerous mitochondria, abundant intermediate filaments and large gap junctions which connect the cells with each other and with smooth muscle cells. Due to their location in the gut and the specific species, the ICC are markedly heterogeneous in appearance, ranging from cells closely resembling smooth muscle cells to those similar to fibroblasts (Table 1). Nevertheless, the above-mentioned morphological features are shared by all types of ICC and serve in identifying them. Recent discoveries on a significant role of c- kit in the maturation of the ICC and their specific immunoreactivity to anti-c-Kit antibody have confirmed the view that the ICC comprise an independent and specific entity of cells. This view is reinforced by the findings of the author's group that the ICC characteristically possess vimentin filaments and are stained with the zinc iodide-osmium tetroxide method which provides a staining affinity similar to methylene blue, the dye used in the original work by Cajal, (1911). Developmental studies indicate that the ICC are derived from a non-neuronal, mesenchymal origin. This paper further reviews advances in the physiological studies on the ICC, in support of the hypothesis by THUNEBERG (1982) that they function as a pacemaker in the digestive tract and a mediator transmitting impulses from the nerve terminals to the smooth muscle cells.

Development and plasticity of interstitial cells of Cajal

Interstitial cells of Cajal (ICC) are the pacemakers in gastrointestinal (GI) muscles, and these cells also mediate or transduce inputs from the enteric nervous system. Different classes of ICC are involved in pacemaking and neurotransmission. ICC express specific ionic conductances that make them unique in their ability to generate and propagate slow waves in GI muscles or transduce neural inputs. Much of what we know about the function of ICC comes from developmental studies that were made possible by the discoveries that ICC express c-kit and proper development of ICC depends upon signalling via the Kit receptor pathway. Manipulating Kit signalling with reagents to block the receptor or downstream signalling pathways or by using mutant mice in which Kit or its ligand, stem cell factor, are defective has allowed novel studies into the specific functions of the different classes of ICC in several regions of the GI tract. Kit is also a surface antigen that can be used to conveniently label ICC in GI muscles. Immunohistochemical studies using Kit antibodies have expanded our knowledge about the ICC phenotype, the structure of ICC networks, the interactions of ICC with other cells in the gut wall, and the loss of ICC in some clinical disorders. Preparations made devoid of ICC have also allowed analysis of the consequences of losing specific classes of ICC on GI motility. This review describes recent advances in our knowledge about the development and plasticity of ICC and how developmental studies have contributed to our understanding of the functions of ICC. We have reviewed the clinical literature and discussed how loss or defects in ICC affect GI motor function.

Control systems of gastrointestinal motility are immature at birth in dogs

Networks of interstitial cells of Cajal (ICC) in the myenteric plexus (Myp) or circular muscle (CM) function as pacemakers for gastrointestinal slow waves. ICC in contact with muscle and closely associated with nerves in the CM may mediate inhibitory neurotransmission. We wondered if ICC in Myp and CM and their connections are immature at birth and mature first in the proximal gut in association with nerves. Tissues from lower esophageal sphincter (LES), pylorus (PYL), small intestine (SI) and colon (CO) of 18 term fetal dogs taken from six females were fixed and prepared for ultrastructural examination and studied. Ganglia were present where expected in the Myp and submucous plexus (SMP). ICC cells were present in the Myp of PYL, SI and CO and appeared to have normal relationships to the outer border of CM as in adults. ICC in CM were found associated with nerves in the LES and in PYL, but not in SI or CO. However, axons in CM were everywhere usually free of glial covering, indicating ongoing migration or development. No organized deep muscular plexus (DMP) in SI or submuscular plexus (SP) in colon was present. Visible gap junctions were absent everywhere except for very rare ones between circular muscle cells. We conclude that at birth the neural and ICC networks of CM are more immature in intestine and colon than in oesophagus and stomach. Development of nerve and ICC of CM in oesophagus and stomach apparently precedes that in the remaining gut. However networks in these regions have not achieved adult organization and ICC and smooth muscle cells are anatomically poorly coupled. These findings suggest the reasons that gut motility at birth will not be adult in pattern are because ICC, nerve and muscle control systems are not fully differentiated. Further developmental delays in ICC and nerve maturation could have serious consequences for feeding of infant animals.

Development of interstitial cells of Cajal and pacemaking in mice lacking enteric nerves

BACKGROUND & AIMS

Development of interstitial cells of Cajal (ICC) requires signaling via Kit receptors. Kit is activated by stem cell factor (SCF), but the source of SCF in the bowel wall is unclear and controversy exists about whether enteric neurons express the SCF required for ICC development.

METHODS

Glial cell line-derived neurotrophic factor (GDNF) knockout mice, which lack enteric neurons throughout most of the gut, were used to determine whether neurons are necessary for ICC development. ICC distributions were determined with Kit immunofluorescence, and function of ICC was determined by intracellular electrical recording.

RESULTS

ICC were normally distributed throughout the gastrointestinal tracts of GDNF-/- mice. Intracellular recordings from aganglionic gastrointestinal muscles showed normal slow wave activity at birth in the stomach and small intestine. Slow waves developed normally in aganglionic segments of small bowel placed into organ culture at birth. Quantitative polymerase chain reaction showed similar expression of SCF in the muscles of animals with and without enteric neurons. Expression of SCF was demonstrated in isolated intestinal smooth muscle cells.

CONCLUSIONS

These data suggest that enteric neurons are not required for the development of functional ICC. The circular smooth muscle layer, which develops before ICC, may be the source of SCF required for ICC development.

Neuronal NOS provides nitrergic inhibitory neurotransmitter in mouse lower esophageal sphincter

To identify the enzymatic source of nitric oxide (NO) in the lower esophageal sphincter (LES), studies were performed in wild-type and genetically engineered endothelial nitric oxide synthase [eNOS(-)] and neuronal NOS [nNOS(-)] mice. Under nonadrenergic noncholinergic (NANC) conditions, LES ring preparations developed spontaneous tone in all animals. In the wild-type mice, electrical field stimulation produced frequency-dependent intrastimulus relaxation and a poststimulus rebound contraction. NOS inhibitor N(omega)-nitro-L-arginine methyl ester (100 microM) abolished intrastimulus relaxation and rebound contraction. In nNOS(-) mice, both the intrastimulus relaxation and rebound contraction were absent. However, in eNOS(-) mice there was no significant difference in either the relaxation or rebound contraction from the wild-type animal. Both nNOS(-) and eNOS(-) tissues showed concentration-dependent relaxation to NO donor diethylenetriamine-NO and there was no difference in the sensitivity to the NO donor in nNOS(-), eNOS(-), or wild-type animals. These results indicate that in mouse LES, nNOS rather than eNOS is the enzymatic source of the NO that mediates NANC relaxation and rebound contraction.

Myofibroblasts. II. Intestinal subepithelial myofibroblasts

Intestinal subepithelial myofibroblasts (ISEMF) and the interstitial cells of Cajal are the two types of myofibroblasts identified in the intestine. Intestinal myofibroblasts are activated and proliferate in response to various growth factors, particularly the platelet-derived growth factor (PDGF) family, which includes PDGF-BB and stem cell factor (SCF), through expression of PDGF receptors and the SCF receptor c-kit. ISEMF have been shown to play important roles in the organogenesis of the intestine, and growth factors and cytokines secreted by these cells promote epithelial restitution and proliferation, i.e., wound repair. Their role in the fibrosis of Crohn's disease and collagenous colitis is being investigated. Through cyclooxygenase (COX)-1 and COX-2 activation, ISEMF augment intestinal ion secretion in response to certain secretagogues. By forming a subepithelial barrier to Na(+) diffusion, they create a hypertonic compartment that may account for the ability of the gut to transport fluid against an adverse osmotic gradient. Through the paracrine secretion of prostaglandins and growth factors (e.g., transforming growth factor-beta), ISEMF may play a role in colonic tumorigenesis and metastasis. COX-2 in polyp ISEMF may be a target for nonsteroidal anti-inflammatory drugs (NSAIDs), which would account for the regression of the neoplasms in familial adenomatous polyposis and the preventive effect of NSAIDs in the development of sporadic colon neoplasms. More investigation is needed to clarify the functions of these pleiotropic cells.

Interstitial cells of cajal generate electrical slow waves in the murine stomach

1. The gastric corpus and antrum contain interstitial cells of Cajal (ICC) within the tunica muscularis. We tested the hypothesis that ICC are involved in the generation and regeneration of electrical slow waves. 2. Normal, postnatal development of slow wave activity was characterized in tissues freshly removed from animals between birth and day 50 (D50). Slow wave amplitude and frequency increased during this period. Networks of myenteric ICC (IC-MY) were present in gastric muscles at birth and did not change significantly in appearance during the period of study as imaged by confocal immunofluorescence microscopy. 3. IC-MY networks were maintained and electrical rhythmicity developed in organ culture in a manner similar to normal postnatal development. Electrical activity was maintained for at least 48 days in culture. 4. Addition of a neutralizing antibody (ACK2) for the receptor tyrosine kinase, Kit, to the culture media caused progressive loss of Kit-immunoreactive cells. Loss of Kit-immunoreactive cells was associated with loss of slow wave activity. Most muscles became electrically quiescent after 3-4 weeks of exposure to ACK2. 5. In some muscles small clusters of Kit-immunoreactive IC-MY remained after culturing with ACK2. These muscles displayed slow wave activity but only in the immediate regions in which Kit-positive IC-MY remained. These data suggest that regions without Kit-immunoreactive cells cannot generate or regenerate slow waves. 6. After loss of Kit-immunoreactive cells, the muscles could not be paced by direct electrical stimulation. Stimulation with acetylcholine also failed to elicit slow waves. The data suggest that the generation of slow waves is an exclusive property of IC-MY; smooth muscle cells may not express the ionic apparatus necessary for generation of these events. 7. We conclude that IC-MY are an essential element in the spontaneous rhythmic electrical and contractile activity of gastric muscles. This class of ICC appears to generate slow wave activity and may provide a means for regeneration of slow waves.

Innervation of the esophagus in mice that lack MASH1

The striated muscle of the esophagus differs from other striated muscle, because it develops by the transdifferentiation of smooth muscle, and the motor end plates receive a dual innervation from vagal (cholinergic) motor neurons and nitric oxide synthase (NOS)-containing enteric neurons. Mash1-/- mice have no enteric neurons in their esophagus and die within 48 hours of birth without milk in their stomachs (Guillemot et al. [1993] Cell 75:463-476). In this study, the innervation of the esophagus of newborn Mash1-/-, Mash1+/- and wild type mice was examined. There was no difference between Mash1-/-, Mash1+/-, and wild type mice in the transdifferentiation of the muscle and the development of nicotinic receptor clusters. However, there were significantly more cholinergic nerve terminals per motor end plate in Mash1-/- mice than Mash1+/- or wild type mice. Each of the Mash1-/- mice had fewer than 50 NOS neurons per esophagus, compared with approximately 3,000 in wild type mice. Newborn Mash1+/- mice also contained significantly fewer NOS neurons than wild type mice. In Mash1-/- mice, NOS nerve fibers were virtually absent from the external muscle but were present at the myenteric plexus. Unlike that of newborn wild type mice, the lower esophageal sphincter of Mash 1-/- mice lacked NOS nerve fibers; this may explain the absence of milk in the stomach. We conclude that 1) the transdifferentiation of the esophageal muscle and the development of the extrinsic innervation do not require enteric neurons or MASH1, 2) extrinsic NOS neurons only innervate the myenteric plexus.

Activation of outward K+ currents: effect of VIP in oesophagus

1. Electrical field stimulations (EFS) of the opossum and canine lower oesophageal sphincters (OLOS and CLOS respectively) and opossum oesophageal body circular muscle (OOBCM) induce non-adrenergic, non-cholinergic (NANC) relaxations of any active tension and NO-mediated hyperpolarization. VIP relaxes the OLOS and CLOS and any tone in OOBCM without major electrophysiological effects. These relaxations are not blocked by NOS inhibitors. Using isolated smooth muscle cells, we tested whether VIP acted through myogenic NO production. 2. Outward currents were similar in OOBCM and OLOS and NO increased them regardless of pipette Ca2+(i), from 50-8000 nM. L-NAME or L-NOARG did not block outward currents in OLOS at 200 nM pipette Ca2+. 3. Outward currents in CLOS cells decreased at 200 nM pipette Ca2+ or less but NO donors still increased them. VIP had no effect on outward currents in cells from OOBCM, OLOS or CLOS under conditions of pipette Ca2+ at which NO donors increased outward K+ currents. 4. We conclude, VIP does not mimic electrophysiological effects of NO donors on isolated cells of OOBCM, OLOS or CLOS. VIP relaxes the OLOS and CLOS and inhibits contraction of OOBCM by a mechanism unrelated to release of myogenic NO or an increase in outward current. 5. Also, the different dependence of outward currents of OOBCM and OLOS on pipette Ca2+ from those of CLOS suggests that different K+ channels are involved and that myogenic NO production contributes to K+ channel activity in CLOS but not in OLOS or OOBCM.

Searching for intrinsic properties and functions of interstitial cells of Cajal

Evidence is mounting that interstitial cells of Cajal may function as pacemaker cells and have a role in NO-mediated neurotransmission. Several colonic motor disorders may be associated with abnormal ICC function.