Electron microscopy of the connective tissues between longitudinal and circular muscle of small intestine of cat

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Interstitial Cells in the Musculature of the Gastrointestinal Tract: Cajal and Beyond

Expression of the receptor tyrosine kinase KIT on cells referred to as interstitial cells of Cajal (ICC) has been instrumental during the past decade in the tremendous interest in cells in the interstitium of the smooth muscle layers of the digestive tract. ICC generate the pacemaker component (electrical slow waves of depolarization) of the smooth musculature and are involved in neurotransmission. By integration of ICC functions, substantial progress has been made in our understanding of the neuromuscular control of gastrointestinal motility, opening novel therapeutic perspectives. In this article, the ultrastructure and light microscopic morphology, as well as the functions and the development of ICC and of neighboring fibroblast-like cells (FLC), are critically reviewed. Directions for future research are considered and a unifying concept of mesenchymal cells, either KIT positive (the "ICC") or KIT negative "non-Cajal" (including the FLC and possibly also other cell types) cell types in the interstitium of the smooth musculature of the gastrointestinal tract, is proposed. Furthermore, evidence is accumulating to suggest that, as postulated by Santiago Ramon y Cajal, the concept of interstitial cells is not likely to be restricted to the gastrointestinal musculature.

The interstitial cells of Cajal and a gastroenteric pacemaker system

In spite of a claim by Kobayashi (1990) that they do not correspond to the cells originally depicted by CAJAL, a particular category of fibroblast-like cells have been identified in the gut by electron microscopy (Faussone-Pellegrini, 1977; Thuneberg, 1980) and by immunohistochemistry for Kit protein (Maeda et al., 1992) under the term of the "interstitial cells of Cajal (ICC)". Generating electrical slow waves, the ICC are intercalated between the intramural neurons and the effector smooth muscular cells, to form a gastroenteric pacemaker system. ICC at the level of the myenteric plexus (IC-MY) are multipolar cells forming a reticular network. The network of IC-MY which is believed to be the origin of electrical slow waves is morphologically independent from but associated with the myenteric plexus. On the other hand, intramuscular ICC (IC-IM) usually have spindle-shaped contours arranged in parallel with the bulk smooth muscle cells. Associated with nerve bundles and blood vessels, the IC-IM possess receptors for neurotransmitters and such circulating hormones as cholecystokinin, suggesting their roles in neuromuscular and hormone-muscular transmissions. In addition, gap junctions connect the IC-MY and IC-IM, thereby realizing the electrically synchronized integrity of ICC as a pacemaker system in the gut. The smooth muscle cells are also coupled with ICC via gap junctions, and the functional unit thus formed enables rhythmically synchronized contractions and relaxations. It has recently been found that a lack of Kit-expressing cells may induce hyper-contractility of the tunica muscularis in vitro, whereas a decrease in Kit expression within the muscle wall causes dysmotility-like symptoms in vivo. The pacemaker system in the gut thus seems to play a critical role in the maintenance of both moderate and normal motility of the digestive tract. A loss of Kit positive cells has been detected in several diseases with an impaired motor activity, including diabetic gastroenteropathy. Pathogenesis of these diseases is thought to be accounted for by impaired slow waves and neuromuscular transmissions; a pacemaker disorder may possibly induce a dysmotility-like symptom called 'gastroenteric arrhythmia'. A knowledge of the structure and function of the ICC and the pacemaker system provides a basis for clarifying the normal mechanism and the pathophysiology of motility in the digestive tract.

Comparative morphology of interstitial cells of Cajal: ultrastructural characterization

The shape, distribution, and ultrastructural features of interstitial cells of Cajal (ICC) of different tissue layers and organs of the rat and guinea-pig digestive tract were described and compared with the corresponding cells in other species including mice, dogs, and humans, as reported in the literature. By light microscopy, the best marker for ICC appeared to be immunoreactivity for c-Kit. Ultrastructurally, ICC were characterized by the presence of many mitochondria, bundles of intermediate filaments, and gap junctions, which linked ICC with each other. However, ICC were morphologically heterogeneous and had particular features, depending on their tissue and organ location and species. ICC in the deep muscular plexus of the small intestine and in the submuscular plexus of the colon were the most like smooth muscle cells, and had a distinct basal lamina and numerous caveolae. In contrast, ICC of Auerbach's plexus at all levels of the gastrointestinal tract were the least like smooth muscle cells. They most closely resembled unremarkable fibroblasts. ICC within the circular muscle layer were intermediate in form. In addition to the tissue specificity, some organ and species specificity could be distinguished. The structural differences between ICC may be determined by their microenvironment, including the effects of mechanical force, type of nerve supply, and spacial relationship with smooth muscle cells.

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.

Comparative morphology of interstitial cells of Cajal: ultrastructural characterization

The shape, distribution, and ultrastructural features of interstitial cells of Cajal (ICC) of different tissue layers and organs of the rat and guinea-pig digestive tract were described and compared with the corresponding cells in other species including mice, dogs, and humans, as reported in the literature. By light microscopy, the best marker for ICC appeared to be immunoreactivity for c-Kit. Ultrastructurally, ICC were characterized by the presence of many mitochondria, bundles of intermediate filaments, and gap junctions, which linked ICC with each other. However, ICC were morphologically heterogeneous and had particular features, depending on their tissue and organ location and species. ICC in the deep muscular plexus of the small intestine and in the submuscular plexus of the colon were the most like smooth muscle cells, and had a distinct basal lamina and numerous caveolae. In contrast, ICC of Auerbach's plexus at all levels of the gastrointestinal tract were the least like smooth muscle cells. They most closely resembled unremarkable fibroblasts. ICC within the circular muscle layer were intermediate in form. In addition to the tissue specificity, some organ and species specificity could be distinguished. The structural differences between ICC may be determined by their microenvironment, including the effects of mechanical force, type of nerve supply, and spacial relationship with 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.

Role of gap junctions in structural arrangements of interstitial cells of Cajal and canine ileal smooth muscle

We examined the structural and functional basis for pacemaking by interstitial cells of Cajal (ICC) in circular smooth muscle of the canine ileum. Gap junctions were found between ICC of myenteric plexus (MyP), occasionally between MyP ICC and outer circular smooth muscle cells, between individual outer circular smooth muscle cells, between them and ICC of the deep muscular plexus (DMP), and between DMP ICC. No visible gap junctions connected MyP ICC to longitudinal muscle cells or inner circular muscle cells. Occasionally contacts occurred between the two muscle layers. No special structures were found to connect MyP and DMP ICC networks. Octanol concentration dependently reduced the amplitude and frequency of, but did not abolish, slow waves in circular muscle in isolated ileum recorded near the MyP or the DMP. Slow waves triggered from MyP ICC by a current pulse also persisted. Contractile activity was abolished, cells were depolarized, and fast inhibitory junction potentials were reduced by octanol. We conclude that ICC pacemakers of the MyP and DMP utilize gap junctional conductances for pacemaking function but may not require them. Coupling between the two ICC networks may utilize the circular muscle syncytium.

Pacemaker cells in the gastrointestinal tract: interstitial cells of Cajal

Interstitial cells of Cajal (ICC) were described a century ago as primitive neurons in the intestines. Through the years, ICC have been mistaken for neurons, glial cells, fibroblasts, smooth muscle cells, and macrophages. We identified ICC in the musculature of mouse small intestine by their characteristic morphology and topography, and we analysed the relation between ICC, autonomic nerves, and smooth muscle. Subsequent morphological and electrophysiological evidence has strongly supported our hypotheses that some ICC populations are gut pacemakers and may hold other fundamental regulatory functions (coordinative, mechanoreceptive, mediating nervous input). Recognition of common principles of ICC organization (confinement to specific locations in relation to smooth muscle layers; formation of extensive cellular networks through tight coupling of overlapping thin processes; innervation patterns; characteristic patterns of contact with smooth muscle cells) and ultrastructure (myoid features: basal lamina, caveolae, rich in sER and mitochondria, often prominent filament bundles and dense bands/bodies) has allowed the identification of ICC in the GI musculature of all species investigated. However, variation in organization and ultrastructure is significant, between both species and regions of the GI tract. Our studies of ICC in human intestine permit an extension of the above hypotheses to man and provide a basis for further studies of ICC pathology and pathophysiology. The latter may become a fruitful area of research in the coming decades.

Identification and classification of interstitial cells in the canine proximal colon by ultrastructure and immunocytochemistry

The ultrastructure and immunocytochemistry of interstitial cells (ICs) in the canine proximal colon were investigated. Three types of ICs were found within the tunica muscularis. (1) ICs were located along the submucosal surface of the circular muscle (IC-SM). These cells shared many features of smooth muscle cells, including myosin thick filaments and immunoreactivity to smooth muscle gamma actin, myosin light chain, and calponin antibodies. IC-SM were clearly different from smooth muscle cells in that contractile filaments were less abundant and intermediate filaments consisted of vimentin instead of desmin. (2) ICs in the region of the myenteric plexus (IC-MY) were similar to IC-SM, but these cells had no thick filaments or immunoreactivity to smooth muscle gamma actin or calponin antibodies. (3) The fine structures and immunoreactivity of ICs within the muscle layers (IC-BU) were similar to IC-MY, but IC-BU lacked a definite basal lamina and membrane caveolae. IC-BU and IC-MY were both immunopositive for vimentin. Since all ICs were immunopositive for vimentin, vimentin antibodies may be a useful tool for distinguishing between ICs and smooth muscle cells. Each class of ICs was closely associated with nerve fibers, made specialized contacts with smooth muscle cells, and formed multicellular networks. A combination of ultrastructural and immunocytochemical techniques helps the identification and classification of ICs by revealing the fine structures and determining the "chemical coding" of each class of ICs.

Immunohistochemical localization of a gap junction protein (connexin43) in the muscularis externa of murine, canine, and human intestine

Electron-microscopic studies have revealed a heterogeneous distribution of gap junctions in the muscularis externa of mammalian intestines. This heterogeneity is observed at four different levels: among species; between small and large intestines; between longitudinal and circular muscle layers; and between subdivisions of the circular muscle layer. We correlated results obtained with two immunomethods, using an antibody to the known gap-junctional protein (connexin43) with ultrastructural findings, and further evaluated the respective sensitivity of these two approaches. For comparative reasons we also included the vascular smooth muscle of coronary arteries into our study. Two versions of the immunotechnique (peroxidase-antiperoxidase and fluorescence methods) were applied to frozen sections of murine, canine, and human small and large intestines, as well as to pig coronary artery. In the small intestine of all three species a very strong reactivity marked the outer main division of the circular muscle layer, while the longitudinal muscle layer as well as the inner thin division of the circular muscle layer were negative. In murine and human colon both muscle layers were negative, while in canine colon the border layer between the circular muscle and the submucosa reacted strongly, and scattered activity was found in the portion of the circular muscle layer (one tenth of its thickness) closest to the submucosa. The remainder of the circular muscle layer and the entire longitudinal muscle layer were negative in the canine colon. In the coronary artery we could not confirm the positive, specific labeling reported by other investigators (l.c.).(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical coupling of circular muscle to longitudinal muscle and interstitial cells of Cajal in canine colon

1. Electrical communication between circular muscle, longitudinal muscle and interstitial cells of Cajal (ICC) was investigated; the hypothesis was tested that the resting membrane potential (RMP) gradient in the circular muscle of canine colon is caused by electrical coupling to neighbouring cells. 2. Isolated longitudinal muscle exhibited spike-like action potentials at a RMP of -45 mV with a frequency and amplitude of 20 cycles/min and 12 mV, respectively. 3. The circular muscle (CM), devoid of longitudinal muscle, myenteric plexus and submuscular ICC-smooth-muscle network, was electrically quiescent at a uniform RMP of -62 mV across the entire circular muscle layer. 4. Preparations consisting of only the submuscular ICC network and a few adjacent layers of circular muscle cells exhibited slow wave-type action potentials at a RMP of about -80 mV. 5. In ICC-CM preparations, consisting of the submuscular ICC network and circular muscle, a RMP gradient of 10 mV was observed near the submucosal border, whereas the RMP was constant at -62 mV in the myenteric half of the circular muscle. 6. In full thickness (FT) preparations, a RMP gradient of 23 mV was observed. The RMP decreased gradually from -71 mV at the submucosal border to -48 mV at the myenteric border of the circular muscle. 7. Coupling of longitudinal muscle to circular muscle caused circular muscle cells at the myenteric surface to depolarize by 14 mV and longitudinal muscle cells to hyperpolarize by 3 mV. 8. In the ICC-CM preparations, the slow wave amplitudes did not decay exponentially away from the ICC network indicating that slow waves propagated actively into the circular muscle; in the FT preparations there was an apparent exponential decay but this was due to the RMP gradient. 9. Spike-like action potentials (SLAPs) superimposed on the plateau phase of slow waves did not decay exponentially away from the myenteric border suggesting that SLAPs were generated within the circular muscle layer. 10. In summary, circular muscle cells possess a uniform intrinsic RMP of -62 mV. The RMP gradient in situ is caused by electrical coupling of circular muscle cells to longitudinal muscle cells and the submuscular network of ICC. In situ, slow wave-type action potentials propagate actively into the circular muscle layer, and, dependent on the level of excitation, circular muscle cells actively generate spikes.

Ultrastructure of interstitial cells of Cajal in circular muscle of human small intestine

BACKGROUND

Interstitial cells of Cajal (ICC) may be important regulatory cells in gut muscle layers. This study examined ICC within the circular muscle of human small intestine.

METHODS

Surgically resected, uninvolved intestine was studied by light microscopy and electron microscopy.

RESULTS

Muscle lamellae were separated by main septa in continuity with submucosa. Smooth muscle cells ran radially in the septa. Two types of ICC were distinguished. One ICC type had abundant intermediate filaments and smooth cisternae and a discontinuous basal lamina. This ICC type was present in the septa and in the outer third of the circular lamellae. The other ICC type had a complete basal lamina and conspicuous caveolae. This ICC type was observed only in the inner third of the circular lamellae. Both ICC types were close to nerves, but only the latter type formed gap junctions with one another and with muscle cells. Junctions between the two ICC types were not observed.

CONCLUSIONS

The arrangement suggests that ICC and radially oriented muscle cells participate in electrical and mechanical coordination of the circular muscle layer of human small intestine.

Use of rhodamine 123 to label and lesion interstitial cells of Cajal in canine colonic circular muscle

The role of interstitial cells of Cajal (ICC) is difficult to determine because these cells are not easily identified by light microscopy, and there are no compounds available to specifically lesion ICC. Ultrastructural studies have shown an abundance of mitochondria in ICC. Therefore, we have used rhodamine 123, a fluorescent dye that is specifically accumulated by mitochondria, to identify ICC in canine proximal colon. This technique provided good discrimination between ICC and smooth muscle cells, but enteric neurons were labeled with rhodamine 123. This compound has cytotoxic properties in some cells. Therefore, we treated intact muscle strips with rhodamine 123 while recording intracellular electrical activity from circular muscle cells. Uptake of rhodamine 123 by ICC was associated with an alteration in electrical rhythmicity. These data suggest that rhodamine 123 may be a useful tool for visualizing and perhaps chemically lesioning ICC.

Organization of electrical activity in the canine pyloric canal

1. The electrical activity of the canine gastroduodenal junction was investigated using cross-sectional muscle preparations and intracellular recording techniques. 2. Spontaneous electrical slow waves were recorded from antral and pyloric cells but not from duodenal cells adjacent to the pyloric region. Slow waves were generated in the antrum and propagated to the pyloric region via the circular layer. Pyloric slow waves consisted of an upstroke phase, a plateau phase and oscillations superimposed upon the plateau, whereas antral slow waves had smooth plateau potentials. 3. Within the pylorus slow waves decayed in amplitude with distance from the myenteric border of the circular muscle; the majority of pyloric circular cells were normally electrically quiescent. 4. The longitudinal muscle in the pylorus was electrically coupled and paced by the circular muscle. In longitudinal cells slow waves were usually of long duration with multiple spikes superimposed upon the plateau phase. 5. Nifedipine (10(-8) to 10(-5) M) decreased slow waves amplitude and duration. Tetraethylammonium ions (TEA; 10 mM) increased the duration of slow waves, caused spiking activity during the plateau phase and also elicited spiking in the quiescent regions. 6. The results suggest that gastric slow waves pace the myenteric portion of the circular muscle layer and the longitudinal layer of the pylorus, but do not traverse the gastroduodenal junction, nor pace the majority of cells within the circular muscle of the pylorus. Other excitatory mechanisms are necessary to activate these regions and to co-ordinate their motility with gastric motility.

Immunocytochemistry of the interstitial cells of Cajal in the rat intestine

There is experimental evidence suggesting that the interstitial cells of Cajal are essential for rhythmic slow waves of the smooth muscle layers of the mammalian small intestine. Different investigators have identified them variously as modified neurons, glia, fibroblasts or modified smooth muscle cells. Since histological categorization bears on understanding their function, we have examined the immunoreactivity of the myenteric plexus of the rat small intestine, paying special attention to the cell type identified as Thuneberg's Type I-ICC. Polyclonal and monoclonal antisera directed against 4 intermediate filament proteins: neurofilament protein, glial fibrillary acidic protein, vimentin and desmin were used. In addition, antisera directed against neuron-specific enolase, substance P and vasoactive intestinal polypeptide were also tested for reactivity. Type I-ICCs were immunonegative to all the antisera directed against intermediate filament proteins and neuropeptides. However, some Type I-ICCs were immunopositive to antisera against neuron-specific enolase. On the basis of these results and the distribution of immunoreactivities to these kinds of antisera in other tissues, we suggest that Type I-ICCs are distinct from typical myenteric neurons, from glia, from fibroblasts and from smooth muscle fibers. Staining with antiserum against neuron-specific enolase suggests a relation to some type of neuron.

Three-dimensional observation of the fibroblast-like cells associated with the rat myenteric plexus, with special reference to the interstitial cells of Cajal

An extensive cellular network becomes visible over the myenteric plexus of the rat after removal of the overlying tissues under the scanning electron microscope. The cells are mainly stellate and have many slender processes via which they interconnect. They form a three-dimensional network and are closely associated with the ganglia and nerve bundles, and also extend over the smooth muscle cells. They are considered to correspond to the interstitial cells of Cajal because of their peculiar arrangement and their topography. Transmission electron-microscopic evidence demonstrates that the majority of those cells have features of fibroblasts. Gap junctions and intermediate junctions are observed between these fibroblast-like cells, and also between them and smooth muscle cells. Examination of serial thin sections reveals that single fibroblast-like interstitial cells connect to both circular and longitudinal muscle cells via gap junctions. It is suggested that the network of interstitial cells conducts electrical signals.

Control of human colonic motor function

Human colonic motility is governed by control mechanisms involving the electrical activity of the smooth muscle cell membranes, the intrinsic and extrinsic nervous activity, and hormonal action. The structural bases for neural and myogenic control have not been demonstrated. However, gap junctions are lacking between muscle cells, and nerves are not close to smooth muscle cells. The myogenic control, as observed in vitro, is described and compared with results obtained from different in vivo techniques. In vitro and in vivo measurements are critically evaluated, and a reconciliation between them attempted. No appropriate animal model is available to help resolve different findings and interpretations. Neural control of colon motility is exerted probably through modulation of myogenic activity as well as directly. The activities of extrinsic nerves, intrinsic motor nerves and afferent nerves are integrated within the colon, at prevertebral ganglia and in the spinal cord in animals, but similar data are not available for the human. There is a lack of studies directly relating transit to motility and conventional beliefs need reexamination.

Electrophysiology of smooth muscle of the small intestine of some mammals

Intracellular recordings were made from cells located in the longitudinal, inner and outer circular muscle layers of the dog, cat, rabbit, opossum and human small intestine. In whole-thickness preparations in all five species, longitudinal muscle cells generated slow waves and spikes. However, in isolated longitudinal muscle preparations, all cells tested were electrically silent. In whole-thickness and in isolated preparations, cells in the inner circular muscle layer generated spontaneous spikes superimposed on slow potentials. However, the occurrence of spikes and slow potentials was more regular in whole-thickness preparations. In whole-thickness preparations, cells in the outer circular muscle layer generated slow waves which were coupled with phasic contractions. However, in isolated outer circular muscle preparations, all cells tested were electrically silent and spontaneous phasic contractions were absent. In whole-thickness preparations, non-neural cells located on the serosal side of the outer circular muscle layer generated slow waves. The data suggest that spontaneous slow waves of the small intestine of the dog, cat, rabbit, opossum and human are generated in non-neural cells located between the longitudinal and outer circular muscle layer and by non-neural cells located between the outer and inner circular muscle layers.

Autonomic innervation of interstitial cells in the nasal respiratory mucosa

We analyzed the microscopic innervations of the pars respiratoria of the nasal mucosa in humans, cats, and rabbits. To this end, the techniques of Jabonero, Champy-Maillet, and Koelle-Friedenwald were employed to detect specific acetylcholinesterase activity. The supremum colli ganglion was also removed from cats in order to observe any tissue changes produced. Using our histochemical techniques, we were able to demonstrate for the first time that Cajal's interstitial cells in the nasal mucosa are acetylcholinesterase-positive. These cells also appear to be totally integrated into the structure of the terminal vegetative neural formations. Additionally, the fibers surrounding these cells were found to show early degeneration after experimental cervical sympathectomies had been performed.

Macrophage-like cells in the muscularis externa of mouse small intestine

In muscularis externa of mouse small intestine, cells with ultrastructural features of macrophages were invariably observed in three layers: in the subserosal layer, between the circular and longitudinal muscle layers, and in association with the deep circular plexus. These macrophage-like cells (MLC) had a single indented nucleus, perinuclear Golgi complex, smooth and rough endoplasmic reticulum, many pits (coated and uncoated) in the plasma membrane, coated vesicles, light vesicles, and primary lysosomes, but rather few heterogeneous lysosomal vacuoles. MLC were partially enveloped by processes of interstitial cells of Cajal. FITC-dextran used in combined fluorescence stereo microscopy, fluorescence microscopy, and electron microscopy was employed as a tracer to study the endocytic qualities of the MLC. The mice were killed 5, 15, 30, and 60 min, 1 day, and 4 days after dextran administration. By fluorescence microscopy after 1 or 4 days MLC were observed as a constant cellular population with a strikingly regular distribution. By electron microscopy dextran-containing vacuoles were conspicuous after 1 h or more. MLC of the subserosal layer and between the circular and longitudinal muscle layers could be distinguished with respect to general appearance, pattern formation, and apparent dextran contents.

Structure of the musculature of the chicken small intestine

The small intestine of the chicken was studied by light and electron microscopy. The musculature, measuring about 180 microns in thickness in the distended intestine, consists of four layers (outer longitudinal, outer circular, inner circular and inner longitudinal) which are directly apposed to one another. There is no layer of connective tissue equivalent to the submucosa of mammalian intestine, and the intestinal glands lie close to the inner longitudinal muscle. Mucosal folds are not formed during isotonic contraction of the intestine. The muscle cells of the chicken small intestine are characterized by large, numerous and sharply outlined dense bodies, by the presence of an extremely thin basal lamina, by prominent dense bands at the cell surface but relatively few intermediate junctions. There are many areas of direct apposition between cell membranes of adjacent cells and little collagen between the muscle cells. The four muscle layers have each distinctive structural features. Gap junctions between muscle cells occur only in the outer circular layer. The outer circular and outer longitudinal layers are closely apposed and numerous junctions of the adherens type link cells of the two layers. Intramuscular blood capillaries are rare and are found virtually only in the outer circular layer; their endothelial cells are joined by tight junctions. In the outer circular layer (but not in the other layers) there are two further cell types, fibroblasts and interstitial cells, which can be clearly distinguished from one another. The latter cells are intimately related to nerve bundles and are connected by gap junctions to some muscle cells.

Cytodifferentiation of the interstitial cells of Cajal related to the myenteric plexus of mouse intestinal muscle coat. An E.M. study from foetal to adult life

The cytodifferentiation of the interstitial cells of Cajal (Type I) related to the myenteric plexus of the mouse intestinal muscle coat was studied in foetuses at term, neonates not yet fed, suckling animals, weaning animals and adult animals. In foetuses at term, interstitial cells of Cajal and their precursor cells are not identifiable. In neonates not yet fed, presumed precursor cells of the interstitial cells of Cajal, i.e. ICC-blasts, can be identified as cells surrounding the developing myenteric plexus and interposed between the circular and longitudinal muscle layers. These ICC-blasts are poorly differentiated but, like the adult interstitial cells, are always related to nerve endings and possess large and numerous mitochondria. In suckling animals these cells gradually develop, and are fully differentiated only after the end of the weaning period.

Excitation-contraction coupling without Ca2+ action potentials in small intestine

Sensitive mechanical and intracellular electrical recordings showed that phasic contractions occurred in response to electrical slow waves in the absence of Ca2+ action potentials. Drugs that either enhanced or depressed slow waves were used to study the relationship between slow-wave amplitude and the amplitude of the phasic contractions. Acetylcholine (Ach) (10(-8) to 3 X 10(-7) M) increased slow waves and contractions without causing action potentials. When ACh was raised to 10(-6) M, action potentials were elicited and accompanying contractions increased in amplitude by at least a factor of five. The Ca2+ channel blocker, Mn2+ (0.5 mM), decreased slow-wave amplitude and the associated phasic contractions. These data agree with a previous study (12), suggesting that an oscillation in intracellular Ca2+ occurs during each slow-wave cycle. The present study suggests that the increase in intracellular Ca2+ during the slow wave is sufficient to activate the contractile apparatus.

Gap junctions of the muscles of the small and large intestine

The distribution of gap junctions (nexuses) in various parts of the small and large intestines of the guinea-pig was studied using the freeze-fracture technique and in thin sections. The percentage area of smooth muscle cell surface occupied by gap junctions varies from 0.50% in the circular muscle of the duodenum to zero in the longitudinal muscle of the ileum. In the circular muscle of the jejunum and ileum the area occupied by nexuses is 0.22% (or about 11 micrometers 2 per cell). The sizes of junctions range from less than 0.01 micrometer 2 to 0.20 micrometer 2, with two-thirds of them being smaller than 0.05 micrometer 2. In the colon, gap junctions are rare, very small and confined to the circular muscle layer. Even the smallest aggregates of intramembrane particles correspond to areas of close apposition between the membranes of adjacent cells; it is therefore justified to interpret them as being gap junctions. Some gap junctions are formed between a smooth muscle cell and an interstitial cell. Gap junctions are not found in the longitudinal muscle of the small intestine; this is in sharp contrast to the abundance of gap junctions in the adjacent circular layer. In the small intestine of cats and rabbits, gap junctions are abundant in the circular muscle layer, whereas they are very small in size and very few in number in the longitudinal muscle layer.

Hypertrophic smooth muscle. III. Increase in number and size of gap junctions

The smooth muscle cells of the circular musculature of the guinea pig ileum are connected by gap junctions (nexuses) which occupy about 0.21% of the cell surface. When the muscle hypertrophies in the portions of the ileum oral to an experimental stenosis, the muscle cells increase in size and number. Gap junctions become markedly larger than in control muscles and occupy 0.49% of the cell surface. While the cells double their surface area, the number of nexuses per unit surface remains unchanged (47--48 per 1000 microns2). The packing density of intramembrane particles (or pits) in the nexuses of hypertrophic muscle cells is 6700 . microns-2, which is slightly less than in control muscle cells (7200 . microns-2). A characteristic grouping of the particles (or the pits) within the nexus is often observed. Nexuses between two processes originating from the same cell are common. Nexuses do not occur in the longitudinal muscle.