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SYMPOSIUM REPORT |
1 School of Human Sciences, Waseda University, Mikajima 2-579-15, Tokorozawa, Saitama, Japan 359-1192
| Abstract |
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(Received 16 July 2006;
accepted after revision 17 August 2006;
first published online 17 August 2006)
Corresponding author T. Komuro: School of Human Sciences, Waseda University, Mikajima 2-579-15, Tokorozawa, Saitama, Japan 359-1192. Email: tkomuro{at}waseda.jp
| Introduction |
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ICC have been found throughout the digestive tract from the oesophagus (Faussone-Pellegrini & Cortesini, 1985; Torihashi et al. 1999) to the inner sphincter region of the anus in the human (Hagger et al. 1998), but they show different distribution patterns and morphological features depending on their anatomical locations, and according to which they are classified into several subtypes (Hanani et al. 2005). The structural arrangement of ICC appears to reflect their physiological tasks and thus provides a clue for critical understanding of intestinal motility. This article deals with the morphological features of each subtypes of ICC revealed by the immunohistochemical and ultrastructural observations of the stomach, small intestine and colon of laboratory animals such as mouse, rat and guinea-pig.
Organization of ICC networks
The location of the different subtypes of ICC is shown schematically in Fig. 1. The cell shape and arrangement of each subtype of ICC is mainly determined by their relationships to local nerve plexuses, the orientation of the smooth muscle layer in which they are contained, and the frequency of connections between ICC themselves.
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ICC of the longitudinal muscle (ICC-LM). ICC-LM are similar to ICC-CM in cell shape but are usually less numerous than the latter in nearly the whole GI tract, i.e. in the stomach, small intestine and colon (Komuro, 2004). ICC-CM and ICC-LM are often collectively termed ICC-IM.
ICC of the deep muscular plexus (ICC-DMP). ICC-DMP are closely associated with the nerve bundles of the deep muscular plexus of the small intestine that extends two-dimensionally in a plane between the inner thin and outer thick sublayers of the circular muscle (Rumessen et al. 1982; Zhou & Komuro, 1992). ICC-DMP are also multipolar cells, but the majority of their processes show a distinct unidirectional orientation along the circumference, due to their close association with both nerve bundles and circular muscle fibres (Komuro, 2004).
ICC of submucosa and sumucosal plexus (ICC-SM and ICC-SMP). ICC-SM and ICC-SMP are found at the interface between the submucosal connective tissue and the innermost circular muscle layer of the pyloric region of the stomach (Horiguchi et al. 2001; Seki & Komuro, 2002; Mitsui & Komuro, 2003) and of the colon, respectively (Berezin et al. 1988; Ishikawa & Komuro, 1996). Their cell axes are parallel with those of adjacent circular muscle cells, but they contain multipolar cells with a few secondary processes and seem to form a loose network with each other (Kunisawa & Komuro, 2004), unlike the adjacent ICC-CM.
ICC of the subserosa (ICC-SS). Stellate interstitial cells in the subserosal layer were observed in the mouse small intestine with supravital methylene blue staining (Thuneberg, 1982) and in the mouse colon by Kit immunohistochemistry (Vanderwinden et al. 2000).
Distribution of ICC
Each subtype of ICC is almost uniformly found in its own tissue layer throughout the entire length of the small intestine and colon, though some differences have been reported in the mouse colon (Ward et al. 2002) and human colon (Horisawa et al. 1998). In this respect, the stomach is unique in that ICC have a different distribution in proximal and distal regions of the same organ (Burns et al. 1997; Seki & Komuro, 2002). In the mouse stomach, for example, ICC-CM and ICC-LM are densely distributed throughout the thick circular and longitudinal muscle layers of the cardia, fundus and most of the squamous epithelial portion of the corpus. However, ICC-MP are completely lacking from the myenteric region in these areas. ICC-MP emerge in the area adjacent to the glandular corpus and become well-developed in the pyloric antrum, while both ICC-CM and ICC-LM decrease in number in this area. Along the circumferential axis of the antrum, ICC-MP are distributed more densely in the greater curvature than in the lesser curvature in the mouse (Hirst et al. 2002) and the guinea-pig (Kunisawa & Komuro, 2004; Mazet & Raynier, 2004).
Another characteristic feature of the pylorus is the presence of ICC-SM at the submucosal border of the circular muscle layer in a confined area directly adjacent to the sphincter (Horiguchi et al. 2001; Mitsui & Komuro, 2003).
Coupling of ICC and smooth muscle cells
Ultrastructural observations revealed that certain types of ICC are intercalated between nerves and smooth muscle cells (Thuneberg, 1982; Zhou & Komuro, 1992; Ishikawa et al. 1997; Seki & Komuro, 1998) and indicated that they may act as a route for neurotransmission. Such ICC make close contacts with nerve varicosities containing many synaptic vesicles on the one-hand and form gap junctions with neighbouring muscle cells on the other (Fig. 3). More concrete morphological evidence for the neuromediatory function of such ICC was recently provided by immunohistochemical staining for synaptic molecules (Beckett et al. 2005).
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Ultrastructure and dependency on the Kit/stem cell factor system
ICC are generally characterized by ultrastructural features such as the presence of numerous mitochondria, abundant intermediate filaments, moderately developed Golgi apparatus, rough and smooth endoplasmic reticulum, close contacts with nerve varicosities and formation of gap junctions with each other and with smooth muscle cells (Komuro, 1999). However, ICC show a certain range of morphological heterogeneity ranging from features similar to the fibroblasts to those specific to smooth muscles cells such as caveolae, a basal lamina and subsurface cistern, depending on anatomical location and species (Table 1; Komuro et al. 1999; Mitsui & Komuro, 2003).
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These ICC share common ultrastructural features and are classified into Type 3 ICC (Table 1), the most similar to smooth muscle cells, in respect of the presence of many caveolae and a distinct basal lamina (Komuro et al. 1999), regardless of the organ or tissue layer concerned. The evidence suggests that the most muscle-like Type 3 ICC can develop and mature cytologically independent of the Kit/SCF system, or that some other system can compensate in their cell maturation. These observations appear to raise important questions for future studies on why ICC show a fairly wide range of phenotypes and which factors determine the ultrastructural features of particular type of ICC, together with studies to demonstrate if ICC and smooth muscle cells are derived from the same mesechymal progenitor cells (Kluppel et al. 1998) and that blockade of Kit signalling induces a smooth muscle cell phenotype (Torihashi et al. 1999).
| Footnotes |
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