The human intestine is divided anatomically into two segments, the small and the large intestine, whereby the first is considerably longer (6-7 m in an adult) than the second (roughly 1.5 m). Both segments are composed of several layers, including the mucosa (glandular epithelium and muscularis mucosa), submucosa, muscularis externa (inner circular and outer longitudinal), and lastly the serosa. As might be expected based on the presence of a considerable variety of cells in this tissue, including epithelial and muscle cells (see section "Introduction"), all three isoforms (caveolin-1, -2, and -3) have been detected in both intestinal segments . Results from the literature suggest that caveolin-1 and -2 are expressed in epithelial cells of the small intestine and colon (Table 2.1).
Most available studies have focused on the analysis of caveolin-1 in the human colon; however, data are controversial. On the one hand, caveolin-1 is reportedly present in normal colon mucosa, as well as stroma and expression is reduced in tumors of different stages , On the other hand, data arguing that caveolin-1 levels are elevated in colon tumor samples has also been provided [37, 107] . Specifically, caveolin-1 expression was increased in samples of adenocarcinomas, but not adenomas and normal mucosa . Studies in rodents investigating the role of caveolin-1 in colon carcinogenesis are generally scarce. In one such study, caveolin-1, but not caveolin-2, expression increased in a rat model of adenocarcinoma induced by azoxymethane [ 107 ] , To the contrary, however, another study using caveolin-1 knock-out mice points to a role for caveolin-1 in controlling proliferation of intestinal crypt stem cells . Clearly more experiments are required to clarify these discrepancies concerning caveolin-1 expression and function in humans and rodent models.
A review of the literature summarizing data available concerning caveolin expression in human, mouse, and rat intestine is shown in Table 2.1. In the majority of these studies, caveolin-1 mRNA and protein were detected in the mucosa of the intestine, specifically in epithelial cells [4, 5, 11, 36, 94]. Studies in tumors also offer controversial data, indicating that caveolin-1 may decrease in mucosa and stromal cells [ 11] or increase in epithelial cells [ 37] and in distant metastases  .
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