Definitive diagnosis of MC rests on histological examination, which is necessary both to rule out other possible causes of chronic diarrhea and to distinguish between LC and CC.
The diagnosis of LC is based on the following features. First, an increase of the density of intra-epithelial lymphocytes (IEL) in the surface epithelium. Physiologically, IEL of the colonic mucosa are less than 5 per 100 epithelial cells, while in LC they are at least 25 per 100 surface epithelial cells[
3,15,67], typically CD8
+, carrying the αβ T-cell receptor[
68-70] and expressing the human mucosal lymphocyte antigen, specific for intestinal lymphocytes[
68,71,72]; lymphocytosis in the crypt epithelium is also seen, but is less constant[
3].
Rarely, an infiltrate of eosinophils in the epithelium can also be found, and in a few reports neutrophils have been described, considered as a sign of acute stage of colitis[
73].
Second, an inflammatory infiltrate in the lamina propria, with prevalence of mononuclear cells such as lymphocytes and plasma cells, but with the sporadic presence of eosinophils, mast cells, macrophages and neutrophils (very rare); unlike those in the surface epithelium, lymphocytes of the lamina propria are mostly CD4
+[68,69]. It has been argued that lymphocytosis in the epithelium and in the lamina propria could be a histological response to a
primum movens inflammatory process, rather than a primitive immunological dysfunction[
16].
Third, surface epithelial damage, manifested with flattening and degeneration of the epithelial cells (with features such as vacuolization of cytoplasm, nuclear irregularity, karyorrhexis and pyknosis) and focal loss and detachment of the epithelium - these features being more common in CC
-[3,74,75]. There is also a minimal distortion of the structure of the crypts, but no crypt abscesses and granulomas[
76]. Moreover, active cryptitis has been reported by Gledhill et al[
49] in 41% of subjects with LC and in 29% with CC.
CC is characterized by a thickening of the subepithelial collagen layer that is absent in LC. The collagen band appears extremely eosinophilic in routine hematoxylin-eosin staining, but is better recognizable with Masson’s trichrome staining; tenascin immunohistochemical stain appears to further improve sensitivity[
77,78].
In the healthy colon, the subepithelial collagen band is thinner than 3 μm[
48]. The diagnostic criterion for CC has been proposed to be a thickness of at least 10 μm by some authors[
15,32,74], at least 7 μm by others[
29,49,76]. However, it is plausible that in most cases the collagen band reaches even 100 μm[
15]. According to Lazenby et al[
3], the thickness of the collagen band alone is neither sufficient nor necessary for the diagnosis of CC: there are also some qualitative abnormalities, such as entrapment of red blood cells and cells of inflammation in the collagen band, and an irregular appearance of the inferior edge of the basement membrane, because of collagen bundles extending into the lamina propria. Some studies report a decreasing gradient of presence of intraepithelial lymphocytes and thickness of collagen band from the cecum to the rectum[
69,76], others suggest that biopsies of the transverse colon give the best chance of diagnosis[
79], but as a general rule left-sided colonic biopsies, easily carried out with a flexible rectosigmoidoscope, are considered sufficient for the diagnosis of MC; if descending colon biopsies are not diagnostic and clinical suspicion is strong, a colonoscopy with random biopsies can be performed.
Studies of immunohistochemistry have shown that the collagen band consists basically of type III collagen - the subtype produced with repair functions - pointing to a reactive origin (the normal basement membrane mainly consists of fibronectin, laminin and type IV collagen)[
80].
The histological features of MC are not specific: CC-like findings have been reported in colon cancer, carcinoid lesions, hyperplastic polyps,
C. difficile infection, Crohn’s colitis, constipation and healthy people[
48,76,80-87], while features resembling LC have been described in human immunodeficiency virus, Crohn’s disease, healthy people[
67,81,88,89].