Anorectal fibrosis in 3 murine models of chronic intestinal fibrosis: a comparative analysis.

BACKGROUND

Anorectal fibrosis is a poorly understood condition of Crohn's disease (CD)-associated perianal complications, characterized by transmural fibrotic remodeling that may contribute to luminal narrowing. Despite well-established animal models for intestinal fibrosis, the anorectal region remains largely unexplored. This study aimed to compare anorectal fibrotic remodeling across 3 commonly used chronic intestinal fibrosis protocols.

METHODS

Male C57BL/6 mice were assigned to control and 3 chronic colitis groups: dextran sulfate sodium (DSS; cyclic oral administration), 2,4,6-trinitrobenzenesulfonic acid (TNBS), and dinitrobenzene sulfonic acid (DNBS; weekly intrarectal administration). Anorectal fibrosis was assessed using pelvic MRI, histopathological analysis (H&E, Masson's trichrome, Sirius red staining), polarized light microscopy for collagen typing, and immunohistochemistry for fibrosis markers (α-SMA, TGF-β1, collagen I) and inflammatory cells (F4/80, MPO, CD3). Molecular analysis included quantitative RT-PCR and hydroxyproline assay.

RESULTS

All 3 models developed comparable colonic inflammation and fibrosis. However, at the anorectal transition zone, DSS-treated mice exhibited significantly greater wall thickness on MRI, more severe collagen deposition, elevated type I/III collagen ratio indicative of mature fibrosis, and enhanced myofibroblast activation with TGF-β1 upregulation, muscularis propria thickening, and reduced luminal area on histologic cross-sections. In the anal squamous epithelium, DSS mice demonstrated significantly increased subepithelial fibrosis, epithelial hyperplasia, and diffuse inflammatory cell infiltration throughout both epithelial and subepithelial layers.

CONCLUSIONS

Under these standard protocols, the chronic cyclic DSS model produced the most prominent anorectal fibrotic remodeling, while TNBS and DNBS showed less severe anorectal changes despite comparable colonic fibrosis. These protocol-dependent differences may inform preclinical investigation of anorectal fibrosis and evaluation of anti-fibrotic therapeutic strategies.