Differential immunoreactivity of alpha-smooth muscle actin in the mouse colon.

Regional differences between left and right sides of the colon are well documented, but variation in smooth muscle cell differentiation remains poorly defined. We examined proximal and distal regions of the mouse colon using immunohistochemistry and immunofluorescence with alpha-smooth muscle actin antibodies (clones 1A4 and CGA7), together with differentiation markers including h-caldesmon, calponin, and tropomyosin. In the muscularis propria, staining did not differ between regions. In the muscularis mucosae, distal segments stained strongly with both alpha-smooth muscle actin antibodies, whereas proximal muscularis mucosae was positive with clone 1A4 but weak or negative with clone CGA7. Proximal muscularis mucosae also showed reduced immunostaining with anti-h-caldesmon, anti-calponin, and anti-tropomyosin. Quantitative immunofluorescence confirmed lower intensity with CGA7 in proximal muscularis mucosae, while 1A4 showed no difference between layers. Lineage co-staining demonstrated that alpha-smooth muscle actin–positive cells were also desmin-positive and vimentin-negative, a pattern typically observed with smooth muscle cells rather than myofibroblasts. These findings indicate that proximal muscularis mucosae contains a less differentiated smooth muscle population and that regional heterogeneity of smooth muscle differentiation may contribute to functional specialization and segment-specific disease susceptibility in the colon.