The small intestine mucosa represents a critical barrier between the inner and outer environment. Although the organization of the intestinal mucosal barrier has been extensively studied in mammals, data from non-mammalian vertebrates remain limited. Here we characterized the mucosal barrier organization in small intestine of the bullfrog (Lithobates catesbeianus) through histological and ultrastructural analyses. Histological investigation showed that small intestinal wall of the L. catesbeianus was thin due to small number of smooth muscle cell layers in the muscularis mucosae (thickness 1.82 ± 0.32 μm) and muscularis (thickness 44.95 ± 3.22 μm). Transmission electron microscopy (TEM) investigation showed the junctional complexes including tight junctions between enterocytes, and the shedding extracellular vesicles were observed in close proximity to the surface of microvilli in the lumen. These structures formed a physical barrier together with the epithelial cells and the mucus layer. In addition, a relatively elementary immune barrier was composed of mucosal resident immune cell types, including lymphocytes, macrophages, mast cells, plasma cells, eosinophils and enteroendocrine cells. Taken together, the mucosal barrier of small intestine in the L. catesbeianus consisted of two parts, physical barrier and immune barrier, which constituted a relatively well-developed small intestine mucosal barrier of L. catesbeianus. These findings provide baseline data for understanding mucosal barrier evolution in vertebrates.

KEY WORDS: Amphibian; Intestine; Ultrastructure; Immune.

YOU, W.; LI, H.; CHEN, H.; ZHAN, X.; LIU, C.; CHEN, S.; YE, Y. & ZHANG, H. Histological evidence of mucosal barrier in small intestine of the bullfrog (Lithobates catesbeianus). Int. J. Morphol., 44(1):60-70, 2026.