<p>Galls alter the tissue organization of host plants, including modifications in cell wall composition. This study investigated tissue development and cell wall dynamics in galls of <i>Sapium glandulosum</i> to identify key steps involved in their establishment. Samples of young, mature, and senescent galls, as well as nongalled leaves, were analyzed using structural and immunocytochemical approaches. For histology, samples were fixed, embedded in resin, sectioned, stained with toluidine blue, and mounted with Entellan^®. For immunocytochemistry, resin-embedded samples were tested for epitopes of cell wall proteins, pectins, and hemicelluloses using antibodies. The leaves of <i>S. glandulosum</i> are glabrous, hypostomatic, and exhibit dorsiventral mesophyll. Gall development alters the typical leaf morphogenetic pattern, giving rise to structures with a parenchymatic cortex. In young galls, hypertrophy and hyperplasia were observed, followed by tissue maturation in mature galls. Senescent galls showed signs of cytoplasmic degradation in most cortical cells. Structural modifications in the side chains of rhamnogalacturonan I and increased cross-linking of pectic polymers affect cell wall properties, playing roles in both development and defense responses. The low immunolabeling with JIM5 in young and mature galls suggests the suppressed activity of pectin methylesterases, which may reflect a strategy by which gall-inducing organisms inhibit host defense signaling. Xyloglucan epitopes were detected in the vascular bundles of mature galls, suggesting the reinforcement of cell walls and possibly supporting the feeding activity of the gall inducer. The combination of anatomical and immunocytochemical data provided a basis for understanding how gall induction modulates cell differentiation and cell wall composition in <i>S. glandulosum</i>.</p>