The ultrastructural expression of laminin and fibronectin was studied in human colonic mucosa with a pre-embedding immunoperoxidase technique. Both antigens were present in epithelial basement membrane. The lamina densa was uniformly and homogeneously immunostained by both fibronectin and laminin, while the lamina rara showed discrete, positive sites occasionally connecting the lamina densa with the epithelial plasma membrane. In the lamina reticularis, fibronectin was diffusely positive, while laminin showed a discontinuous positivity only in the uppermost region in vicinity of the lamina densa. Collagen fibrils were regularly decorated by fibronectin-positive material. Basement membranes in the capillary wall showed a similar pattern of reactivity both with laminin and fibronectin. These results point out that laminin as well as fibronectin are integral components of intestinal mucosa basement membranes
