SUMMARYThe oligosaccharides of the mucous gastric glycoproteins are involved in the protection of the gastric mucosa and are altered in different diseases. Therefore, it is important to know their composition in health, to better determine the alterations induced by the disease. Moreover, analysis of the molecular composition of the fundic gland cells has been previously used to obtain new insights into the origin of the different cell types. The aim of the present study was the localization in the subcellular structures of the fucose residues of the oligosaccharides in human fundic glands. For this, lectin cytochemical methods were used at the light and electron microscopic levels. They were combined with enzymatic and chemical treatments to characterize the nature of the oligosaccharide chains containing the fucose residues. The presence of this carbohydrate belonging to N-or O-linked oligosaccharides has been demonstrated in the secretory granules of the surface, gastric pit, mucous neck, and transitional cells of the fundic mucosa, and in the intracellular canaliculi and tubulovesicular system of the parietal cells. These fucose residues were added in the trans-Golgi regions to the elongating chains. The term "mucosal defense" refers to the factors that permit the gastric mucosa to withstand frequent exposure to substances with a wide range of properties including, e.g., pH, osmolarity, and detergent properties. The first level of defense consists of the factors secreted into the lumen, including acid, bicarbonate, mucus, immunoglobulins, and other antibacterial substances (Wallace and Granger 1996). Mucous secretions form a barrier that traps bicarbonate secreted by the epithelium and stops acid retrodiffusion from the gastric lumen. Moreover, bacteria become trapped in the mucus and are eventually excreted in the feces (Wallace and Granger 1996).