Final answer:
Tight junctions form a watertight seal mainly on the apical side of epithelial cells, which is essential in maintaining the integrity of barriers like the urinary bladder. Proteins such as claudins and occludins are crucial in establishing this seal.
Step-by-step explanation:
Tight (occluding) junctions link epithelial cells primarily on the apical side, which is the side facing the lumen or the cavity that the epithelial tissue lines. These junctions form a watertight seal, preventing materials from leaking between the cells and thus preserving the integrity of barriers such as the urinary bladder or the intestinal lining. Proteins like claudins and occludins are integral in forming this seal by holding the cells tightly against each other, creating a physical barrier that controls the selective movement of substances through the epithelial layer.