Final answer:
Tight junctions are complete barriers found between epithelial cells, formed by proteins called claudins and occludins, creating a watertight seal.
Step-by-step explanation:
Complete barriers that typically occur between cells of the epithelial type are known as tight junctions. These junctions form a watertight seal between two adjacent animal cells and are crucial in maintaining the selective permeability of epithelial layers in organs such as the intestines and lungs.
The proteins responsible for the formation of tight junctions are predominantly claudins and occludins. These proteins hold cells tightly against each other, preventing the leakage of extracellular fluids and ensuring that substances must be transported through the cell membranes to pass between sides of an epithelial layer.