A tight junction is a watertight seal between two adjacent animal cells (Figure 17b). Proteins hold the cells tightly against each other. This tight adhesion prevents materials from leaking between the cells. Tight junctions are typically found in the epithelial tissue that lines internal organs and cavities and composes most of the skin. For example, the tight junctions of the epithelial cells lining the urinary bladder prevent urine from leaking into the extracellular space.
Also found only in animal cells are desmosomes, which act like spot welds between adjacent epithelial cells (Figure 17c). They keep cells together in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles.