Which cell junction are regions where membranes of adjacent cells fuse?

Tight junctions are specialized structures that form a barrier between adjacent cells by tightly fusing their membranes together. This fusion creates a seal that prevents molecules and ions from passing between the cells, thereby maintaining the distinct environments of the adjacent cellular spaces. They are crucial in tissues where selective permeability is essential, such as in epithelial cells lining the intestines or blood-brain barrier.

In contrast, desmosome junctions provide mechanical stability between cells by anchoring them together using intermediate filaments, but they do not create a fused membrane barrier. Anchoring junctions, which include adherens junctions and desmosomes, serve to mechanically link cells but do not create the impermeable barrier characteristic of tight junctions. Communicating cell junctions, or gap junctions, allow for the transfer of ions and small molecules between cells, facilitating intercellular communication, but again do not involve the fusion of cell membranes.

Thus, the defining feature of tight junctions is indeed their ability to fuse cell membranes to restrict paracellular movement, making them the correct choice in this context.