Which of the following is not classified as a cell junction?

In the context of cell biology, cell junctions are specialized structures that facilitate communication and adhesion between adjacent cells. Tight junctions, gap junctions, and desmosomes are all well-defined types of cell junctions that play critical roles in maintaining tissue integrity and facilitating intercellular communication.

Tight junctions create a seal between cells to prevent the passage of molecules through the intercellular space, thus contributing to the barrier function of epithelial tissues. Gap junctions are channels that allow for the direct transfer of small molecules and ions between adjacent cells, enabling communication and coordination of responses in tissues. Desmosomes provide strong adhesion between cells, helping to resist mechanical stress and maintain tissue architecture.

The term "regulatory junction" does not correspond to a recognized category of cell junctions in cell biology. Therefore, this choice does not reflect a known structure or functionality associated with intercellular connections, distinguishing it from the other three options that are prominent and well-characterized types of cell junctions.