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The apical membrane of a polarized cell is the part of the plasma membrane that forms its lumenal surface, distinct from the basolateral membrane. This is particularly evident in epithelial cells, but also describes other polarized cells, such as neurons.
Tight junctions that join epithelial cells near their apical surface prevent the migration of proteins to the apical membrane. The basal and lateral surfaces thus remain roughly equivalent to one another, yet distinct from the apical surface.
Referring to the tip (apex) of a cell, an organ or other body structure. For example, epithelial cells have their apical surface exposed to the body exterior, or (depending on their location) an internal open space like the intestinal lumen.