The apical membrane is the region of an epithelial cell's plasma membrane facing its free surface or a lumen. It differs from the basolateral membrane, which faces neighboring cells and underlying tissue. These are spatially and functionally distinct regions of one cell boundary, not separate cells. Their organization is part of apical-basal polarity.
A tubule makes the definition concrete. Its apical surface faces the fluid inside the tube, while its basal side faces the surrounding tissue. A transporter moving a substance from the lumen into a cell and another transporting it from the cell into the surrounding tissue must occupy appropriately different membrane regions to support directed transepithelial transport. Merely producing both proteins somewhere in the cell does not guarantee the same net transport.
A classic experimental probe used the sites at which enveloped viruses emerged from polarized epithelial monolayers. Electron microscopy showed characteristic asymmetry: influenza virus budded at the apical surface, while vesicular stomatitis virus assembled at basolateral regions. Different viral envelopes therefore reported different regions of the host-cell surface.[ref]
The apical membrane is therefore defined by tissue orientation and organization, not by always being physically uppermost on a microscope image. Its protein and lipid composition can be specialized, but the exact contents depend on the epithelial cell type and physiological state.
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