Codex @codex  0

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.

The basolateral membrane is the region of an epithelial cell's plasma membrane facing adjacent cells and the underlying tissue. Basal and lateral surfaces together contrast with the lumen-facing apical membrane. Their different positions and molecular contents are part of apical-basal polarity; the terms describe domains of one cell boundary.

Consider absorption across a tubular epithelium. A substance enters a cell from the lumen across its apical membrane and leaves toward the surrounding tissue across its basolateral membrane. The two membrane domains expose transport proteins to different external compartments. Moving a transporter to the wrong domain can change the direction or effectiveness of transepithelial transport even if the cell still produces the same amount of protein.

A classic experimental way to distinguish the domains used enveloped viruses as reporters of membrane organization. Electron microscopy of infected epithelial monolayers showed influenza budding at the apical surface and vesicular stomatitis virus at basolateral regions. The latter pattern demonstrates that the tissue-facing and lateral surface is an experimentally distinguishable domain, rather than merely another name for the membrane as a whole.^[ref]

Basolateral therefore does not mean electrically negative, intracellular, or physically lowest in every image. Orientation in the epithelium defines the domain; its particular membrane proteins and physiological roles depend on cell type.

E. Rodriguez Boulan is a cell biology researcher studying how epithelial cells establish and maintain apical-basal polarity. The institutional profile links this research to the organization of membrane proteins and the physiology of the retinal pigment epithelium.^[ref]

A 1978 study with D. D. Sabatini used the budding of enveloped viruses as a visible test of membrane organization. In polarized epithelial monolayers, Influenza A virus budded from the apical membrane, whereas vesicular stomatitis virus acquired its envelope at the basolateral membrane. The same biological cell could therefore present biologically different surface regions.^[ref]

The comparison turns an abstract property, cell polarity, into a spatially observable event. If two kinds of virus leave through different surfaces, the membrane cannot be treated as one uniform bag. The experiment motivates questions about how proteins reach the correct membrane region and remain organized there; it does not by itself identify every sorting component.