= Basolateral membrane
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.<Asymmetric budding of viruses in epithelial monlayers: a model system for study of epithelial polarity by E. Rodriguez Boulan and D. D. Sabatini>[1978, polarized-budding observations and abstract]{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.
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