An apical surface is a surface that faces the lumen which is the inner cavity of an organ. An organ of structure also has a basal side which faces the basement membrane and is commonly in contact with connective tissue or muscle tissue below it.

What is the apical surface of a cell?

The apical surface of epithelial cells, which lines the lumen of sac- and tube-shaped organs and the inner surfaces of the body cavities, forms the interface between the extracellular milieu and underlying tissues.

What is the difference between the basolateral and apical surface?

In the context of renal tubule physiology, the term basolateral membrane or serosal membrane refers to the cell membrane which is oriented away from the lumen of the tubule, whereas the term luminal membrane or apical membrane refers to the cell membrane which is oriented towards the lumen.