MICROANATOMY

Detailed explanation-1: -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.

Detailed explanation-2: -Epithelial cells have a free surface called the apical surface. We often think of this part of the epithelial cell as the top of the cell, however, the free surface can face any direction so long as it is facing the external environment, such as the opening of a cavity or the surface of the skin.