The free unattached surface of epithelial tissues is known as the apical surface. This surface is not attached to any other structure and is often exposed to an open space, such as the lumen of a duct or the external environment. The apical surface may have specialized structures like microvilli or cilia, which aid in functions such as absorption and movement of substances.
In contrast, the other surfaces of epithelial cells are attached to underlying tissues or other cells. The basal surface is attached to the basement membrane, which provides structural support and connects the epithelium to the underlying connective tissue. The lateral surfaces are attached to neighboring epithelial cells through specialized junctions.
Understanding the different surfaces of epithelial tissues is crucial for comprehending their roles in protection, secretion, absorption, and other physiological functions.