Epithelium lines the surface of organs, the lumen of hollow organs, body cavities, joint cavities, and the body itself. Examples of various types of lining epithelium are pictured on the following pages. The basal surface of the cell is in contact with the basement membrane or is directed toward the basement membrane whereas the apical surface is directed away from the basement membrane. Two apical specializations include cilia and microvilli. Microvilli increase the apical surface area and are typically observed in absorptive cells.
Cilia are much larger and have a core of microtubules.
These extend into the cilia from a centriole at the base of the individual cilium. This centriole is known as a basal body. Cilia are not absorptive in nature but are motile and used to move material across the apical surface of the cell. Growth of cilia or microvilli is induced by the basement membrane which is an epithelial structure that separates epithelium from connective tissue. The basement membrane is comprised of a special type of collagen.