Epithelial tissue forms sheets of cells that line all external and internal surfaces, creating a selective barrier between the body and the outside world or the hollow spaces within organs. Understanding the apical surface is fundamental to understanding how these barriers function in organs like the lungs, gut, and kidneys. This specific face of the cell acts as the primary interface where interaction with the exterior environment takes place.
Defining the Apical Surface
The apical surface is the “free” or exposed face of an epithelial cell. This surface always points toward the external environment or the lumen, the interior open space of an organ or tube. This specialized domain allows the cell to interact directly with substances passing through the body, such as digested food or inhaled air.
The epithelial cell is distinctly organized, a feature known as polarity. The apical surface is clearly separated from the other two domains. The lateral surfaces abut neighboring cells, and the basal surface rests upon the basement membrane, which connects the cell to the underlying connective tissue. This arrangement ensures that every function performed by the epithelial layer is directional.
Specialized Structures of the Apical Domain
The apical domain is often specialized with microscopic extensions designed to enhance the cell’s function. Two common structures are microvilli and cilia. Microvilli are tiny, finger-like projections of the cell membrane, containing a core of actin filaments anchored into the terminal web. When dense, they resemble a “brush border,” which dramatically increases the surface area available for uptake in absorptive tissues like the small intestine.
Cilia are longer, motile extensions with a more complex internal structure. They are composed of a “9+2” arrangement of microtubules, known as the axoneme. This precise arrangement allows them to beat in a coordinated, sweeping motion, functioning like microscopic oars.
The Functional Roles of the Apical Surface
The physical specializations of the apical surface enable the physiological tasks of epithelial tissue. One main role is absorption, seen in the small intestine and kidney tubules. Microvilli provide an expanded surface area for transport proteins to capture nutrients or water from the lumen. Nutrients are actively transported across the apical membrane into the cell cytoplasm.
Another role is secretion, where cells release substances onto the surface of the epithelium. In the stomach, epithelial cells release mucus and digestive enzymes, while tracheal cells secrete mucus to trap foreign particles. The apical domain also facilitates mechanical clearance, evident in the respiratory tract. Motile cilia propel the mucus layer and trapped debris out of the lungs, a process known as mucociliary clearance.
The apical surface acts as the first line of protection, forming a selective barrier that prevents harmful substances, pathogens, and toxins from entering the underlying tissues. This function is highly developed in stratified epithelia, such as the skin, where the outermost apical layers are shed and replaced.
Maintaining Cell Organization Through Polarity
The ability of epithelial cells to perform directional tasks, such as moving nutrients from the gut lumen to the bloodstream, depends on maintaining cellular polarity. This requires the apical domain to remain chemically and structurally distinct from the basolateral domain (the combined basal and lateral surfaces). This distinction ensures that specific transport proteins are localized only to the correct membrane face.
The structures responsible for maintaining this separation are tight junctions. They form a continuous seal around the cell near the apical surface, acting like a belt. Tight junctions prevent the mixing of proteins and lipids between the apical and basolateral membranes, ensuring the functional integrity of each domain. They also control paracellular permeability, restricting the passage of molecules through the space between adjacent cells.