Many cells in the body have distinct sides with different jobs, much like a brick in a wall has a defined top and bottom. The apical side of a cell is its “top” or outermost surface. This surface is exposed to either the external environment or an internal cavity within the body. This organization of functionally different surfaces is how complex tissues carry out their specific roles.
Locating the Apical Side in Tissues
The presence of an apical side is a defining characteristic of epithelial cells. These are the cells that form tissues, called epithelium, which line the surfaces and cavities throughout the body, acting as protective barriers. Epithelial tissues cover the skin, line the digestive tract, and form the inner surfaces of the respiratory and urinary systems. This organization creates a clear distinction between different cellular regions.
This sidedness of epithelial cells is known as cell polarity, which means the cell has a defined top (apical) and bottom (basal). The apical side is the surface that faces an open space, called a lumen. For instance, in the small intestine, the apical surfaces of the epithelial cells face the central tube where digested food passes. In the airways, the apical side of the cells faces the passage through which air flows.
This strategic positioning is directly tied to the function of the tissue. By having a surface dedicated to interacting with an internal passage, the cell can specialize its activities. The rest of the cell, its sides and bottom, are then freed up to handle internal communications and structural support, anchoring the cell to the deeper tissues of the body.
The apical surface can be found in a single sheet of cells, known as a simple epithelium, or as the topmost layer in a multi-layered tissue, called a stratified epithelium. In both arrangements, it is always the surface that is exposed and unattached to other cells on its outward-facing side. This orientation allows the body to form highly organized and functional linings.
Distinct Functions of the Apical Surface
The apical surface of an epithelial cell is focused on two major processes: secretion and absorption. Because this surface directly interfaces with a lumen or the external environment, it is positioned to either release substances into that space or pull substances out of it. The specific function depends on the location and role of the tissue in the body.
In tissues geared toward absorption, the apical membrane is responsible for the initial uptake of molecules. A prominent example is in the small intestine, where epithelial cells absorb nutrients from digested food. The apical surfaces of these cells are equipped with transporter proteins that grab molecules like glucose and amino acids from the lumen and bring them into the cell. This is the first step in delivering nutrients to the rest of the body.
Conversely, in secretory tissues, the apical side is designed to release substances. A clear example is in the respiratory tract, where epithelial cells lining the trachea and bronchi secrete mucus from their apical surfaces. This mucus acts as a trap for dust, pollen, and other inhaled debris, preventing it from reaching the delicate tissues of the lungs. The action of these secreting cells maintains a protective layer.
Other examples of apical secretion include the release of enzymes from cells in the pancreas into digestive ducts and the secretion of sweat from glands onto the skin. The apical surface acts as a controlled gateway. It ensures that these substances are released only where they are needed, preventing them from damaging the cell itself or adjacent tissues.
Specialized Apical Structures
To perform its duties of absorption and secretion efficiently, the apical surface often features specialized structural modifications. These are not merely passive surfaces; they are shaped to maximize their functional capacity. The two most common of these specializations are microvilli and cilia, each tailored to a specific task.
Microvilli are microscopic, finger-like projections of the cell membrane that extend from the apical surface. Their primary purpose is to increase the surface area available for absorption. While they may look like tiny hairs, they are not motile. They form a dense, brush-like border that can amplify the cell’s absorptive capacity, which is important in the small intestine.
Cilia are longer, hair-like appendages that are capable of movement. They are found on the apical surface of cells in locations like the respiratory tract and the fallopian tubes. Cilia beat in coordinated, rhythmic waves to propel substances along the surface of the tissue. In the airways, this movement sweeps the layer of mucus with its trapped debris out of the lungs.
These structures are directly linked to the functions discussed previously. The presence of dense microvilli is a hallmark of an absorptive cell, while the presence of motile cilia indicates a cell involved in surface clearance. The specific architecture of the apical surface is a direct reflection of the job that the epithelial tissue performs.
The Basolateral Counterpart
To understand the apical side, one must consider its counterpart: the basolateral surface. This term combines the “basal” or bottom surface, which sits on a foundation called the basement membrane, and the “lateral” sides, which are in contact with neighboring cells. This region of the cell has functions that are distinct from, yet complementary to, the apical domain.
The basolateral surface is responsible for anchoring the cell and for intercellular communication. The basal side connects the epithelium to the underlying connective tissue, providing structural stability. The lateral sides contain junctions that bind the cells tightly together. After the apical side absorbs substances, the basolateral membrane transports these molecules out of the cell and into the bloodstream.
A feature separating these two domains are tight junctions. These are complexes of proteins that form a continuous, watertight seal near the apical end of the cell, encircling it like a plastic ring on a six-pack of cans. This seal physically separates the apical membrane from the basolateral membrane, preventing molecules from leaking between the cells.
This separation is important to epithelial function. It ensures that substances absorbed at the apical surface do not simply diffuse back out through the basolateral side, and vice versa. By creating these distinct compartments, the tight junctions maintain the cell’s polarity and allow the apical and basolateral surfaces to specialize in their different roles.