The apical side of a cell is the exposed surface of an epithelial cell layer that interacts directly with the external environment or the space within an internal organ, known as a lumen. This surface is the uppermost membrane of cells that form epithelial tissue, which covers surfaces and lines body cavities. The apical surface acts as the interface between the body’s internal systems and the surrounding world. Its distinct molecular makeup and physical features enable the cell to perform specific functions, such as nutrient uptake or particle movement.
The Foundation of Cellular Polarity
Cells that form linings and barriers, known as epithelial cells, are organized with an intrinsic asymmetry called cellular polarity. This arrangement means the cell has structurally and functionally different sides, which is necessary for directional processes like transport. The apical membrane faces the lumen, such as the digestive tract or an airway, allowing it to interface with contents that are technically outside the body.
The opposite side of the cell is the basolateral domain, which is oriented toward underlying tissues, blood vessels, and the rest of the body. The basal face is anchored to the basal lamina, a thin layer of extracellular matrix. This defined polarity allows the cell to regulate the movement of substances, ensuring that nutrients absorbed from the lumen are specifically transported across the cell toward the bloodstream.
Specialized Apical Structures and Features
The apical membrane features unique physical extensions and molecular complexes. Microvilli are common modifications, appearing as numerous minute, finger-like projections that extend from the surface. Each microvillus contains a core of bundled actin microfilaments anchored to the terminal web, a network of proteins within the cell.
This dense arrangement of microvilli is often referred to as a brush border, and its purpose is to dramatically increase the surface area of the cell membrane. Cilia are longer, hair-like appendages that are motile in certain tissues, possessing a complex internal structure of microtubules organized in a “9+2” pattern. These motile cilia beat in a coordinated, sweeping motion across the apical surface, powered by motor proteins.
A specialized structure called the tight junction is situated just beneath the apical surface, completely encircling the cell and fusing it to its neighbors. The tight junction forms a seal that prevents fluids and small molecules from passing between the cells. This seal forces substances to be actively transported through the cell itself, maintaining the barrier’s integrity and enforcing directional flow.
Primary Physiological Roles
The specialized apical structures enable the cell layer to carry out three general functions: absorption, secretion, and protection. Absorption is a primary role, exemplified by the cells lining the small intestine, where the brush border microvilli maximize nutrient uptake. These cells take in digested sugars, amino acids, and fats from the lumen before transporting them to the basolateral side for entry into the circulation.
Secretion involves releasing specific substances from the cell onto the apical surface. This process is carried out by epithelial cells in various glands, which release enzymes, hormones, or mucus into ducts or onto a lining. Protection is carried out by forming a physical barrier against pathogens, toxins, and fluid loss, such as on the skin’s surface.
In the respiratory tract, the apical cilia work together to perform a cleansing action called the mucociliary escalator. These beating projections move a layer of mucus, which traps inhaled dust and microbes, upward and out of the lungs. This continuous, coordinated physical action against the external environment is a direct consequence of the unique structural modifications found only on the cell’s apical surface.