What Is the Apical Domain and Why Is It Important?

The apical domain is the outward-facing surface of a cell. Many cells, particularly epithelial cells that line surfaces or cavities, have this distinct top surface. An epithelial cell can be compared to a building with a specific roof designed to interact with the outside environment. This “roof” is the apical domain, and it is structurally and functionally different from the building’s foundation and walls. This specialization allows the cell to perform tasks directed toward the open space it faces, such as a body cavity or the interior of an organ.

The Foundation of Cell Polarity

For tissues to function, their cells must exhibit polarity. This means the cell has different regions on its surface. In epithelial cells, this manifests as two domains: the apical domain, which faces an external environment or an internal open space called a lumen, and the basolateral domain, which contacts adjacent cells and underlying tissue. This division is maintained by tight junctions, which form a seal between cells and prevent the mixing of components from the two domains.

The separation of the apical and basolateral domains is necessary for directional processes. In the lining of the small intestine, cells must absorb nutrients from the lumen and transfer them into the bloodstream. Cell polarity ensures that proteins for taking up nutrients are on the apical surface, while proteins that export those nutrients toward the blood supply are on the basolateral surface. This organization creates a one-way path for nutrients, preventing them from leaking back into the intestine.

Distinctive Structures of the Apical Surface

The apical surface is often equipped with structures that facilitate its functions, and the type varies depending on the cell’s role. One of the most common specializations is the presence of microvilli. These are microscopic, finger-like projections of the plasma membrane that increase the cell’s surface area. In cells of the small intestine, thousands of microvilli form a dense layer called a brush border, which enhances the capacity for nutrient absorption.

Other cells feature cilia, which are longer, hair-like structures. Motile cilia move in a coordinated, wave-like fashion to propel substances across the cell surface, such as clearing mucus from the respiratory tract. Non-motile, or primary, cilia act as cellular antennae, sensing chemical or mechanical signals in the environment. A protective layer called the glycocalyx, a coating of sugar and protein molecules, also covers the apical surface, aiding in protection and cell-to-cell recognition.

Core Functions of the Apical Domain

The structures on the apical domain enable its functions, which allow epithelial tissues to manage the exchange of substances between different body compartments. The main functions are:

• Absorption: The increased surface area provided by microvilli is populated with transporter proteins that facilitate the uptake of nutrients, ions, and water. For example, cells in the kidney tubules use their apical surfaces to reabsorb substances from the filtrate that will become urine.
• Secretion: The apical membrane serves as the exit point for products synthesized within the cell. Cells in the stomach lining secrete digestive enzymes and acid from their apical surface. Similarly, goblet cells in the respiratory and intestinal tracts release mucus to protect and lubricate the tissue surface.
• Transport: The coordinated beating of motile cilia in the airways is an example of active transport, moving a layer of mucus to clear debris.
• Sensory Reception: Primary cilia in the kidney tubules sense the flow of fluid, a signal that helps regulate cell function and tissue maintenance. This capability allows cells to respond to changes in their local environment.

Impact on Organ Systems and Health

The proper functioning of the apical domain is linked to the health of entire organ systems. In the digestive system, the absorptive capacity of intestinal cells is fundamental for nutrition. In the urinary system, the reabsorptive and secretory functions of apical membranes in kidney tubule cells are necessary for filtering waste and maintaining fluid balance.

When the structures or functions of the apical domain are compromised, it can lead to significant diseases. Cystic fibrosis is an example where a defective protein on the apical membrane of epithelial cells impairs ion transport. The CFTR protein, a channel for chloride ions, does not function correctly, leading to abnormally thick mucus in the lungs and digestive tract because water cannot properly hydrate the secretions.

In celiac disease, the immune response to gluten damages the microvilli on the apical surface of intestinal cells. This destruction of the absorptive surface leads to malabsorption, causing malnutrition and a range of gastrointestinal symptoms. These examples show how disruptions at a subcellular level can have profound consequences for overall health.