The apical surface represents a specialized outer boundary found on certain types of cells, particularly those that form linings or coverings within the body, known as epithelial cells. This surface directly faces an external environment or the interior space of an organ, often referred to as a lumen. This distinct cellular face serves as the primary interface for various interactions with the surrounding environment.
The Concept of Cell Polarity
The existence of the apical surface is rooted in a fundamental biological principle called cell polarity, which means a cell possesses distinct structural and functional regions. Epithelial cells, for instance, are organized with a clear top-to-bottom orientation, ensuring specific tasks are performed at the correct cellular location. The apical surface, facing the external space, is structurally and biochemically different from the basolateral surface, which faces the underlying tissues and blood supply. These two distinct membrane domains are separated and maintained by specialized cellular structures known as tight junctions. These junctions form a continuous band around the cell, acting like a fence that prevents molecules and proteins from freely moving between the apical and basolateral domains, thus preserving the cell’s organized structure and specialized functions.
Key Functions of the Apical Surface
The apical surface is highly specialized to perform particular roles, primarily involving the controlled movement of substances. One main function is absorption, which involves taking in materials from the external environment into the cell. For example, in the digestive system, cells lining the small intestine absorb nutrients from digested food across their apical membranes. Another important function is secretion, where cells release substances from their interior onto the apical surface, including enzymes for digestion or mucus to lubricate and protect a surface. The apical surface also provides a protective barrier, shielding underlying tissues from potentially harmful external factors or pathogens.
Structural Specializations
The apical surface’s unique tasks are facilitated by specialized modifications that increase its efficiency.
Microvilli
Microvilli are one such modification, appearing as numerous, minute, finger-like protrusions extending from the cell membrane. These structures dramatically increase the surface area available for absorption or secretion, sometimes by as much as 25 to 40 times. When viewed under a microscope, a dense collection of microvilli can resemble the bristles of a brush, a characteristic appearance often termed a “brush border”.
Cilia
Cilia are another type of hair-like projection found on the apical surface, typically longer and thicker than microvilli, measuring about 1 to 10 micrometers in length. Unlike microvilli, many cilia are motile, meaning they can move in a coordinated, wave-like fashion. This rhythmic beating motion helps to propel fluids or particles across the cell surface, such as moving mucus and trapped debris along the lining of the respiratory tract.
Stereocilia
Stereocilia, a less common variation, are non-motile, elongated microvilli found in specific locations like the inner ear, where they play a role in sensory perception by converting mechanical stimuli into electrical signals.
Examples in the Human Body
The specialized nature of the apical surface is evident across various organ systems in the human body. In the small intestine, the epithelial cells lining its inner surface possess a dense array of microvilli on their apical side. This extensive “brush border” significantly expands the surface area, facilitating the efficient absorption of digested nutrients, such as carbohydrates and amino acids, into the body.
The respiratory tract, particularly the trachea, provides another example where the apical surface plays a distinct role. Here, cells are covered with numerous motile cilia. These cilia work in conjunction with mucus-producing goblet cells, rhythmically beating to sweep mucus and trapped foreign particles upwards and out of the airways, maintaining clear passageways for breathing. Within the kidney tubules, the apical surface of cells is involved in the precise reabsorption of water, ions, and other solutes back into the bloodstream, a function supported by specialized transport proteins embedded within its membrane.