Epithelial tissues form linings throughout the body, serving as a barrier between different environments. A specialized type of this tissue is the ciliated epithelium, distinguished by hair-like projections called cilia on its free surface. These cilia are not static; they are motile structures capable of coordinated movement to propel substances across the tissue layer. This tissue acts as a biological conveyor belt, transporting fluids and particles through various internal passageways.
Cellular Structure and Composition
The cells that make up ciliated epithelium are arranged in a single layer and are columnar or cuboidal in shape. Projecting from the apical, or top, surface of these cells are the cilia, which are microscopic, membrane-bound extensions. Each cell can have up to 300 cilia. The core of each cilium contains a complex arrangement of microtubules, protein filaments that provide structural support and are responsible for generating movement.
Interspersed among the ciliated cells are specialized goblet cells. These cells are modified columnar epithelial cells that synthesize and secrete mucus. The mucus they produce forms a protective, sticky layer that sits atop the ciliated epithelium. This close association is important in the respiratory tract, where the mucus traps inhaled debris and pathogens.
Primary Functions and Locations
The primary role of ciliated epithelium is to move substances over its surface, a function directly tied to its specific locations. In the respiratory system, it lines the majority of the airways, including the trachea and bronchial tubes. Here, it performs a process known as mucociliary clearance. Inhaled particles like dust, pollen, and bacteria are trapped in the mucus layer, and the cilia sweep this mucus upward toward the throat to be swallowed or expelled.
In the female reproductive system, ciliated epithelium is found lining the uterine tubes, also known as fallopian tubes. The cilia in this location generate a gentle current that helps transport the ovum, or egg cell, from the ovary toward the uterus after ovulation. A less commonly known location is the central nervous system, where ependymal cells line the ventricles of the brain and the central canal of the spinal cord, assisting in the circulation of cerebrospinal fluid.
The Coordinated Movement of Cilia
The movement of cilia is a synchronized and energy-dependent process. The cilia on a sheet of epithelium beat in a coordinated, wave-like pattern, a phenomenon known as metachronal rhythm. This rhythmic beating creates a continuous and directional current across the surface of the tissue. The effect is similar to the coordinated strokes of oars on a rowing boat, allowing for efficient propulsion of fluid or mucus in a single, consistent direction.
This motion is powered by specialized motor proteins within the cilia that use cellular energy to cause the internal microtubules to slide past one another. This sliding action results in the cilium bending in a rapid, whip-like stroke, followed by a slower recovery stroke. The coordination between adjacent cilia ensures that the force generated is cumulative and effective for moving overlying substances.
Conditions Related to Ciliary Dysfunction
When the structure or function of cilia is compromised, health problems can arise. These issues can be either genetic or acquired. Primary Ciliary Dyskinesia (PCD) is an inherited genetic disorder where cilia are either immotile or their beating pattern is disorganized and ineffective. In the respiratory tract, the failure of mucociliary clearance leads to chronic and recurrent infections, such as sinusitis, bronchitis, and pneumonia. In the reproductive system, PCD can lead to infertility in both males and females.
Ciliary function can also be damaged by external factors, such as exposure to cigarette smoke. The toxic chemicals in smoke can paralyze and, over time, destroy the cilia lining the respiratory tract. This damage disables the mucociliary escalator, causing mucus and trapped particles to accumulate in the lungs. The body’s attempt to clear this buildup results in a persistent, deep cough known as “smoker’s cough.”