Cilia and flagella are hair-like appendages that project from the surface of nearly all eukaryotic cells, from single-celled organisms to complex human tissues. These microscopic structures are important for the survival and function of diverse life forms. While they differ in length and movement patterns, both cilia and flagella serve as cellular machines responsible for locomotion and environmental sensing.
The Core Architecture
The internal skeleton of eukaryotic cilia and flagella is known as the axoneme. This structure is composed of microtubules, which are hollow cylindrical polymers of the protein tubulin. Motile cilia or flagella possess a characteristic “9+2” arrangement: nine pairs of fused microtubules arranged in a ring around two central, single microtubules.
Each of the nine peripheral doublets consists of one complete microtubule and one partial microtubule. This microtubule framework is encased by an extension of the cell’s plasma membrane. The core provides the mechanical strength and the structural tracks necessary for generating movement.
Anchoring the appendage to the cell is the basal body, which is structurally similar to a centriole. The basal body organizes the microtubule assembly and exhibits a “9+0” structure, consisting of nine peripheral microtubule triplets with no central microtubules. This region acts as the organizing center from which the axoneme is built.
The energy for movement is supplied by the motor protein dynein. Dynein arms are attached to the outer microtubule doublets, extending toward the adjacent doublet. By hydrolyzing adenosine triphosphate (ATP), dynein converts chemical energy into mechanical force, which drives all ciliary and flagellar motion.
Generating Motion
Movement in motile cilia and flagella is achieved through the sliding microtubule model. This model explains that the microtubules within the axoneme do not shorten or contract to create a bend. Instead, the dynein motor proteins “walk” along the surface of the neighboring microtubule doublet.
The sliding force generated by the dynein arms is converted into a bending motion. Structural components, such as nexin links and radial spokes, cross-link the doublets and anchor the structure to the basal body. These connections prevent the microtubules from sliding completely apart, resulting in a controlled, rhythmic bending that propels the cell or moves fluid over the cell surface.
The movement patterns of cilia and flagella differ based on their function. Flagella are typically long and few in number, generating a continuous, undulating wave that moves along the appendage from base to tip. This whip-like motion propels a cell through a liquid medium, such as the forward motion of sperm.
Cilia are shorter and more numerous, covering large cellular surfaces. Their movement involves a distinct, repetitive pattern: a stiff, forceful power stroke followed by a flexible recovery stroke, similar to an oar rowing a boat. Cilia often beat in a synchronized, wave-like motion known as a metachronal wave, which allows them to efficiently move substances in a specific direction.
Essential Functions in Biology
Cilia and flagella function in both the transport of materials and the sensing of the environment. In humans, motile cilia are responsible for transport systems, such as those in the respiratory tract. Millions of cilia line the respiratory tract, forming the mucociliary escalator that sweeps mucus and trapped foreign particles out of the lungs.
Motile cilia lining the fallopian tubes generate fluid currents that transport the egg toward the uterus. The most recognized flagellum role is the propulsion of sperm cells, where the appendage drives the cell through the reproductive tract for fertilization. Defects in these motile structures can lead to conditions like chronic respiratory disease and infertility.
Many cells possess a single, non-motile structure known as a primary cilium. Unlike motile cilia, primary cilia lack the central pair of microtubules, giving them a “9+0” arrangement that prevents movement. These organelles function as cellular antennae, detecting chemical and mechanical signals from the surrounding environment.
Primary cilia are important in various sensory pathways and developmental signaling. In the kidney, they sense the flow of fluid through the tubules, helping to regulate calcium signaling and organ function. The photoreceptors in the eye are modified primary cilia that detect light.
Structural or functional defects in cilia are collectively termed ciliopathies, resulting in a wide spectrum of human diseases. Conditions such as polycystic kidney disease, retinal degeneration, and developmental disorders related to left-right body symmetry are linked to ciliary dysfunction.