Cilia are microscopic, hair-like projections extending from the surface of many eukaryotic cells. They line the respiratory tract, helping clear mucus and trapped particles, and are present in the fallopian tubes, assisting in egg cell movement. Cilia are classified into two main types: motile, which are capable of movement, and non-motile, which serve sensory roles.
The Axoneme Microtubule Skeleton
At the core of every cilium lies an internal scaffold called the axoneme. This structure is primarily composed of microtubules, hollow, cylindrical polymers made of tubulin. In motile cilia, the axoneme exhibits a highly conserved “9+2 arrangement,” with nine outer doublets surrounding two central single microtubules. This intricate arrangement forms the framework for ciliary function.
Each outer doublet microtubule consists of two fused microtubules, designated A and B tubules, while the central pair microtubules are single, complete microtubules. This precise architectural pattern provides the structural integrity necessary for the cilium’s form and movement. The microtubules within the axoneme are remarkably stable and resistant to depolymerization, unlike many other cellular microtubules. Their stability is a result of unique biochemical and biophysical properties.
Electron microscopy studies have been instrumental in revealing this detailed “9+2” architecture. The axoneme acts as the fundamental skeleton, providing support and binding sites for various accessory proteins. Without this array, the cilium would lack its characteristic shape and specialized roles. The consistent arrangement across different species highlights its functional importance.
Accessory Proteins and Ciliary Movement
Attached to the axoneme’s microtubule skeleton are accessory proteins that enable the movement of motile cilia. Dynein arms are motor proteins that generate force for ciliary beating. These arms are anchored to the A-tubule of one outer doublet microtubule and “walk” along the B-tubule of an adjacent doublet, causing the microtubules to slide past each other in an ATP-dependent manner. This sliding motion is then converted into the bending that characterizes ciliary movement.
Nexin links provide elastic connections between adjacent outer doublet microtubules. These links limit microtubule sliding, transforming the sliding force generated by dynein into a bending motion of the entire cilium. Without these links, the microtubules would simply slide apart without producing a controlled bend. Radial spokes are T-shaped protein complexes that project inward from the outer doublet microtubules towards the central pair.
Radial spokes interact with the central pair microtubules and dynein arms, coordinating dynein activity and influencing the cilium’s bending pattern, or “waveform.” They convert dynein action into a propulsive waveform. The interplay among dynein arms, nexin links, and radial spokes ensures rhythmic and synchronized beating for effective ciliary function, such as fluid movement or cell locomotion.
Structural Variations for Different Functions
While motile cilia adhere to the “9+2” axoneme structure, a variation exists in non-motile, or primary, cilia. These cilia feature a “9+0 arrangement,” possessing nine outer doublet microtubules but lacking the central pair. This difference means they lack the dynein arms and radial spokes associated with movement.
The absence of these motor proteins and coordinating structures prevents primary cilia from generating a beating motion. Instead, primary cilia function as sensory organelles, acting like cellular antennae to detect signals from the extracellular environment. They are found on nearly all mammalian cell types, usually as a single projection per cell.
For example, primary cilia in kidney tubules sense fluid flow, and their bending can trigger calcium influx into the cell, signaling changes in urine flow. Similarly, olfactory neurons possess many non-motile cilia that host G-protein coupled receptors, allowing them to detect odorants and facilitate the sense of smell. This adaptation to a “9+0” arrangement highlights how modifications to the ciliary design enable specialized sensory roles.
The Basal Body Anchor
Every cilium is anchored to the cell’s interior by a structure known as the basal body. This foundational component is a modified centriole, an organelle involved in cell division. The basal body consists of nine triplet microtubules arranged in a pinwheel pattern, forming a cylindrical shape. These triplets are composed of three fused microtubules, distinct from the doublets found in the axoneme.
The basal body serves as the nucleation site for the growth of the axoneme’s outer doublet microtubules. Two of the three microtubules from each triplet in the basal body extend upward to become the A and B tubules of the axoneme’s outer doublets. This process occurs at a specialized region called the transition zone, which connects the basal body to the axoneme.
Beyond its role in anchoring the cilium to the cell membrane, the basal body also organizes the assembly of the axoneme. It acts as a template, guiding the precise arrangement of microtubules as the cilium elongates. The basal body’s position and orientation within the cell are also important, as they can influence the direction of ciliary beating in motile cilia.