Cilia and Flagella: Structure, Movement, and Function

Posted on Mar 14, 2025 in Biology

Cilia and flagella are mobile extensions found in various cell types. A cilium is typically 5-10 μm in length and found in tubular epithelia, measuring about 2 μm in thickness. Their pedaling or undulating movements help to drag particles, agitate, and circulate fluids on the cell surface.

Ciliary Structure

Structurally, a cilium is integrated with the ciliary membrane and the axoneme, which is embedded in the ciliary matrix. Key components include the stem, transition zone, and ciliary basal body. The microtubular structure consists of a central pair of individual microtubules and nine peripheral pairs, along with associated proteins.

Microtubule Arrangement

  • Central Microtubules: Two individual microtubules located in the center of the axoneme.
  • Peripheral Microtubules: Nine pairs of microtubules arranged around the central pair.

Key Proteins

  • Nexin: An elastic protein forming interdoublet bridges that connect neighboring microtubule pairs, spanning from microtubule A to microtubule B with a periodicity of 86 μm.
  • Central Sheath: A dense, oval-shaped structure surrounding the two central microtubules.
  • Radial Fibers: Fibers extending from the A tubules of the outer doublets to the central sheath.
  • Ciliary Dynein: A motor protein within the ciliary axoneme, belonging to a large family of motor proteins. Dynein arms facilitate the sliding of adjacent microtubule doublets over each other.

Ciliary Movement

Purpose of Movement

  • In free cells (e.g., ciliated protozoa), movement enables locomotion within a liquid medium.
  • In ciliated epithelia (e.g., respiratory epithelium), movement facilitates the transport of mucus and particles across the cell surface.

Speed and Frequency

  • A single cilium can perform between 600 and 1300 movements per minute.
  • A complete movement cycle lasts approximately 1/25th of a second.
  • The angular velocity is about 12° per millisecond.
  • A 12 μm cilium moves at a speed of approximately 2.5 nm/sec.

Direction

The direction of ciliary movement is determined by the cilium’s structure and is perpendicular to the plane through the two central microtubules.

Types of Movement

  • Pendulum Movements: Less common, primarily seen in protozoa. The stem remains rigid except at the base, allowing the cilium to bend and return.
  • Hook (Uniform) Movements: These involve an active phase and a return phase, moving in one direction within a plane perpendicular to the cell membrane. During the active phase, the stem extends and curves slightly, describing an angle of 180 degrees around a fixed point. As the distal end continues to move, the proximal segment begins its return, creating a curvature that allows the movement to gradually reach the distal end, straightening the stem back to its starting position.

Coordination in Epithelia

In epithelia with a brush border, effective movement requires precise coordination:

  • Isochronous Rhythm: Cilia beat together in unison.
  • Metachronous Rhythm: Each cilium moves slightly ahead of the preceding one, forming waves across the epithelial surface.

Ciliary movement is always directed in the same direction, ensuring coordinated function across the epithelium. Lesions can be grouped into several categories:

  1. Compound cilia with multiple axonemes.
  2. Swollen cilia.
  3. Fine cilia with structural alterations.
  4. Cilia with intracytoplasmic microtubules.
  5. Cilia with atypical basal bodies.