Cilia: Structure, Function, and Movement Mechanisms

Posted on Apr 2, 2025 in Biology

Cilia: Structure, Function, and Movement

Cilia are mobile cytoplasmic extensions, typically 5 to 10 μm in length and 0.2 μm thick. They contain an axoneme and project from the free surface of many cells. Cilia function to move crawling or rolling particles, agitate and circulate fluids across surface epithelia, such as in the respiratory tract and certain excretory tubes of the testis.

Cilia originate from basal corpuscles. Structurally, a cilium is composed of the axoneme and ciliary membrane, immersed in the ciliary matrix. Key components include:

  • Stem: Average diameter of 0.2 μm and a length of 5-10 μm. It is bounded by the ciliary membrane and contains the axoneme and ciliary matrix.
  • Transition Zone: Located between the stem and the basal corpuscle. The basal cell membrane in this zone is characterized by a clustering of membrane proteins, forming the ciliary necklace.
  • Basal Corpuscle: A cylindrical structure, approximately 500 nm in length, structurally corresponding to a centriole.
  • Ciliary Roots: Composed of macrofilaments arranged parallel to each other, exhibiting alternating dense and clear areas, giving them a striated appearance. They are part of the basal corpuscle and coordinate ciliary movement.
  • Axoneme (Axial Filament Complex): The core structure formed by microtubules. It consists of 9 peripheral microtubule pairs and a set of associated proteins, forming the axis of the cilium and responsible for its movements.

Axoneme Components

  • Central Microtubules: Two individual microtubules located in the central region of the axoneme.
  • Peripheral Microtubules: Nine pairs of peripheral microtubules arranged around the central pair. The axis through the center forms a 10° angle to the tangent of the ciliary surface. The doublets are composed of two tubules designated A and B.
  • Nexin: An elastic protein that forms interdoublet bridges, connecting neighboring microtubule pairs, extending from microtubule A to the B neighbor.
  • Central Sheath: A dense, oval-shaped structure surrounding the two central microtubules.
  • Radial Fibers: Fibers extending radially from the A tubules of the outer doublets to the central sheath.
  • Ciliary Dynein: A protein of the ciliary axoneme, forming side arms that enable adjacent microtubule doublets to slide over each other.

Ciliary Movement

Purpose of Movement

In ciliated epithelia, such as the respiratory epithelium, the purpose is to move mucus and particles across the cell surface.

Speed and Frequency

Cilia beat rapidly, between 600 and 1300 strokes per minute, with each complete motion taking approximately 1/25 of a second.

Direction

The direction of movement is specific to the type of cilium.

Characteristics of Motion

Types of movements include:

  • Pendulous Relocations: Rare, except in protozoa.
  • Hook Movements: Characterized by an active phase and a return phase, allowing fluids or particles to be swept in one direction. Effective motion requires perfect coordination. These can be:
    • Isochronous Rhythm: All cilia beat at once.
    • Metachronous Rhythm: Each cilium is slightly ahead of the preceding one, forming waves that move across the epithelial surface.

Mechanism of Movement

Movement is due to the sliding of microtubule pairs over each other, as demonstrated by electron microscopy studies. The doublets maintain a constant length. During bending, doublets on the inner side of the bend are closer together, while those on the outer side are more distant. Dynein arms are involved in this movement, with the sliding occurring through successive on-off attachments of the dynein arms from microtubule A to B. These connections require energy, released through ATP hydrolysis. Dynein arm activity requires the presence of cations such as Co2+ and Mg2+.

The central pair of microtubules is also involved in ciliary movement. Their absence is observed in Kartagener syndrome, described in 1933, which is associated with infertility, chronic sinusitis, bronchiectasis, and situs inversus. This syndrome is often due to the total or partial absence of dynein arms.