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Structure and Function of Cilia

What are cilia?

Slender microscopic cilia
Source: Andy Forge, UCL Ear Institute

Cilia are slender, microscopic, hair-like structures or organelles that extend from the surface of nearly all mammalian cells (multiple or single). They are primordial.

The ciliary apparatus is connected to cell cycle progression and proliferation, and cilia play a vital part in human and animal development and in everyday life.

The length of a single cilium is 1-10 micrometres and width is less than 1 micrometre.

Cilia are broadly divided into two types. They function separately and sometimes together:

Motile Cilia
Source: Amelia Shoemark

'Motile' (or moving) cilia are found in the lungs, respiratory tract and middle ear. These cilia have a rhythmic waving or beating motion (see right). They work, for instance, to keep the airways clear of mucus and dirt, allowing us to breathe easily and without irritation. They also help propel sperm.

'Non-motile or 'primary' cilia were long thought to be evolutionary vestigial organs (since being first described in 1898). They are now recognised as playing crucial roles in a number of organs. Some act as a sensory antenna for the cell, receiving signals from other cells or fluids nearby.

  • In the kidney, for example, cilia bend with urine flow and send a signal to alert the cells that there is a flow of urine.
  • In the eye, non-motile cilia are found inside the light-sensitive cells (photoreceptors) of the retina. These cilia act like microscopic train-tracks, and allow the transport of vital molecules from one end of the photoreceptor to the other.

Structurally, each cilium comprises a microtubular backbone - the ciliary axoneme - surrounded by plasma membrane (see figure below).

Motile cilia are characterized by a typical ’9+2’ architecture with nine outer microtubule doublets and a central pair of microtubules (e.g bronchi).

Primary cilia appear typically as single appendages microtubules on the apical surface of cells and lack the central pair of microtubules (e.g. in kidney tubules).

cilium

Source: Miriam Schmidts, Institute of Child Health

Ciliary proteins are synthesized in the cell body and must be transported to the tip of the axoneme. This is achieved by Intraflagellar Transport (IFT), an ordered and highly regulated anterograde and retrograde translocation of polypeptide complexes (IFT particles) along the length of the ciliary axoneme.

See figure below.

Intraflagellar Transport within a Cilium
Source: L.B. Pedersen and J.L. Rosenbaum. Current Topics in Developmental Biology.85:23-61, 2008

 

Dysfunction or defects in motile and primary cilia are now understood to underlie a number of devastating genetic conditions - termed ciliopathies - which carry a heavy economic and health burden on individuals, families and society.

Much is still unknown about the structure and function of motile and primary cilia, but we believe that more research into these critically important cellular organelles will eventually bring about better ways to treat and help people whose lives are impacted by defective cilia.