What are cilia?

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

Cilia

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 (moving) 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. 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.

SEM of kidney primary cilium
Source: UT Southwestern MC

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).

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.

Structure and Function of Cilia

Cilium structure
Source: Miriam Schmidts, Institute of Child Health

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).

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.

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.

Impact of Defective Cilia

Defective and dysfunctional functioning in motile and non-motile cilia result in a large number of symptoms and disorders which have significant impact on those affected.

Defective and dysfunctional functioning in motile and non-motile cilia affects multiple systems, causing blindness, deafness, chronic respiratory infections, kidney disease, heart disease, infertility, obesity and diabetes.  These symptoms have significant impact on those affected; some are devastating, most are life-threatening.

The diagram indicates the effects and a non-exhaustive summary of some of these symptoms is provided below. For information on known and identified ciliopathy syndromes and diseases, see the Ciliopathies section of this site.

Cognitive impairment Cochlea: Hearing loss Lung / airway abnormalities chest infections Thoracic skeleton Liver disease cysts & fibrosis Renal anomalies cysts & kidney failure Limb: Polydacty Retinal degeneration blindness Anosmia Craniofacial abnormalities Congenital heart defects Pancreatic cysts Truncal obesity diabetes Sterility Skeletal abnormalities dwarfism