Cilium

Cilium
SEM micrograph of the cilia projecting from respiratory epithelium in the lungs
Details
Identifiers
Latin	Cilium
Code	TH H1.00.01.1.01014
TH	H1.00.01.1.01014
Anatomical terminology []

A cilium (Latin for eyelash; the plural is cilia) is an organelle found in eukaryotic cells. Cilia are thick protuberances that project from the much larger cell body.

There are two types of cilia: motile cilia and nonmotile, or primary cilia, which typically serve as sensory organelles. In eukaryotes, motile cilia and flagella together make up a group of organelles known as undulipodia. Eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length. Biologists have various ideas about how the various flagella may have evolved.

Cilia can be divided into primary and motile forms.

In humans, primary cilia are found on nearly every cell in the body.

In comparison to motile cilia, non-motile (or primary) cilia usually occur one per cell; nearly all mammalian cells have a single non-motile primary cilium. In addition, examples of specialized primary cilia can be found in human sensory organs such as the eye and the nose:

Although the primary cilium was discovered in 1898, it was largely ignored for a century. Only recently has great progress been made in understanding the function of the primary cilium. Until the 1990s, the prevailing view of the primary cilium was that it was merely a vestigial organelle without important function. Recent findings regarding its physiological roles in chemical sensation, signal transduction, and control of cell growth, have led scientists to acknowledge its importance in cell function, with the discovery of its role in diseases not previously recognized to involve the dysgenesis and dysfunction of cilia, such as polycystic kidney disease,congenital heart disease, and an emerging group of genetic ciliopathies. The primary cilium is now known to play an important role in the function of many human organs. The current scientific understanding of primary cilia views them as "sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation.".