Cilium
The cilium is a short hair-like membrane protrusion from many types of eukaryotic cell. The cilium has the shape of a slender threadlike projection that extends from the surface of the much larger cell body. Eukaryotic flagella found on sperm cells and many protozoans have a similar structure to motile cilia that enables swimming through liquids, but they are longer than cilia and have a different undulating motion.
There are two major classes of cilia: motile and non-motile cilia, each with two subtypes, giving four types in all. A cell will typically have one primary cilium or many motile cilia. The structure of the cilium core, called the axoneme, determines the cilium class. Most motile cilia have a central pair of single microtubules surrounded by nine pairs of double microtubules called a 9+2 axoneme. Most non-motile cilia have a 9+0 axoneme that lacks the central pair of microtubules. Also lacking are the associated components that enable motility including the outer and inner dynein arms, and radial spokes. Some motile cilia lack the central pair, and some non-motile cilia have the central pair, hence the four types.
Most non-motile cilia, termed primary cilia or sensory cilia, serve solely as sensory organelles. Most vertebrate cell types possess a single non-motile primary cilium, which functions as a cellular antenna. Olfactory neurons possess a great many non-motile cilia. Non-motile cilia that have a central pair of microtubules are the kinocilia present on hair cells.
Motile cilia are found in large numbers on respiratory epithelial cells – around 200 cilia per cell, where they function in mucociliary clearance, and also have mechanosensory and chemosensory functions. Motile cilia on ependymal cells move the cerebrospinal fluid through the ventricular system of the brain. Motile cilia are also present in the oviducts of female mammals, where they function in moving egg cells from the ovary to the uterus. Motile cilia that lack the central pair of microtubules are found in the cells of the embryonic primitive node; termed nodal cells, these nodal cilia are responsible for the left-right asymmetry of bilaterians.
Structure
Cilia can be between one and five micrometers in length. A cilium is assembled and built from a basal body on the cell surface. From the basal body, the ciliary rootlet forms ahead of the transition plate and transition zone where the earlier microtubule triplets change to the microtubule doublets of the axoneme.Basal body
The foundation of the cilium is the basal body, a modified mother centriole on the cell surface. Mammalian basal bodies consist of a barrel of nine triplet microtubules, subdistal appendages and nine strut-like structures, known as distal appendages, which attach the basal body to the membrane at the base of the cilium. Two of each of the basal body's triplet microtubules extend during growth of the axoneme to become the doublet microtubules.Ciliary rootlet
The ciliary rootlet is a cytoskeleton-like structure that originates from the basal body at the proximal end of a cilium. Rootlets are typically 80-100 nm in diameter and contain cross striae distributed at regular intervals of approximately 55-70 nm. A prominent component of the rootlet is rootletin a coiled coil rootlet protein coded for by the CROCC gene.Transition zone
To achieve its distinct composition, the proximal-most region of the cilium consists of a transition zone, also known as the ciliary gate, that controls the entry and exit of proteins to and from the cilium. At the transition zone, Y-shaped structures connect the ciliary membrane to the underlying axoneme. Control of selective entry into cilia may involve a sieve-like function of transition zone. Inherited defects in components of the transition zone cause ciliopathies, such as Joubert syndrome. Transition zone structure and function is conserved across diverse organisms, including vertebrates, Caenorhabditis elegans, Drosophila melanogaster and Chlamydomonas reinhardtii. In mammals, disruption of the transition zone reduces the ciliary abundance of membrane-associated ciliary proteins, such as those involved in Hedgehog signal transduction, compromising Hedgehog-dependent embryonic development of digit number and central nervous system patterning.Axoneme
Inside a cilium is a microtubule-based cytoskeletal core called the axoneme. The axoneme of a primary cilium typically has a ring of nine outer microtubule doublets, and the axoneme of a motile cilium has, in addition to the nine outer doublets, two central microtubule singlets. This is the same axoneme type of the flagellum. The axoneme in a motile cilium acts as a scaffold for the inner and outer dynein arms that move the cilium, and provides tracks for the microtubule motor proteins of kinesin and dynein. The transport of ciliary components is carried out by intraflagellar transport which is similar to the axonal transport in a nerve fibre. Transport is bidirectional and cytoskeletal motor proteins kinesin and dynein transport ciliary components along the microtubule tracks; kinesin in an anterograde movement towards the ciliary tip and dynein in a retrograde movement towards the cell body. The cilium has its own ciliary membrane enclosed within the surrounding cell membrane.Types
Non-motile cilia
In animals, non-motile primary cilia are found on nearly every type of cell, blood cells being a prominent exception. Most cells only possess one, in contrast to cells with motile cilia, an exception being olfactory sensory neurons, where the odorant receptors are located, which each possess about ten cilia. Some cell types, such as retinal photoreceptor cells, possess highly specialized primary cilia.Although the primary cilium was discovered in 1898, it was largely ignored for a century and considered a vestigial organelle without important function. Recent findings regarding its physiological roles in chemosensation, signal transduction, and cell growth control, have revealed its importance in cell function. Its importance to human biology has been underscored by the discovery of its role in a diverse group of diseases caused by the dysgenesis or dysfunction of cilia, such as polycystic kidney disease, congenital heart disease, mitral valve prolapse, and retinal degeneration, called ciliopathies. The primary cilium is now known to play an important role in the function of many human organs. Primary cilia on pancreatic beta cells regulate their function and energy metabolism. Cilia deletion can lead to islet dysfunction and type 2 diabetes.
Cilia are assembled during the G1 phase and are disassembled before mitosis occurs. Disassembly of cilia requires the action of aurora kinase A.
The current scientific understanding of primary cilia views them as "sensory cellular antennae that coordinate many cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."
The cilium is composed of subdomains and enclosed by a plasma membrane continuous with the plasma membrane of the cell. For many cilia, the basal body, where the cilium originates, is located within a membrane invagination called the ciliary pocket. The cilium membrane and the basal body microtubules are connected by distal appendages. Vesicles carrying molecules for the cilia dock at the distal appendages. Distal to the transition fibers form a transition zone where entry and exit of molecules is regulated to and from the cilia. Some of the signaling with these cilia occur through ligand binding such as Hedgehog signaling. Other forms of signaling include G protein-coupled receptors including the somatostatin receptor 3 in neurons.
Modified non-motile cilia
In the inner ear, kinocilia that are termed as specialized primary cilia, or modified non-motile cilia are found on the hair cells. They possess the 9+2 axoneme of the motile cilia, but lack the inner dynein arms that give movement. They do move passively following the detection of sound, allowed by the outer dynein arms.Motile cilia
s also have motile cilia or secondary cilia that are usually present on a cell's surface in large numbers, and beat in coordinated metachronal waves. Multiciliated cells are found lining the respiratory tract where they function in mucociliary clearance sweeping mucus containing debris away from the lungs. Each cell in the respiratory epithelium has around 200 motile cilia.In the reproductive tract, smooth muscle contractions help the beating of the cilia in moving the egg cell from the ovary to the uterus.
In the ventricles of the brain ciliated ependymal cells circulate the cerebrospinal fluid.
The functioning of motile cilia is strongly dependent on the maintenance of optimal levels of periciliary fluid bathing the cilia. Epithelial sodium channels are specifically expressed along the entire length of cilia in the respiratory tract, and fallopian tube or oviduct that apparently serve as sensors to regulate the periciliary fluid.
Modified motile cilia
Motile cilia without the central pair of singlets are found in early embryonic development. They are present as nodal cilia on the nodal cells of the primitive node. Nodal cells are responsible for the left-right asymmetry in bilateral animals. While lacking the central apparatus there are dynein arms present that allow the nodal cilia to move in a spinning fashion. The movement creates a current flow of the extraembryonic fluid across the nodal surface in a leftward direction that initiates the left-right asymmetry in the developing embryo.Motile, multiple, 9+0 cilia are found on the epithelial cells of the choroid plexus. Cilia also can change structure when introduced to hot temperatures and become sharp. They are present in large numbers on each cell and move relatively slowly, making them intermediate between motile and primary cilia. In addition to 9+0 cilia that are mobile, there are also solitary 9+2 cilia that stay immobile found in hair cells.