Cilium
Overview
A cilium is an organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

 found in eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

 cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s. Cilia are slender protuberances that project from the much larger cell body
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

.

There are two types of cilia: motile cilia and non-motile, or primary cilia, which typically serve as sensory organelles. In eukaryotes, 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.
Cilia are rare in most plants, occurring most notably in cycad
Cycad
Cycads are seed plants typically characterized by a stout and woody trunk with a crown of large, hard and stiff, evergreen leaves. They usually have pinnate leaves. The individual plants are either all male or all female . Cycads vary in size from having a trunk that is only a few centimeters...

s.

Cilia can be divided into primary and motile forms.
Larger eukaryotes, such as mammals, have motile cilia as well.
Encyclopedia
A cilium is an organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

 found in eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

 cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s. Cilia are slender protuberances that project from the much larger cell body
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

.

There are two types of cilia: motile cilia and non-motile, or primary cilia, which typically serve as sensory organelles. In eukaryotes, 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.

Types and distribution

Cilia are rare in most plants, occurring most notably in cycad
Cycad
Cycads are seed plants typically characterized by a stout and woody trunk with a crown of large, hard and stiff, evergreen leaves. They usually have pinnate leaves. The individual plants are either all male or all female . Cycads vary in size from having a trunk that is only a few centimeters...

s.

Cilia can be divided into primary and motile forms.

Motile cilia

Larger eukaryotes, such as mammals, have motile cilia as well. Motile cilia are usually present on a cell's surface in large numbers and beat in coordinated waves.
  • In human
    Human
    Humans are the only living species in the Homo genus...

    s, for example, motile cilia are found in the lining of the trachea
    Vertebrate trachea
    In tetrapod anatomy the trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus...

     (windpipe), where they sweep mucus and dirt out of the lungs.
  • In female
    Female
    Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...

     mammals, the beating of cilia in the Fallopian tube
    Fallopian tube
    The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges are two very fine tubes lined with ciliated epithelia, leading from the ovaries of female mammals into the uterus, via the utero-tubal junction...

    s moves the ovum
    Ovum
    An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

     from the ovary
    Ovary
    The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

     to the uterus
    Uterus
    The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

    .


Ciliate
Ciliate
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

s are microscopic organisms that possess motile cilia exclusively and use them for either locomotion or to simply move liquid over their surface.

Primary/immotile cilium

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:
  • The outer segment of the rod photoreceptor cell in the human eye is connected to its cell body with a specialized non-motile cilium.
  • The dendritic knob
    Dendrite
    Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...

     of the olfactory neuron, where the odorant receptors are located, also contains non-motile cilia (about 10 cilia per dendritic knob).


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
Dysgenesis
Dysgenesis may refer to:* Dysgenesis , which indicates abnormal organ development during embryonic growth and development* Anterior segment dysgenesis, a failure of the normal development of the tissues of the anterior segment of the eye...

and dysfunction of cilia, such as polycystic kidney disease
Polycystic kidney disease
Autosomal dominant polycystic kidney disease is an inherited systemic disorder that predominantly affects the kidneys, but may affect other organs including the liver, pancreas, brain, and arterial blood vessels...

 congenital heart disease, and an emerging
Emergence
In philosophy, systems theory, science, and art, emergence is the way complex systems and patterns arise out of a multiplicity of relatively simple interactions. Emergence is central to the theories of integrative levels and of complex systems....

 group of genetic
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....

 ciliopathies
Ciliopathy
A ciliopathy is a genetic disorder of the cellular cilia or the cilia anchoring structures, the basal bodies, or of ciliary function.Although ciliopathies are usually considered to involve proteins that localize to the primary cilia or centrosomes, it is possible for ciliopathies to be associated...

. 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
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."

Structure, assembly, maintenance, and function

"Inside cilia and flagella is a microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

-based cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...

 called the axoneme
Axoneme
Numerous eukaryotic cells carry whip-like appendages whose inner core consists of a cytoskeletal structure called the axoneme....

. The axoneme of primary cilia typically has a ring of nine outer microtubule doublets (called a 9+0 axoneme), and the axoneme of a motile cilium has two central microtubule singlets in addition to the nine outer doublets (called a 9+2 axoneme). The axonemal cytoskeleton acts as a scaffolding for various protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 complexes and provides binding sites for molecular motor
Molecular machine
A molecular machine, or nanomachine, is any discrete number of molecular components that produce quasi-mechanical movements in response to specific stimuli . The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level...

 proteins such as kinesin II
Kinesin
A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments, and are powered by the hydrolysis of ATP . The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular...

, that help carry proteins up and down the microtubules."

The building blocks of the cilia such as tubulins and other partially assembled axonemal proteins are added to the ciliary tips which point away from the cell body. In most species bi-directional motility called intraflagellar transport
Intraflagellar transport
Intraflagellar transport or IFT is a bidirectional motility along axonemal microtubules that is essential for the formation and maintenance of most eukaryotic cilia and flagella. It is thought to be required to build all cilia that assemble within a membrane projection from the cell surface...

 (IFT) plays an essential role to move these building materials from the cell body to the assembly site. IFT also carries the disassembled material to be recycled from the ciliary tip back to the cell body. By regulating the equilibrium between these two IFT processes, the length of cilia can be maintained dynamically. The disassembly of the cilia requires the action of the protein kinase Aurora A .

Exceptions where IFT is not present include Plasmodium falciparum
Plasmodium falciparum
Plasmodium falciparum is a protozoan parasite, one of the species of Plasmodium that cause malaria in humans. It is transmitted by the female Anopheles mosquito. Malaria caused by this species is the most dangerous form of malaria, with the highest rates of complications and mortality...

which is one of the species of Plasmodium
Plasmodium
Plasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was described in 1885 by Ettore Marchiafava and Angelo Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known...

that cause malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

 in humans. In this parasite, cilia assemble in the cytoplasm.

At the base of the cilium where the cilia attaches to the cell body is the microtubule organizing center, the basal body
Basal body
A basal body is an organelle formed from a centriole, and a short cylindrical array of microtubules. It is found at the base of a eukaryotic undulipodium and serves as a nucleation site for the growth of the axoneme microtubules...

. Some basal body proteins as CEP164
CEP164
Centrosomal protein of 164 kDa, also known as CEP164, is a protein that in humans is encoded by the CEP164 gene.Its function appears two be twofold: CEP164 is required for primary cilium formation . Furthermore, it is an important component in the response to DNA damage by UV light....

, ODF2
ODF2
Outer dense fiber protein 2, also known as cenexin, is a protein that in humans is encoded by the ODF2 gene.-Further reading:...

 
and CEP170
CEP170
Centrosomal protein 170kDa, also known as CEP170, is a protein that in humans is encoded by the CEP170 gene.- Function :The product of this gene is a component of the centrosome, a non-membraneous organelle that functions as the major microtubule-organizing center in animal cells...

, regulate the formation and the stability of the cilium. A transition zone between the basal body and the axoneme "serves as a docking station for intraflagellar transport and motor proteins."

"In effect, the [cilium] is a nanomachine
Molecular machine
A molecular machine, or nanomachine, is any discrete number of molecular components that produce quasi-mechanical movements in response to specific stimuli . The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level...

 composed of perhaps over 600 proteins in molecular complexes, many of which also function independently as nanomachines."

Sensing the extracellular environment

"Some epithelial cells are ciliated, and they commonly exist as a sheet of polarised cells forming a tube or tubule with cilia projecting into the lumen
Lumen (anatomy)
A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...

." Primary cilia on epithelial cells provide chemosensation
Chemosensor
A chemoreceptor, also known as chemosensor, is a sensory receptor that transduces a chemical signal into an action potential. In more general terms, a chemosensor detects certain chemical stimuli in the environment.- Classes :...

, thermosensation and mechanosensation
Mechanosensation
Mechanosensation is a response mechanism to mechanical stimuli. The physiological foundation for the senses of touch, hearing and balance, and pain is the conversion of mechanical stimuli into neuronal signals: mechanosensation. Mechanoreceptors of the skin, called cutaneous mechanoreceptors, are...

 of the extracellular environment by playing "a sensory role mediating specific signalling cues, including soluble factors in the external cell environment, a secretory
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...

 role in which a soluble protein is released to have an effect downstream of the fluid flow, and mediation of fluid flow if the cilia are motile."

Cilium-related disease

Ciliary defects can lead to a number of human diseases. Genetic mutations compromising the proper functioning of cilia, ciliopathies
Ciliopathy
A ciliopathy is a genetic disorder of the cellular cilia or the cilia anchoring structures, the basal bodies, or of ciliary function.Although ciliopathies are usually considered to involve proteins that localize to the primary cilia or centrosomes, it is possible for ciliopathies to be associated...

, can cause chronic disorders such as primary ciliary dyskinesia
Primary ciliary dyskinesia
Primary ciliary dyskinesia , also known as immotile ciliary syndrome or Kartagener Syndrome ', is a rare, ciliopathic, autosomal recessive genetic disorder that causes a defect in the action of the cilia lining the respiratory tract and fallopian tube, and also of the flagella of sperm in...

 (PCD), nephronophthisis
Nephronophthisis
Nephronophthisis is a genetic disorder of the kidneys which affects children. It is classified as a medullary cystic kidney disease.The disorder is inherited in an autosomal recessive fashion and, although rare, is the most common genetic cause of childhood kidney failure.It is a form of...

 or Senior-Loken syndrome. In addition, a defect of the primary cilium in the renal tube cells can lead to polycystic kidney disease
Polycystic kidney disease
Autosomal dominant polycystic kidney disease is an inherited systemic disorder that predominantly affects the kidneys, but may affect other organs including the liver, pancreas, brain, and arterial blood vessels...

 (PKD). In another genetic disorder called Bardet-Biedl syndrome
Bardet-Biedl syndrome
The Bardet–Biedl syndrome is a ciliopathic human genetic disorder that produces many effects and affects many body systems. It is characterized principally by obesity, retinitis pigmentosa, polydactyly, mental retardation, hypogonadism, and renal failure in some cases.-Summary of the...

 (BBS), the mutant gene products are the components in the basal body and cilia.

Lack of functional cilia in female Fallopian tubes can cause ectopic pregnancy
Ectopic pregnancy
An ectopic pregnancy, or eccysis , is a complication of pregnancy in which the embryo implants outside the uterine cavity. With rare exceptions, ectopic pregnancies are not viable. Furthermore, they are dangerous for the parent, since internal haemorrhage is a life threatening complication...

. A fertilized ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

 may not reach the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

 if the cilia are unable to move it there. In such a case, the ovum will implant in the Fallopian tubes, causing a tubal pregnancy, the most common form of ectopic pregnancy.

Since the flagellum of human sperm is actually a modified cilium, ciliary dysfunction can also be responsible for male infertility.

Of interest, there is an association of primary ciliary dyskinesia with left-right anatomic abnormalities such as situs inversus
Situs inversus
Situs inversus is a congenital condition in which the major visceral organs are reversed or mirrored from their normal positions. The normal arrangement is known as situs solitus...

 (a combination of findings known as Kartagener's syndrome) and other heterotaxic defects. These left-right anatomic abnormalities can also result in congenital heart disease. In fact, it has been shown that proper cilial function is responsible for the normal left-right asymmetry in mammals.

Ciliopathy as an origin for many multi-symptom genetic diseases

Recent findings in genetic research have suggested that a large number of genetic disorder
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....

s, both genetic syndromes
Syndrome
In medicine and psychology, a syndrome is the association of several clinically recognizable features, signs , symptoms , phenomena or characteristics that often occur together, so that the presence of one or more features alerts the physician to the possible presence of the others...

 and genetic diseases
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

, that were not previously related in the medical literature, may be, in fact, highly related in the root cause
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

 of the widely-varying set of medical symptoms
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 that are clinically visible in the disorder. These have been grouped as an emerging
Emergence
In philosophy, systems theory, science, and art, emergence is the way complex systems and patterns arise out of a multiplicity of relatively simple interactions. Emergence is central to the theories of integrative levels and of complex systems....

 class of diseases called ciliopathies
Ciliopathy
A ciliopathy is a genetic disorder of the cellular cilia or the cilia anchoring structures, the basal bodies, or of ciliary function.Although ciliopathies are usually considered to involve proteins that localize to the primary cilia or centrosomes, it is possible for ciliopathies to be associated...

. The underlying cause may be a dysfunctional molecular mechanism in the primary cilia structures, organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

s which are present in many diverse cellular types throughout the human
Human
Humans are the only living species in the Homo genus...

 body. Cilia defects adversely affect "numerous critical developmental signaling pathways" essential to cellular development and thus offer a plausible hypothesis for the often multi-symptom nature of a large set of syndromes and diseases. Known ciliopathies include primary ciliary dyskinesia
Primary ciliary dyskinesia
Primary ciliary dyskinesia , also known as immotile ciliary syndrome or Kartagener Syndrome ', is a rare, ciliopathic, autosomal recessive genetic disorder that causes a defect in the action of the cilia lining the respiratory tract and fallopian tube, and also of the flagella of sperm in...

, Bardet-Biedl syndrome
Bardet-Biedl syndrome
The Bardet–Biedl syndrome is a ciliopathic human genetic disorder that produces many effects and affects many body systems. It is characterized principally by obesity, retinitis pigmentosa, polydactyly, mental retardation, hypogonadism, and renal failure in some cases.-Summary of the...

, polycystic kidney
Polycystic kidney disease
Autosomal dominant polycystic kidney disease is an inherited systemic disorder that predominantly affects the kidneys, but may affect other organs including the liver, pancreas, brain, and arterial blood vessels...

 and liver disease
Polycystic liver disease
Polycystic liver disease usually describes the presence of multiple cysts scattered throughout normal liver tissue, in association with polycystic kidney disease.-Pathophysiology:Associations with PRKCSH and SEC63 have been described....

, nephronophthisis
Nephronophthisis
Nephronophthisis is a genetic disorder of the kidneys which affects children. It is classified as a medullary cystic kidney disease.The disorder is inherited in an autosomal recessive fashion and, although rare, is the most common genetic cause of childhood kidney failure.It is a form of...

, Alstrom syndrome
Alstrom syndrome
Alström syndrome is a rare genetic disorder caused by mutations in the gene ALMS1. It is among the rarest genetic disorders in the world, as currently it has only 266 reported cases in medical literature and over 501 known cases in 47 countries. It was first described by Carl-Henry Alström in...

, Meckel-Gruber syndrome, Sensenbrenner syndrome
Sensenbrenner syndrome
Sensenbrenner syndrome is a rare multisystem disease first described in 1975. It is inherited in an autosomal recessive fashion, and a number of genes appear to be responsible...

 and some forms of retinal degeneration
Retinopathy
Retinopathy is a general term that refers to some form of non-inflammatory damage to the retina of the eye. Frequently, retinopathy is an ocular manifestation of systemic disease.-Pathophysiology:Causes of retinopathy are varied:...

.

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