SEM micrograph of the cilia projecting from respiratory epithelium in the lungs

A cilium (plural cilia) is an organelle found in eukaryotic cells. Respiratory Epithelium is a type of Epithelium found lining the Respiratory tract, where it serves to moisten and protect the airways In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called Cilia are tail-like projections extending approximately 5–10 micrometers outwards from the cell body. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre

There are two types of cilia: motile cilia, which constantly beat in a single direction, and non-motile cilia, which typically serve as sensory organelles. Along with flagella, they make up a group of organelles known as undulipodia. A flagellum ( plural flagella) is a tail-like structure that projects from the Cell body of certain Prokaryotic and Eukaryotic cells and it An undulipodium or 9+2 organelle is an extracellular projection of a Eukaryotic cell containing a Microtubule array

Types and distribution

Cilia are rare in plants, occurring most notably in cycads. Cycads are a group of Seed plants characterized by a large crown of compound leaves and a stout trunk. Ciliates possess motile cilia exclusively and use them for either locomotion or to simply move liquid over their surface. The ciliates are one of the most important groups of Protists common almost everywhere there is water — lakes ponds oceans rivers and soils with many ectosymbiotic Some ciliates bear groups of cilia that are fused together into large mobile projections called cirri (singular, cirrus).

Larger eukaryotes, such as mammals, have motile cilia as well. Motile cilia are rarely found alone, usually present on a cell's surface in large numbers and beating in coordinated waves. In humans, for example, motile cilia are found in the lining of the trachea (windpipe), where they sweep mucus and dirt out of the lungs. Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus The traceartes, or windpipe, is a tube that has an inner diameter of about 20-25 mm and a length of about 10-16 cm in humans In female mammals, the beating of cilia in the Fallopian tubes moves the ovum from the ovary to the uterus. Female (♀ is the Sex of an Organism, or a part of an organism which produces ova (egg cells Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. "Ovaria" redirects here This is also a proposed section and a Synonym of Solanum. The uterus (from the Latin word for womb) is the major Female reproductive organ of most Mammals including Humans One end the

In comparison to motile cilia, non-motile cilia usually occur one per cell. The outer segment of the rod photoreceptor cell in the human eye is connected to its cell body with a specialized non-motile cilium. A photoreceptor, or photoreceptor cell, is a specialized type of Neuron (nerve cell found in the Eye 's Retina that is capable of The dendritic knob of the olfactory neuron, where the odorant receptors are located, is also carrying non-motile cilia (about 10 cilia / dendritic knobs). Olfaction (also known as olfactics or smell) refers to the Sense of smell. Aside from these specialized examples, almost all mammalian cells have a single non-motile primary cilium. Although the primary cilium has historically been one of the oldest cellular organelles to be studied (at least since 1898), only a small group of devotees have followed it until its importance began to become clear in the late 1990s. 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^[1] and congenital heart disease^[2]. Polycystic kidney disease ( PKD, also known as polycystic kidney syndrome is a progressive ciliopathic, Genetic disorder of the Kidneys A congenital heart defect (CHD is a defect in the structure of the Heart and Great vessels of a Newborn.

Assembly and maintenance

To grow a cilium, 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. A Tubulin is one of several members of a small family of globular Proteins The most common members of the tubulin family are α-tubulin and β-tubulin the proteins that In most species bi-directional motility called intraflagellar transport or IFT plays an essential role to move these building materials from the cell body to the assembly site. Intraflagellar transport or IFT is the cellular process essential for the formation and maintenance of Eukaryotic Cilia and Flagella. 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 proceses, the length of cilia can be maintained dynamically.

Exceptions where IFT is not present include Plasmodium falciparum which is one of the species of Plasmodium that cause malaria in humans. Plasmodium falciparum is a Protozoan Parasite, one of the species of Plasmodium that cause Malaria in humans A plasmodium is also the macroscopic form of the Protist known as a Slime mould. Malaria is a vector -borne Infectious disease caused by Protozoan Parasites It is widespread in tropical and subtropical regions including In this parasite, cilia assemble in the cytoplasm. ^[3]

Cilium-related disease

Ciliary defects can lead to several human diseases. Genetic mutations compromising the proper functioning of cilia can cause chronic disorders such as primary ciliary dyskinesia (PCD). Primary ciliary dyskinesia ( PCD) also known as immotile ciliary syndrome or Kartagener Syndrome (KS, is a rare Autosomal recessive In addition, a defect of the primary cilium in the renal tube cells can lead to polycystic kidney disease (PKD). Polycystic kidney disease ( PKD, also known as polycystic kidney syndrome is a progressive ciliopathic, Genetic disorder of the Kidneys In another genetic disorder called Bardet-Biedl syndrome (BBS), the mutant gene products are the components in the basal body and cilia. Laurence-Moon-Biedl syndrome and Laurence-Moon-Biedl-Bardet redirect here

Lack of functional cilia in mammalian Fallopian tubes can cause ectopic pregnancy. The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges ( singular salpinx) are two very fine tubes lined with ciliated An ectopic pregnancy is a Complication of pregnancy in which the fertilized Ovum is implanted in any tissue other than the uterine wall A fertilized ovum may not reach the uterus if the cilia are unable to move it there. An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. The uterus (from the Latin word for womb) is the major Female reproductive organ of most Mammals including Humans One end the In such a case, the ovum will implant in the Fallopian tubes, causing a tubal pregnancy, the most common form of ectopic pregnancy. An ectopic pregnancy is a Complication of pregnancy in which the fertilized Ovum is implanted in any tissue other than the uterine wall

Since the flagellum of human sperm is actually a modified cilium, ciliary dysfunction can also be responsible for male infertility. ^[4]

Of interest, there is an association of primary ciliary dyskinesia with left-right anatomic abnormalities such as situs inversus (a combination of findings known as Kartagener's syndrome) and other heterotaxic defects. Situs inversus (also called situs transversus) is a congenital condition in which the major Visceral organs are reversed or mirrored Primary ciliary dyskinesia ( PCD) also known as immotile ciliary syndrome or Kartagener Syndrome (KS, is a rare Autosomal recessive These left-right anatomic abnormalities can also result in congenital heart disease^[5]. A congenital heart defect (CHD is a defect in the structure of the Heart and Great vessels of a Newborn. In fact, it has been shown that proper cilial function is responsible for the normal left-right asymmetry in mammals. ^[6]

References

1. ^ Wagner CA (2008). "News from the cyst: insights into polycystic kidney disease. ". J. Nephrol. 21 (1): 14–6. PMID 18264930.  
2. ^ Brueckner M (2007). "Heterotaxia, congenital heart disease, and primary ciliary dyskinesia.". Circulation 115 (22): 2793–5. doi:10.1161/CIRCULATIONAHA.107.699256. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 17548739.  
3. ^ Of cilia and silliness (more on Behe) - The Panda's Thumb
4. ^ Ichioka K, Kohei N, Okubo K, Nishiyama H, Terai A (2006). "Obstructive azoospermia associated with chronic sinopulmonary infection and situs inversus totalis. ". Urology 68 (1): 204. e5–7. doi:10.1016/j.urology.2006.01.072. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16850538.  
5. ^ Kennedy MP, Omran H, Leigh MW, et al (2007). "Congenital heart disease and other heterotaxic defects in a large cohort of patients with primary ciliary dyskinesia.". Circulation 115 (22): 2814–21. doi:10.1161/CIRCULATIONAHA.106.649038. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 17515466.  
6. ^ McGrath J, Brueckner M (2003). "Cilia are at the heart of vertebrate left-right asymmetry.". Curr. Opin. Genet. Dev. 13 (4): 385–92. doi:10.1016/S0959-437X(03)00091-1. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 12888012.  

External links

• Brief summary of importance of cilia to many organs in human physiology
• The Ciliary Proteome Web Page at Johns Hopkins