Cilium
Cilium, a cellular organelle resembling hair, is found in the cells of mammals and plays crucial roles in various biological functions. There are two main types of cilia: primary cilia and motile cilia. Primary cilia, often overlooked until recently, are essential for sensing environmental information, while motile cilia exhibit a back-and-forth movement that helps in fluid transport and cellular navigation. Cilia are constructed around a central structure called the axoneme, which facilitates their movement through the action of motor proteins like dynein.
These organelles are not just pivotal for movement; they also have sensory functions that enable cells to detect motion, light, and chemicals, sending this information to the cell's nucleus. Dysfunctional cilia are linked to various medical conditions known as ciliopathies, which can lead to serious health issues such as ectopic pregnancies and certain genetic syndromes. Furthermore, research suggests that cilia play a significant role in cancer development and embryonic growth, as they help establish cellular orientation during early development. Understanding cilia's structure and function is essential for comprehending their impact on health and disease.
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Cilium
A cilium (plural cilia) is a cellular organelle, a microscopic structure found in the cells of mammals. It resembles a hair and can occur either singly or in great numbers. Its name comes from the Latin word for “eyelid.” Cilia that appear singly are known as “primary cilia.” Primary cilia were little studied until the late twentieth century because most scientists thought they served no real function, but it is now believed that they play an important role in enabling the cell to gather information about its surroundings.
The other type of cilia are called “motile cilia.” These cilia move back and forth in a waving motion that serves some function for the cell or organism, such as keeping an airway free of debris or propelling a cell through a medium. An example of this can be seen in the paramecium, a single-celled freshwater organism that, when viewed under a microscope, looks like an oval-shaped cell surrounded by a fringe of short, rapidly waving hairs. These hairs are the cilia, and they allow the paramecium to navigate through its environment.
Background
Almost all cells have a primary cilium, but only certain types of cells have motile cilia. Motile cilia operate to move liquid across the surface of the cell. This has different effects, depending on whether the cell is attached to tissue or floating without being anchored. Motile cilia on an unattached cell make it possible for the cell to “swim” through its environment, seeking food or avoiding danger. Motile cilia on cells that are attached to tissue help move fluid, debris, and other matter along the surface of the tissue.
In many cases, cilia also serve sensory functions in addition to those pertaining to movement. Cilia of different types are capable of sensing motion (mechanoreceptors), light (photoreceptors), or chemicals (chemoreceptors). The cilia then send information derived from this sensory input to the nucleus of the cell.
Overview
A cilium is built around a central, skeleton-like structure called an axoneme. A cross-section view of a motile cilium reveals that the axoneme is a ring of nine doublet microtubules that encircle two singlet microtubules. The cilium has been described as a kind of nanomachine made up of proteins rather than mechanical parts, almost like a combination of a motor and an antenna.
The characteristic movement of cilia is caused by changes in the configuration of protein molecules within the axoneme. Motor proteins called “dynein” detach, move down or up the microtubules, and then reattach. This movement translates into the larger-scale movement of the cilium as a whole.
It is believed that many troublesome diseases can trace their origins, at least in part, to problems with the formation or the functioning of cilia. Ectopic pregnancy, for example, in which an egg is fertilized in the fallopian tube and remains in the tube rather than traveling to the uterus, is caused by the failure of cilia to move the fertilized egg to the uterus.
Certain types of diseases related to the cilia are called “ciliopathies.” Scientists suspect that certain illnesses, which present such a wide variety of symptoms that they can initially baffle the physician confronted with them, may be caused by problems with the primary cilium found in most cells. The theory is that the primary cilium is such a basic part of most cells’ ability to move, sense, and function that if it is not working properly, this could lead to all manner of problems in the organism. Some ciliopathies include Joubert syndrome, some types of retinal degeneration, polycystic kidney disease, and Bardet-Biedl syndrome.
Cilia have also gained attention as part of research into the causes and treatment of cancer. Since the primary cilium is responsible for receiving sensory information from the cell’s surrounding environment and conveying that information to the nucleus so the cell can respond to the information appropriately, it is believed that the primary cilium may play an important role in understanding how cancer develops at the molecular level.
In keeping with their close association with the most fundamental steps in human development, cilia are thought to play a major role in the growth of the embryo. When a human embryo begins to develop, it is a symmetrical ball of reproducing cells. Before long, the ball manifests a head end and a tail end, and a left side and a right side. Ultimately, the embryo will grow to have a number of asymmetrical features, such as the liver on the right side, even though the organism appears symmetrical to the outside observer, since eyes, arms, legs, et cetera, appear to be mirror images of each other. At one time, scientists were greatly puzzled at how the cells could differentiate left from right as they reproduced, but the study of cilia seems to have provided an answer. The motile cilia on embryonic cells appear to be responsible for moving the fluid surrounding the embryo in a leftward direction. The embryonic cells then use the direction of this flow as a point of reference with which to orient themselves. If anything happens to interrupt the cilia as they generate this flow, then the organism’s development can suffer severe and even debilitating consequences..
Bibliography
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