Motile cilium

A cilium (from Latin, meaning 'eyelash'; the plural is cilia) is an organelle found on eukaryotic cells and are slender protuberances that project from the much larger cell body. A cilium (from Latin, meaning 'eyelash'; the plural is cilia) is an organelle found on eukaryotic cells and are slender protuberances that project from the much larger cell body. There are two types of cilia: motile cilia and non-motile, 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 or length. Biologists have various ideas about how the various flagella may have evolved. Cilia can be divided into non-motile (primary) forms and motile forms. In animals, non-motile primary cilia are found on nearly every cell. In comparison to motile cilia, non-motile 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. It is also known that the cilium must be disassembled before mitosis can occur. However, the mechanisms that control this process are still largely unknown. 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.'.The primary non-motile cilia has a “9+0” axonemal structure and is divided into subdomains. The entire structure is enclosed by a plasma membrane continuous with the plasma membrane of the cell. The basal body, where the cilia originates, is located within the ciliary pocket. The cilium membrane and the basal body microtubules are connected by transition fibers. Vesicles carrying molecules for the cilia dock at the transition fibers. The transition fibers form a transition zone where entry and exit of molecules is regulated to and from the cilia. Molecules can move to the tip of the cilia with the aid of anterograde IFT particles and the kinesin-2 motor. Molecules can also use retrograde IFT particles and the cytoplasmic dynein motor to move toward the basal body. Some of the signaling with these cilia occur through ligand binding such as Hedgehog signaling. Other forms of signaling include G-coupled receptors including the somatostatin receptor 3 in neuronal cells. 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. The functioning of motile cilia is strongly dependent on the maintenance of optimal levels of periciliary fluid bathing the cilia. Epithelial sodium channels ENaC that are specifically expressed along the entire length of cilia apparently serve as sensors that regulate fluid level surrounding the cilia. Ciliates are microscopic organisms that possess motile cilia exclusively and use them for either locomotion or to simply move liquid over their surface.