what are the structures found in a nucleus of a ciliated cell
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Cilia are elongated motile cylindrical projections from the apical cell membrane, approximately 0.25 mm in diameter, that contain microtubules and cytoplasm in continuity with that of the cell. ... The microtubules, radial spokes, and nexin links are considered to form a “skeletal” structure for the cilium.
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STRUCTURE OF RESPIRATORY CILIA
The human tracheobronchial tree is lined by pseudostratified, ciliated epithelium from the larynx down to the level of the terminal bronchiole. The nasal cavity (apart from its most anterior portion) and the paranasal sinuses are also lined by ciliated epithelium, the predominant direction of clearance being toward the oropharynx. Respiratory epithelium undergoes a continual turnover, with cells being replaced approximately once every 4 to 8 weeks.
Human respiratory tract ciliated cells bear 200 to 300 cilia on their surface. Cilia are elongated motile cylindrical projections from the apical cell membrane, approximately 0.25 mm in diameter, that contain microtubules and cytoplasm in continuity with that of the cell. Human tracheal cilia are 5 to 8 mm long, becoming shorter in more distal airways. Ultrastructural examination of a transverse section of a respiratory tract cilium reveals that the axoneme consists of a characteristic 9+2 structure of nine pairs of microtubules (composed of the proteins, a- and b-tubulin) arranged around two single central microtubules that are surrounded and held together by the central sheath. The peripheral doublets are linked to neighbouring doublets by fine filaments (nexin links) composed of a protein called nexin. Radial spokes are thicker filaments that project centrally from the “A” microtubule and end in a terminal knob, a short distance from the central sheath and central microtubules. Projecting at regular intervals from the “A” microtubule toward the “B” microtubule of the adjacent outer doublet are the inner and outer dynein arms, which are curved or hooklike filamentous structures. The dynein arms contain proteins with adenosine triphosphatase (ATPase) activity and, according to the sliding microtubule hypothesis of ciliary bending, attach intermittently to the adjacent “B” microtubule, and change shape, with resultant sliding of doublets each other. Such a cyclical movement of different microtubule pairs about each other causes ciliary bending, first in one direction and then in the other. The microtubules, radial spokes, and nexin links are considered to form a “skeletal” structure for the cilium. This confers properties of elasticity and rigidity on the cilium and may transfer bending forces through the cilium and help the cilium to resume its initial shape after each beat.
The peripheral doublets extend along the length of the cilium and, at the base, continue into the cell in a modified form, becoming triplets and, in association with several accessory structures, form a basal body. Striated or ciliary rootlets extend from the basal extremity of the basal body into the apical cytoplasm. Together with the basal body, they anchor the cilium and maintain the orientation of its direction of beating. Projecting laterally from the basal body in the direction of the active stroke of the cilium is a structure called the basal foot. All basal feet in a cell and in the whole epithelium are normally oriented in approximately the same direction so that the effective strokes of all cilia move in the same direction.
Much of the molecular regulation of ciliary beating has been elucidated from studies of the unicellular, biflagellate green alga Chlamydomonas reinhardtii, for which the flagella have been characterized biochemically, and there are numerous genetic mutants that have assisted in identifying genes involved in flagellar motility. From such studies, it is known that the axoneme consists of approximately 250 proteins,20 and considerable information is available on the composition of the dynein arms and how mutations of the genes encoding these pro teins affect flagellar motility. Dynein arms, which generate ciliary motility, are large polypeptide assemblies, variably composed of different dynein chains, including 14 heavy (400-500kDa), 7 intermediate (45-110kDa), and 15 light (8-55kDa) chains. Moreover, the outer and inner dynein arms have different compositions: the outer arms are composed consistently of three heavy, two intermediate, and eight light chains, whereas the inner dynein arms have a much more diverse composition of one or two heavy, two or three intermediate, and two or three light chains.
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