Histology Lecture 3: Cytoskeleton PDF

Histology lecture (3) Cytoskeleton Histology Cell organelles Membranous Non-membranous 1. Golgi apparatus. 1. Ribosomes. 2. Mitochondria....

Histology lecture (3) Cytoskeleton Histology Cell organelles Membranous Non-membranous 1. Golgi apparatus. 1. Ribosomes. 2. Mitochondria. 2. Centriole. 3. Endoplasmic reticulum (smooth & rough) 3. Cilia & flagella. 4. Lysosomes 4. Microfilament. 5. Peroxisomes. 5. Microtubules. 6. Cell membrane. Cytoskeleton (Non-membranous cell organelles) Definition They are a type of cell organelles not surrounded by unit membrane : a) Cytoplasmic filaments (6-16nm in diameter) b) Microtubules (25 nm in diametre) They are important as a cyto-skeleton of the cell. A Cytoplasmic Filaments Definition Thread-like structures act as a part of cytoskeleton to the cells. L/M Only seen when present in bundles by special stain ( silver). E/M Classified according to their diameter into : Thin filaments Intermediate filaments Thick filaments (Microfilaments) Actin Myosin 6 nm 8-10 nm 12-16 nm Histology 1 Thin filaments Distribution & function They are formed of actin with tropomyosin and troponin. Muscle In the micro- Amoeboid In cleavage In blood contraction villi, for their movement of furrow during platelets, for with myosin movement motile cells. cell division. clot retraction. 2 thick filaments (Myosin) Diameter 12-16 nm (thicker than myosin). In muscle in association with actin filaments forming the myofibrils for Distribution contraction. Histology 3 intermediate filaments Diameter 8-10 nm. There are about 50 types could be identified by using immuno- cytochemical techniques. Not capable of producing contraction. Muscle Types epithelial cells connective tissue & muscle. Nerve cells Glial cells Play a major role in supporting and maintaining the shape of the cells. Functions Important for tumor identifications. Adhesion between epithelial cells so that they could with-stand rough treatments. Histology b Microtubules Structure Non branching, hollow tubules variable length, but fixed diameter Formed of tubulin protein (in free dimeric or in a polymerized forms) Tubulin dimers ↔ polymerized state forming microtubules. Polymerization is directed by a microtubule organizing centers (MTOCs) which include: a) Centrioles. b) Centromeres of chromosomes L/M They are difficult to see them by L.M except by using special stains. E/M Hollow tubules, in cross section they appear as tiny circles 25 nm in diameter. The wall consists of 13 protofilaments that are composed of tubulin dimers. The length of microtubules can be changed by adding or removal of tubulin molecules at their end Histology Functions 1. Maintain and stabilizing the shape of the cell. 2. Intracellular transport. 3. Formation of mitotic spindle during cell division. 4. Structure of cilia, flagella and centriole. clinical hint Cytotoxic drugs as colchicine prevent the formation of new microtubules. Therefore, used in malignant diseases as they arrest mitosis. Histology 1 Centrioles They are derived from the microtubules. They are responsible for cell division so absent in non-dividing cells: Origin a) RBCs b) nerve cells. Site Usually present near the nucleus in an area called centrosome. H.&E: not demonstrated. Iron H.: dark blue. LM During G1 of the cell cycle, the cell has a single pair of centrioles. During G2 of the cell cycle, the cell contains two pairs of centrioles. Two short hollow cylinders perpendicular to each others. The wall of each centriole is formed of : EM  27 microtubules longitudinally arranged in 9 bundles.  Each bundle consists of 3 micro-tubules (triplets) Share in formation of mitotic spindle during cell division. function They are responsible for formation of cilia & flagella. Histology 2 Cilia Definition Motile hair like processes projecting over the cell surface capable of moving fluids & particles along the surface in one direction. L/M The cilia appear as short, fine, hair like structures arising from free surface of the cell may be hundreds/cell The cytoplasm of the cell underneath them appears refractile and densely stained. E/M Each cilium is formed of a shaft, basal body and rootlets. It is a finger like projection over the cell surface covered by cell membrane. It contains 9 doublets & 2 central single microtubules The microtubules of the nine doublets Formed of:  subunit-A → Complete Shaft of  subunit-B → incomplete Cilium  The microtubules of the singlets are complete. Two arms arise from subunit-A formed of a protein dynein & have ATPase activity. There are connections between subunit-A and subunit-B of adjacent doublet called nexin. Basal body Similar to centriole in structure formed of nine triplets It is formed of longitudinally arranged protofilaments, containing Rootlets of rootletin protein which anchor the basal body to the surrounding Cilium cytoplasm. Histology Function 1. Cilia move in a wave like manner to move secretions or particles over the tissue surface e.g. in respiratory system &female genital system. 2. Cilia can modify & act as receptors for receiving light. 3. Flagellum: it is a long cilium forming the tail of the sperm → motility. 3 Flagella Cytoplasmic processes, e.g. tail of a spermatozoan. Similar to cilia, having 9 peripheral doublets of micro-tubules and two central singlet. 1. They are much longer than cilia (200um). 2. Each outer doublets has a large outer fiber, which is absent in cilia. 3. whip-like swimming movement. 4. Number: one flagellum/cell Cilia Flagella Number Several hundred per cell One flagellum per cell Length Short long Movement Beating rhythmically in one direction Have whip-like swimming movement Basal body Similar to cilia, having 9 peripheral Shaft doublets of micro-tubules and two Structure Rootlet central singlet. Each outer doublets has a large outer fiber which is absent in cilia. Respiratory system: trachea, Tail of sperms. bronchi, Tail of sperm Sites Genital system: uterus and fallopian tube