المحاضرة الثالثة هستو (قبل التعديل)

Cytoskeleton

• A type of cell organelle not surrounded by a unit membrane.
• Plays an important role as the cell's internal framework.
• Composed of cytoplasmic filaments (6-16nm in diameter) and microtubules (25 nm in diameter).

Cytoplasmic Filaments

• Thread-like structures that are part of the cytoskeleton.
• Only visible when present in bundles under special stains (silver).
• Classified into thin filaments, intermediate filaments, and thick filaments.

Thin Filaments (Microfilaments)

• Composed of actin with tropomyosin and troponin.
• Involved in muscle contraction with myosin.
• Found in microvilli for movement.
• Involved in amoeboid movement of motile cells.
• Involved in the formation of the cleavage furrow during cell division.
• Found in blood platelets for clot retraction.

Thick Filaments (Myosin)

• Diameter of 12-16 nm (thicker than myosin).
• Found in muscle in association with actin filaments, forming myofibrils for contraction.

Intermediate Filaments

• Diameter of 8-10 nm.
• Over 50 types can be identified using immunocytochemical techniques.
• Not capable of producing contraction.
• Found in muscle, epithelial cells, connective tissue, muscle, nerve cells, and glial cells.
• Play a major role in supporting and maintaining the shape of cells.
• Important for adhesion between epithelial cells, allowing them to withstand harsh treatments.
• Used for tumor identification.

Microtubules

• Non-branching, hollow tubules of variable length but fixed diameter.
• Composed of tubulin protein (in free dimeric or polymerized forms).
• Polymerization is directed by microtubule organizing centers (MTOCs), which include centrioles and centromeres of chromosomes.
• Difficult to see with light microscopy except using special stains.
• Appear as tiny circles, 25 nm in diameter, under electron microscopy.
• The wall consists of 13 protofilaments composed of tubulin dimers.
• The length of microtubules can be changed by adding or removing tubulin molecules at their end.

Functions of Microtubules

• Maintain and stabilize the shape of the cell.
• Intracellular transport.
• Formation of the mitotic spindle during cell division.
• Structure of cilia, flagella, and centrioles.

Centrioles

• Derived from microtubules.
• Responsible for cell division, so they are absent in non-dividing cells such as red blood cells and nerve cells.
• Typically present near the nucleus in an area called the centrosome.
• Not demonstrable with H&E stain; appear dark blue with iron hematoxylin stain under light microscopy.
• A cell has a single pair of centrioles during G1 of the cell cycle.
• A cell has two pairs of centrioles during G2 of the cell cycle.
• Two short, hollow cylinders perpendicular to each other.
• The wall of each centriole is formed by 27 microtubules arranged longitudinally into 9 bundles, each bundle consisting of 3 microtubules (triplets).

Functions of Centrioles

• Participate in the formation of the mitotic spindle during cell division.
• Involved in the formation of cilia and flagella.

Cilia

• Motile, hair-like processes projecting over the cell surface, capable of moving fluids and particles along the surface in one direction.
• Appear as short, fine, hair-like structures arising from the free surface of the cell, with hundreds per cell.
• The cytoplasm of the cell underneath them appears refractile and densely stained.
• Each cilium consists of a shaft, basal body, and rootlets.
• Finger-like projection over the cell surface covered by the cell membrane.
• Contains 9 doublets and 2 central single microtubules.
• The microtubules of the nine doublets are formed of subunit-A (complete) and subunit-B (incomplete).
• The microtubules of the singlets are complete.
• Two arms arise from subunit-A formed of a protein dynein and have ATPase activity.
• There are connections between subunit-A and subunit-B of adjacent doublet called nexin.
• The basal body is similar to the centriole in structure, formed of nine triplets, longitudinally arranged protofilaments, containing rootletin protein, which anchors the basal body to the surrounding cytoplasm.

Functions of Cilia

• Move in a wave-like manner to move secretions or particles over the tissue surface, for example, in the respiratory system and female genital system.
• Can act as receptors for receiving light.

Flagella

• Cytoplasmic processes, for example, the tail of a spermatozoan.
• Similar to cilia, having 9 peripheral doublets of micro-tubules and two central singlets.
• Much longer than cilia (~200 um).
• Each outer doublet has a large outer fiber, which is absent in cilia.
• Whip-like swimming movement.

Cytoskeleton

• A type of cell organelle not surrounded by a unit membrane.
• Comprises cytoplasmic filaments and microtubules.
• Important for cell shape and structure.

Cytoplasmic Filaments

• Thread-like structures that act as part of the cytoskeleton within cells.
• Only visible in bundles by special staining using silver.
• Classified based on their diameter: thin filaments (microfilaments), intermediate filaments, and thick filaments.

Thin filaments

• Also known as microfilaments
• Composed of actin, tropomyosin, and troponin.
• Play roles in muscle contraction with myosin, movement of microvilli, amoeboid movement, formation of the cleavage furrow during cell division, and clot retraction in blood platelets.

Thick filaments

• Also known as myosin
• Diameter: 12-16 nm (thicker than actin)
• Found in muscle cells alongside actin filaments, forming myofibrils responsible for contraction.

Intermediate filaments

• Diameter: 8-10 nm
• Around 50 different types that can be identified using immunocytochemical techniques.
• Don't actively produce contraction.
• Important for supporting and maintaining cell shape, cell adhesion in epithelial tissues to withstand harsh treatments, and tumor identification.

Microtubules

• Non-branching, hollow tubules with varying lengths, but a fixed diameter.
• Composed of the protein tubulin, which can exist in free dimeric or polymerized forms.
• Tubulin dimers can polymerize into microtubules, a process directed by microtubule organizing centers (MTOCs)
• MTOCs include centrioles and centromeres of chromosomes.
• Difficult to see under a light microscope (L/M) unless special stains are used.
• Visible as tiny circles (25 nm in diameter) under an electron microscope (E/M).
• Their walls consist of 13 protofilaments of tubulin dimers.
• Their length is dynamic, increasing or decreasing by adding or removing tubulin molecules at their ends.

Microtubule Functions

• Maintaining and stabilizing cell shape.
• Performing intracellular transport.
• Forming the mitotic spindle during cell division.
• Structuring cilia, flagella, and centrioles.

Centrioles

• Derived from microtubules
• Responsible for cell division, so absent in non-dividing cells like red blood cells (RBCs) and nerve cells.
• Located near the nucleus in a region called the centrosome.
• Visible as dark blue under an iron hematoxylin stain (L/M) but not under a hematoxylin and eosin (H&E) stain.
• Cells usually have a single pair of centrioles during the G1 stage of the cell cycle, but two pairs during the G2 stage.
• Appear as two short, hollow cylinders perpendicular to each other.
• Each centriole wall is composed of 27 microtubules arranged longitudinally in nine bundles, with each bundle containing three microtubules (triplets) under an electron microscope (E/M).

Centriole Functions

• Forming the mitotic spindle during cell division.
• Responsible for the formation of cilia and flagella.

Cilia

• Motile hair-like processes projecting from the cell surface that move fluids and particles in one direction.
• Visible as short, fine, hair-like structures arising from the free surface of the cell, often hundreds per cell.
• The cytoplasm beneath cilia appears refractile and densely stained.
• Each cilium consists of a shaft, basal body, and rootlets.
• Finger-like projections from the cell surface covered by the cell membrane.
• Contains 9 doublets and 2 central single microtubules.
• The microtubules of the nine doublets are formed of a complete subunit-A and an incomplete subunit-B.
• The central single microtubules are complete.
• Two arms arise from subunit-A, composed of the protein dynein with ATPase activity.
• Connections between subunit-A and subunit-B of adjacent doublets are called nexin.

Basal Body of Cilia

• Similar in structure to a centriole, composed of nine triplets.
• Formed of longitudinally arranged protofilaments containing rootletin protein, which anchors the basal body to the surrounding cytoplasm.

Cilium Function:

• Cilia move in a wave-like manner to move cellular secretions or particles across a tissue surface. Examples include the respiratory and female genital systems.
• Cilia can function as receptors for receiving light.
• Flagellum: a long cilium that forms the tail of sperm, facilitating motility.

Flagella

• Cytoplasmic processes like the tail of a spermatozoon.
• Similar to cilia, with 9 peripheral doublets of microtubules and two central singlets.
• Much longer than cilia (200 µm).
• Each outer doublet has a large outer fiber, which is absent in cilia.
• Show a whip-like swimming movement.