Cytology and Histology Chapter 2: The Cell Structure PDF

Cytology and histology Chapter 2 : The cell structure  Cell theory : History Because cells are so small, they were not discovered until the invention of the microscope in the 17th century In 1665, Robert Hooke (English philosopher) was the first to observe...

Cytology and histology Chapter 2 : The cell structure  Cell theory : History Because cells are so small, they were not discovered until the invention of the microscope in the 17th century In 1665, Robert Hooke (English philosopher) was the first to observe cells naming the shapes he saw cellulae (Latin, “small rooms”) Anton van Leeuwenhoek (Dutch microscopist), observed living cells, which he termed “animalcules,” or little animals In 1838, Matthias Schleiden (German botanist) stated that all plants “are aggregates of fully individualized, independent, separate beings, namely the cells themselves” In 1839, Theodor Schwann (German physiologist) reported that all animal tissues also consist of individual cells Thus, the cell theory was born  Cell theory is the unifying foundation of cell biology  The cell theory includes the following three principles: 1. All organisms are composed of one or more cells & the life processes of metabolism & heredity occur within these cells 2. Cells are the smallest living things, the basic units of organization of all organisms 3. Cells arise only by division of a previously existing cell Each cell is a complete living being; a cell can: Absorb and process food Reject waste Breath Secret substances Reproduce Repair if damaged The cells of all living organisms are: Very similar to each other Very few differences between species A cell represents :  A structural unit : support of biological activities  A reproduction unit : development or repairing of organs  An information unit: contains hereditary information  A unit of function: ensures the realization of the biological activities necessary for life  Prokaryote Vs Eukaryotes The world of cells is subdivided into two large groups which are fundamentally different in their internal structure & general organization There are two main types: Prokaryotic cells , Eukaryotic cells Prokaryotic cells (bacteria, archeobacteria): Eukaryotic cells : 1 to 10 μm in general 10 to 100 μm in general All unicellular Uni or multi cellular Genetic material not enclosed in a defined Genetic material delimited by a membrane = membrane (nucleoid) nucleus Do not have an internal membrane system Contains numerous membrane-bounded organelles Cell division in prokaryotes takes place Cell division takes place by mitosis and mainly by binary fission , which does not use involves spindles made up of microtubules. a spindle  Prokaryotic Cell Division by Binary Fission  Mitosis  All cells share many structural features All cells resemble one another in certain fundamental ways Four major features all cells have in common: 1. Nucleoid (prokaryotes) or Nucleus (eukaryotes) where genetic material is located. The DNA contains the genes that code for the proteins synthesized by the cell 2. Cytoplasm : A semifluid (jelly) matrix fills the interior of the cell. It contains sugars, amino acids, and proteins that the cell uses to carry out its everyday activities 3. Ribosomes to synthesize proteins 4. Plasma membrane : encloses a cell and separates its contents from its surroundings. The plasma membrane is a phospholipid bilayer with proteins embedded in it. Proteins performs several functions such as receptors, enzymes, channels , pumps  Prokaryotic cells : simple organization Pro = without and Karyon = nucleus Prokaryotes are the simplest organisms Prokaryotic cells are small (1 to 10 μm) Prokaryotic cells can be categorized based on cell shape They consist of cytoplasm surrounded by a plasma membrane. The processes of replication, transcription and translation take place in the cytoplasm They are encased within a rigid cell wall (except mycoplasma) Do contain ribosomes Lack the membrane- bounded organelles characteristic of eukaryotic cells Prokaryotic cells : simple organization  Prokaryotic cells : simple organization Nucleoid region : - Prokaryotes lack nuclei - do not possess linear chromosomes (except Borrelia Burgdorferi chromosome linear) - Single double-stranded ring of DNA that is highly condensed (associated with proteins) to form a visible region (nucleoid region) - Many prokaryotic cells also possess plasmids, which are small, independently replicating circles of DNA - Plasmids contain only a few genes that confer a selective advantage, they are not essential for the cell’s survival  Prokaryotic cells : simple organization The plasma membrane of bacteria resembles that of eukaryotic cells in many ways: - Formed of a double layer of phospholipids, with membrane proteins (peripheral and intrinsic) and a few chains of carbohydrates. However, it does not contain cholesterol - It contains many proteins with multiple functions (transporters, receptors, respiratory chain proteins……) - It is the site of several metabolic reactions (respiration and ATP synthesis) and fulfills a role of control of exchanges between internal and external environments - Allow the supply of nutrients and elimination of waste as well as the detection of environmental signals - Play an important role in cell division - Unable to deform (no endo & exocytosis)  Prokaryotic cells : simple organization  The basic structure of the plasma membrane is the phospholipid bilayer  The plasma membrane contains many intrinsic and peripheral proteins  Prokaryotes have three basic shape: Cocci Bacilli Spirilla  Bacterial cell walls consist of peptidoglycan Most bacterial cells are encased by a strong cell wall (except mycoplasma) The bacterial cell wall is the single most important contributor to cell shape. Bacteria that normally lack cell walls, such as the mycoplasmas, do not have a set shape This cell wall is composed of peptidoglycan, which consists of a carbohydrate matrix (polymers of sugars) that is cross-linked by short polypeptide units Cell walls protect the cell, maintain its shape, and prevent excessive uptake or loss of water  The drugs, interfere with the ability of bacteria to cross-link the peptides in their peptidoglycan cell wall -> this destroys the integrity of the structural matrix, which can no longer prevent water from rushing in & swelling the cell to bursting Bacterial cell walls consist of peptidoglycan Bacterial cell walls consist of peptidoglycan  Gram-positive and gram-negative bacteria Two types of bacteria can be identified using a staining process called the Gram stain Gram-positive bacteria have a thicker peptidoglycan wall (traps crystal violet dye) and stain a purple color Gram-negative bacteria (the more common) contain less peptidoglycan (do not retain the crystal violet dye) and do not retain the purple-colored dye  Structure of gram-positive & gram-negative cell wall  The outer membrane layer makes Gram-negative bacteria resistant to many antibiotics that interfere with cell-wall synthesis in Gram- positive bacteria  S layer, capsule, flagella and pili  S-layer Consist an additional glycoprotein layer that forms a rigid para- crystalline surface Found in some bacteria (outside of the peptidoglycan or outer membrane layers of gram-positive and gram-negative bacteria) Among the archaea, the S-layer is almost universal The functions of S-layers are diverse and variable but often involve adhesion to surfaces or protection  S layer, capsule, flagella and pili  The capsule The capsule (gelatinous layer), surrounds the other wall layers (often the pathogenic bacteria ) Found in some bacteria A capsule enables a prokaryotic cell to adhere to surfaces and to other cells, and to evade an immune response by interfering with recognition by phagocytic cells -> often contributes to the ability of bacteria to cause disease (virulence)  Flagella and pili  Flagella : Many kinds of prokaryotes have slender, rigid, helical flagella composed of the protein flagellin These flagella are anchored in the cell wall , moving the cell through a liquid environment  Pili (singular, pilus) are other hairlike structures. They are common in Gram negative bacteria and rare in Gram positive They are shorter than prokaryotic flagella Two categories, of distinct morphology and function: the common pili and the sexual pili - Common pili : are numerous (from 100 to 200), short and rigid. They are formed by polymerization of a protein: pilin. They have a binding role that allows bacteria to attach to cells (adhesion) - Sexual pili : are longer and less numerous (from 1 to 4) They allow the exchange of genetic material between two bacteria by the phenomenon of bacterial conjugation Flagella and pili Bacterial conjugation  Cell divison : Binary division Bacteria reproduce asexually in a mode of cell division called binary fission The genetic material is first duplicated (by DNA replication), then the bacteria daughter cells identical to the mother cell Thus, the progeny of a bacterial cell is a clone of genetically identical cells, called colony  Archaea have unusual membrane lipids A common feature distinguishing archaea from bacteria is the nature of their membrane lipids Peptidoglycan, is not found in the cell wall of archaea Archaea have cell walls made from a variety of polysaccharides and peptides, as well as membranes containing unusual lipids (confer thermal stability) Archaea share a similar overall cellular architecture with bacteria However, the cellular machinery that replicates DNA and related to synthesized proteins in archaea is more closely eukaryotes  Eukaryotic cells Eukaryotic cells are far more complex than prokaryotic cells The hallmark of the eukaryotic cell is compartmentalization (endomembrane system & organelles)  There is a wide range of eukaryotic organisms, including all animals, plants, fungi, and protists, as well as most algae Eukaryotes may be either :  Single-cells : yeast, paramecium, amoeba  Multicellular : animal, plant and human Eukaryotic animal cells varie in : - Shape - Size - Internal structure - Turnover rate  Shape : Varies from one species to another  Examples : Globular : isolated cells Lenticular : erythrocytes Flagellate : spermatozoides With an extension : nervous cell  Size : Visible to the naked eye - Example :hen’s egg (3cm) Not visible to the naked eye - Examples : - leukocytes ~ 5 µm - Muscular cells ~ 250 µm Internal structure : Varies by specialty and level of cellular activity - Examples : - Young cells : rich in ribosomes -> protein synthesis - Muscular cells : rich in microfilaments & mitochondria -> energy for contraction Turnover rate - Variable depending on cell types - Examples : - WBC -> 1-2 weeks - Platelets -> 10 – 12 days - Hepatocytes & pancreatic cells -> few months  Organization eukaryotic cell  eukaryotic cell consists of a plasma membrane enclosing a number of organelles suspended in a watery fluid called cytosol Organelles, have individual & highly specialized functions  They include: the nucleus, mitochondria, ribosomes Reticulum Endoplasmic, Golgi apparatus, lysosomes & the cytoskeleton  Cell organization  The cell is divided in two mains parts : 1. Nucleus 2. Cytoplasm : cytosol + other organelles  The nucleus Nuclei are roughly spherical in shape They are typically located in the central region of the cell Contains the genetic information that enables the synthesis of nearly all proteins of a living eukaryotic cell (DNA) Many nuclei exhibit a dark-staining zone called the nucleolus, which participates in the synthesis and assembly of ribosome components  The nuclear envelope Double membrane (phospholipid bilayer) perforated with pores The pore allows ions and small molecules to diffuse freely between nucleoplasm and cytoplasm, while controlling the passage of proteins and RNA - protein complexes Transport across the pore is controlled and consists mainly of the import of proteins that function in the nucleus, and the export to the cytoplasm of RNA and RNA–protein complexes formed in the nucleus  The nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum The inner surface of the nuclear envelope is covered with a network of fibers that make up the nuclear lamina The nuclear lamina gives the nucleus its shape and is also involved in the deconstruction and reconstruction of the nuclear envelope that accompanies cell division  The nuclear envelope b. Cell nucleus, showing many nuclear Pores c. Nuclear membrane showing a single nuclear pore d. The nuclear lamina is visible as a dense network of fibers made of intermediate filaments  The nuclear pores A nuclear pore is a complex "basketball hoop" structure ‘It consists of two rings of orthogonal radial symmetry: - one on the nuclear side - one on the cytoplasmic side (relationship with microfilaments of the cytoskeleton) Each of the two rings is connected to the central transporter by 8 radial arms On the nucleoplasmic face is a third ring of smaller diameter. It is connected to the ring on the nuclear side by microfilaments organized in a cage The nuclear lamina is interrupted at the level of the pore  The nuclear pores  Exchange: Is done in both directions between the cytosol and the nucleoplasm : - Passive diffusion: concerns nucleotides, ions and small proteins. These molecules cross the pores by diffusion, without energy consumption. They use the lateral channels - Active : concerns molecules of MW >40 kDa. These molecules use the central transporter & their transport consumes energy  Nucleolus  Inside the nucleus  active in interphase and absent in mitosis  The clusters of ribosomal RNA genes, the rRNAs (they produce) & the ribosomal proteins all come together within the nucleus during ribosome production  This are easily visible within the nucleus as dark-staining regions called nucleolus nucleolus  The nucleolus contains the part chromosome called nucleolar organizer (NO) and which contains the rRNA genes  Chromatin: DNA packaging DNA is the molecule that stores genetic information In eukaryotes, the DNA is divided into multiple linear chromosomes, which are organized with proteins into a complex structure called chromatin Chromatin is composed of DNA (DeoxyriboNucleic acid) wrapped around proteins called histones Two forms: Heterochromatin : generally not active Euchromatin : active  In non dividing cell, DNA is present in the form of a thin network of threads called chromatin  when the cell prepares to divide the chromatin forms distinct structures called chromosomes  Primary Structure of DNA (DeoxyriboNucleic Acid) Definition : a nucleotide polymer A nucleotide is composed of : -Phosphate -Sugar (deoxyribose) -Nitrogenous bases (A, T, C, G) Two types of nitrogenous bases: - Pyrimidine base: C (Cytosine), T (Thymine for DNA ) et U (Uracil for RNA) - Purine base : A (Adenine ), G (Guanine) 4 types of Nucleotides Adenosine Thymidine Guanosine Cytidine  Secondary structure of DNA  Consists of two complementary strands joined to each other via a hydrogen bond  Hydrogen bonds combines the purine base with the pyrimidine base - A=T (2 hydrogen bonds) - C=G (3 hydrogen bonds) In 1953, Watson & Crick proposed an architecture model of the DNA molecule - Double helical structure - Polynucleotide chain twisted around a common axis - Antiparallel  Stucture of chromatin DNA Composed of DNA (DesoxyriboNucleic acid) wrapped around proteins called histones Two forms: Heterochromatin : generally not active Euchromatin : active  Structure of chromatin  Chromatin compaction levels  Each DNA molecule wraps around proteins (histones) and forms chromatin fibers (pearl necklace) Pearl necklace  From chromatin  chromosome  The chromosome is the result of condensation (very strong spiralization) of chromatin  Ribosomes Ribosomes are among the most complex molecular assemblies found in cells Each ribosome is composed of two subunits; each of which is composed of a combination of RNA called ribosomal RNA (rRNA) and proteins They are the cell’s protein synthesis machinery Ribosomes are found either free in the cytoplasm (polysome) or associated with endoplasmic reticulum  Ribosomes Free ribosomes synthesize cytoplasmic, nuclear, mitochondrial, proteins as well as proteins of other organelles (not derived from the endomembrane system) endoplasmic reticulum associated ribosomes synthesize membrane proteins, proteins found in the endomembrane system, and proteins destined for export from the cell Ribosomes subunits join to form a functional ribosome only when they are actively synthesizing proteins Protein synthesis process requires two other main forms of RNA: 1- messenger RNA (mRNA) : which carries coding information from DNA 2- transfer RNA (tRNA) : which carries amino acids Ribosomes translate mRNA, which is transcribed from DNA in the nucleus, into polypeptides that make up proteins  mRNA The genetic information carried by a DNA gene is transferred from the nucleus to the cytoplasm in the form of mRNA to the protein synthesis machinery Each mRNA serves as a matrix on which a specific sequence of amino acids is polymerized to form a specific protein molecule (the end product of a gene)  Brin matrice Transcription (non sens)  Takes place in the nucleus  Separation of the 2 strands Brin sens  One of the two strands of DNA serves as a template for the synthesis of an RNA strand  The rule of complementarity of the nitrogenous bases is respected Transcription takes place using an enzyme :  The synthesized mRNA passes into the cytoplasm to be translated into protein ARN polymerase  Translation  The synthesized mRNA migrates into the cytoplasm & attaches to a ribosome  The mRNA passes in front of the ribosome allowing the successive reading of the codons The passage of each codon corresponds to the addition of a specific amino acid  Amino acids are added via tRNA which carries an anticodon (sequence complementary to that of the mRNA codon) AUG : Initiation code stop codon : UGA , UAA, UAG  Expression of genetic information  Each triplet of nucleotides on DNA corresponds to a codon of the mRNA  Each codon of the mRNA corresponds to a specific anti-codon of the tRNA  Each anti-codon corresponds to a specific amino acid  The endomembrane system The interior of a eukaryotic cell is packed with membranes that form an endomembrane system The presence of these membranes in eukaryotic cells marks one of the fundamental distinctions between eukaryotes and prokaryotes This endomembrane system : - Fills the cell and dividing it into compartments - Channeling the passage of molecules through the interior of the cell - Providing surfaces for the synthesis of lipids and some proteins  The endomembrane system Nuclear membrane Endoplasmic reticulum Golgi apparatus Lysosomes vacuoles  Endoplasmic reticulum (ER) The ER is the largest of the internal membranes Is composed of a phospholipid bilayer embedded with proteins It has functional subdivisions & forms a variety of structures from folded sheets to complex tubular networks Two types of endoplasmic reticulum - Rough Endoplasmic Reticulum (RER) - Smooth Endoplasmic Reticulum (SER)  Rough Endoplasmic reticulum (RER) RER gets its name from its pebbly surface appearance Composed primarily of flattened sacs Rough endoplasmic reticulum has ribosomes embedded within its structure, giving a “rough” appearance It synthesizes and secretes proteins in the liver, hormones and other substances in the glands Rough ER is prominent in cells where protein synthesis happens (such as hepatocytes)  Functions of Rough Endoplasmic reticulum (RER) The majority of the functions of rough ER is associated with protein synthesis plays a vital role in protein folding Also ensures quality control (regarding correct protein folding) The second most important function after protein synthesis and protein folding is protein sorting  Smooth endoplasmic reticulum (SER) Regions of the ER with relatively no bound ribosomes Structures ranging from a network of tubules, to flattened sacs, to higher order tubular networks SER membranes contain numerous enzymes involved in the synthesis of a variety of carbohydrates, lipids and steroid hormones The majority of membrane lipids are assembled in the SER and then sent to all parts of the cell that require membrane components  Functions of SER :  Storage of intracellular calcium : keeps the cytoplasmic level low, allowing Ca2+ to be used as a signaling molecule. For example, in muscle , Ca2+ is used to trigger muscle contraction smooth ER store and releases calcium ions. These are quite important for the nervous system and muscular systems  Synthesis of carbohydrates, lipids and steroid hormone synthesis of essential lipids such as phospholipids and cholesterol is also responsible for the production and secretion of steroid hormones is also responsible for the metabolism of carbohydrates  Detoxification (in hepatocytes): Liver cells have extensive SER as well as enzymes that carry out this detoxification  Smooth endoplasmic reticulum (SER) The ratio SER / RER is not fixed but depends on the function of a cell. Ex: Cells that synthesize secreted proteins, such as antibodies, have a much larger RER Cells active in steroid synthesis (Leydig cells) have a very large SER.  Golgi apparatus The Golgi apparatus is a smooth, concave, membranous structure (component of the edomembrane system) It is composed of Flattened stacks of membranes Individual stacks of membrane are called cisternae (Latin, “collecting vessels”) They are especially abundant in glandular cells, which manufacture and secrete substances  Golgi apparatus The Golgi apparatus functions is the collection, packaging & distribution of molecules synthesize and used within the cell or even outside it Golgi has a front and a back, with distinctly different membrane compositions Thereby 3 zones are defined : - The cis face : in the front, or receiving end and is usually located near the ER - The Medial or intermediate - The Trans face  Materials arrive at the cis face in transport vesicles that bud off the ER, exit the trans face, where they are discharged in secretory vesicles  Functions of Golgi Maturation of proteins synthesized in the ER: cleavage of polypeptide precursors Post-translational modifications : glycosylation, phosphorylation, sulfatation Membrane traffic to and from the plasma membrane (secretion and endocytosis) Protein addressing control (return to the RE, addressing to the lysosome) Modifications of lipids synthesized by the SER (addition or modification of short sugar chains, thus forming glycolipids) production of complex carbohydrates (e.g. non-cellulosic polysaccharides which are part of the cell wall of plants)  Transport through the Golgi Proteins and lipids manufactured on the RER and SER transported into the Golgi apparatus and modified as they pass through it 1. Vesicles from RER or SER 2. Fusion of vesicles with Golgi cistarnea on the Cis face 3. Content progression from the Cis face to the Trans face 4. Budding of vesicles on the trans side 5. Vesicles then diffuse to other locations in the cell, distributing the newly synthesized molecules to their appropriate destinations  Lysosomes Lysosomes are membrane-bounded digestive vesicles They arise from the Golgi apparatus They contain a variety of enzymes involved in breaking down all kinds of biomolecules (RNA, DNA, carbohydrates, proteins) inside the cell into smaller particles that are either recycled, or expelled from the cell as waste Throughout the lives of eukaryotic cells, lysosomal enzymes break down old organelles and recycle their component molecules. For example, mitochondria are replaced in some tissues every 10 days 2 types of lysosomes according to their functional states: 1. Primary: newly formed organelles homogeneous, not involved in degradation 2. Secondary: fusion of primary lysosomes + phagocytic vacuoles  Lysosomes The digestive enzymes in the lysosome are optimally active at acid pH Lysosomes are activated by fusing with phagocytic vacuole The fusion event activates proton pumps in the lysosomal membrane, resulting in a lower internal pH As the interior pH falls, the arsenal of digestive enzymes contained in the lysosome is activated. This leads to the degradation of macromolecules Lysosomes  Peroxisomes The peroxisome is a type of microbody rich in oxydatives enzymes (oxydase, catalase) Spherical organelles formed from the fusion of ER- derived vesicles. These vesicles then import peroxisomal proteins to form a mature peroxisome They are surrounded by a simple membrane, in the form of a lipid bilayer. They are present in all eukaryotic cells (except in reticulocytes) Like mitochondria, peroxisomes are essential sites for the use of oxygen O2 (oxidation reactions) Peroxisomes get their name from the hydrogen peroxide produced as a by-product of the activities of oxidative enzymes  Peroxisomes  Functions : Detoxification : oxidase enzymes remove free hydrogen atoms (oxidation reaction) from specific organic substrates (R) These substrates linked to hydrogen atoms, are potentially toxic to the cell. The oxidation of these molecules detoxifies them. RH2 + O2 → R + H2O2 Peroxidation : catalase uses the hydrogen peroxide H2O2 generated by other enzymes to oxidize a variety of other toxic substrates (R’). This type of reaction is very important in the liver, kidney cells, where peroxisomes detoxify certain toxins passing through the blood. H2O2 + R'H2 ⤇ R '+ 2 H2O Conversion of ROS (reactive species oxygen) : conversion of H2O2 into water molecules (catalase) Oxydation of fatty acids  Mitochondria Mitochondria are sausage-shaped organelles in the cytoplasm about the size of bacteria They are found in all types of eukaryotic cells They are in all cell types except red blood cells Each cell contains 1000 to 3000 mitochondria according to cell types Move through interactions with the cytoskeleton Functionally, they are the central of cellular respiration the processes by which chemical energy (ATP) are produce  Mitochondria Contain their own DNA (circular) and protein synthesis machinery Mitochondrial DNA contains several genes that produce proteins essential to the mitochondrion’s role in oxidative metabolism Mitochondria are not fully autonomous, because most of the genes that encode the enzymes used in oxidative metabolism are located in the cell nucleus A eukaryotic cell does not produce new mitochondria each time the cell divides Instead, the mitochondria themselves divide in two, doubling in number, and these are partitioned between the new cells Most of the components required for mitochondrial division are encoded by genes in the nucleus and are translated into proteins by cytoplasmic ribosomes  Mitochondria Mitochondria are bounded by two membranes: - A smooth outer membrane - An inner folded membrane with numerous contiguous layers called cristae The cristae partition the mitochondrion into two compartments: - A matrix : lying inside the inner membrane - An intermembrane space : lying between the two mitochondrial membranes  Mitochondria The outer membrane : - It is a lipid bilayer - Composition close to that of the plasma membrane - Contains more protein 50 to 60% protein and 50 to 40% lipids The intermembrane space - it is a space of 4 to 7 nm - it contains : H + protons (role in ATP synthesis), cytochrome c molecules (role in apoptosis) The inner membrane : - It is a 5-6nm lipid bilayer - Organization very different from that of the external membrane - 80 % of proteins and 20% of lipids  Mitochondria functions 1. Cellular respiration : ATP synthesis 2. Synthesis functions : - synthesis of steroid hormones : Participate, with the reticulum endoplasmic in the biosynthesis of steroid hormones from cholesterol 3.Thermogenesis 4. Calcium regulation : Mitochondria, with the reticulum endoplasmic are the main reservoir calcium  regulation of intracellular calcium concentration 5. Cell death (apoptosis)  The cytoskeleton The cytoplasm of all eukaryotic cells is crisscrossed by a network of protein fibers that supports the shape of the cell and anchors organelles to fixed locations This network is called the cytoskeleton The cytoskeleton is a dynamic system, constantly assembling and disassembling  The cytoskeleton The cytoplasm of all eukaryotic cells is crisscrossed by a network of protein fibers which is responsible for: - the internal and external shape of the cell - cell movements - the transport of different organelles or vesicles inside the cell - anchors organelles to fixed locations This network is called the cytoskeleton Individual fibers consist of polymers of identical protein subunits that attract one another and spontaneously assemble into long chains Fibers disassemble in the same way, as one subunit after another breaks away from one end of the chain The cytoskeleton is a dynamic system, constantly assembling and disassembling The cytoskeleton is present in the cytoplasm, nucleoplasm and at the cell periphery (under the plasma membrane)  Types of cytoskeletal fibers Eukaryotic cells contain three types of cytoskeletal fibers, each formed from a different kind of subunit: 1- Actin filaments, sometimes called microfilaments 2- Microtubules 3- Intermediate filaments  Actin filaments (microfilaments) Actin filaments are long fibers about 7 nm in diameter They exist in almost all cells and especially in muscle cells Microfilaments are G-actin polymers in a single-stranded helical arrangement Actin exists in the form: - monomeric -> called G-actin (Globular) - filament -> called F-actin (Fibrillary)  Location of microfilaments - Actin is distributed throughout the cytoplasm, in the perinuclear region and under the plasma membrane (cellular cortex) - In muscle cells - the microvilli of absorbent cells - the stereocilia - the contractile ring (at the end of the division allowing the separation of the daughter cells) - the various cellular extensions (lamellipodia) of mobile cells  Role of actin microfilaments - Role in mobility (cells, proteins, organelles, endocytosis, exocytosis) - Role in mitosis (formation of the contractile ring) - Mechanical role (microvilli, junctions) - Role in adhesiveness (attachment to cell adhesion molecules) - Role in muscle contraction  Microtubules Microtubules are the largest of the cytoskeletal elements They are hollow tubes about 25 nm in diameter, each composed of a ring of 13 protein protofilaments (fig below) Proteins consisting of dimers of α- and β-tubulin subunits that polymerize to form the 13 protofilaments The protofilaments are arrayed side by side around a central core, giving the microtubule its characteristic tube shape A protofilament has an α-tubulin at one end and a β-tubulin at the other  Microtubules In many cells, microtubules form from nucleation centers (centrosome) near the center of the cell and radiate toward the periphery They are in a constant state of flux, continually polymerizing and depolymerizing (unstable microtubules) The average half-life of a microtubule ranges from 10 minutes in a non dividing animal (in relax) cell to 20 seconds in a dividing animal cell The ends (extremity) of the microtubule are designated as plus (+) (away from the nucleation center) or minus (−) (toward the nucleation center)  Microtubules Microtubules are involved:  During cell division, they allow the movement of chromosomes  Maintain cell shape  Migration of endocytosis and exocytosis vesicles  Movement of organelles in the cell  Facilitate cell movement  Centrosomes and centrioles Centrioles are barrel-shaped organelles A centriole is formed by 9 groups of 3 microtubules They occur in pairs, usually located near the nuclear membranes they are enveloped by a dense and amorphous material, composed of many proteins and called "pericentric material" (PCM). the centrosome = pairs of centrioles + PCM  Centrosomes The centrosome is a microtubule organizing center (MTOC) It is from the MTOC that the nucleation of microtubules takes place microtubule nucleation: the event that initiates the de novo formation of microtubules. They "grow" from this center  Centrioles are considered as stable microtubules (polymerization & depolymerization dynamics are blocked)  Intermediate filaments The intermediate filaments are the most durable element of the cytoskeleton in animal cells Formed of fibrous protein molecules twined together in an overlapping arrangement (tetramers) Intermediate filaments are characteristically 8 to 10 nm in diameter, between the size of actin filaments and microtubules Once formed, intermediate filaments are stable and usually do not break down Intermediate filaments are composed of overlapping staggered tetramers of protein. These tetramers are then bundled into cables. This molecular arrangement allows for a ropelike structure that imparts mechanical strength to the cell  Intermediate filaments Intermediate filaments constitute a mixed group of cytoskeletal fibers The most common type, composed of protein subunits called the vimentin, provides structural stability for many kinds of cells Keratin,another class of intermediate filament, is found in epithelial cells (cells that line organs and body cavities) and associated structures such as hair and fingernails The intermediate filaments of nerve cells are called neuro-filaments They are not directly involved in cell movements but are involved in maintaining cell morphology, in resistance to mechanical stress and in maintaining cohesion between cells.