Cytoskeletal Structure (Microtubules ,Microfilaments )

 




What are Cytoskeletal Structure?

●  Need for Cytoskeleton: It seems likely that, as the cell size increased during the course of evolution of eukaryotic cell, a framework of fibrous protein elements became necessary to support the extensive system of membranes developed in the relatively fluid cytoplasmic matrix. These elements collectively form cytoskeleton of the cell.



  
     


  Special Feature of Cytoskeleton: The cytoskeleton formed by these organelles is not permanent unlike our skeleton. Its components can disassemble, move to a new location and reform as needed.

     Functions of Cytoskeleton: The cytoskeleton provides mechanical support to the cell. The skeletal organelles also keep other organelles,  such as mitochondria and ribosomes, separated from one another to avoid interference in one another's activities. They maintain the cell shape too. The microtubes and microfilaments are also responsible for the cellular and intracellular movements.



   
   


  ● Types of Cytoskeletal Elements: The Cytoskeletal elements include three types of organelles microtubules, microfilaments and intermediate filaments. 





Microtubules:

They are unbranched hollow submicroscope tubules  of protein tubulin. They develop on specific nucleating region and can undergo quick growth or dissolution  at their ends by assembly or disassembly of monomers. Ca²+, Mg²+, GRP and calcium  binding protein calmodulin play important role in assembly of microtubules. Colchicine  which prevents  assembly  of microtubules prevents spindle formation during cell division . Microtubules occur widely in Eukaryotic cells except in Slime Moulds and Amoebae. Microtubules are of indefinite length with their wall formed of 13 laterally  associated and helically arranged longitudinal strands called protofilaments. Protofilament strands are made of alternated spirals, of two related proteins  called æ_ and ß_ tubulins. The surface of  a microtubule may also possess  lateral projections called arms which may help in forming cross bridges among themselves  and various types of cellular structures like plasmalemma,  endoplasmic reticulum, nucleus envelope and other organelles and also seem to be involved  in movement  of cytosol in the area of microtubules. 




The microtubules are noticeable with electron microscope only.

Discovery: The microtubules were first seen in 1953 by Robertis and Franchi in the axons of modulated nerve fibres. He called them neurotubules.
  
   Location: The microtubules are found in the cytoplasmic matrix of all eukaryotic cells. They also occur in the cell organelles, such as cilia and flagella, centrioles and basal bodies, mitotic apparatus, sperm tail, processes of nerve cells, and supporting elements of protozoans. The microtubules are absent in certain cells, such as amoebae and slime moulds. Prokaryotic cells lack microtubules. Certain blue__ green algae have been recently reported to contain microtubules. 

  Structure of Microtubules:

The microtubules are hollow, unbranched cylinders, generally about 25 nm wide and 0.2 to 25 ųm long. The microtubules may occur singly or in bundles and radiate from the centrioles to the periphery of th cell. The wall of a microtubule is composed of 13 parallel protofilaments that enclose a central lumen about 150nA° wide.Each protofilament is made up of a row of globular subunits formed mainly of a protein tubulin. A tubulin subunit contains one œ _ tubulin molecules and one ß__ tubulin molecules. These œ and ß molecules are arranged alternately in a helical manner. An œß dimer,to be more precise heterodimer as its components differ,is 80 to 100 A° long.



        Origin: The microtubules appear to assemble spontaneously from tubulin subunits that are abundant in the cytosol of all cells. They grow by addition of tubulin subunits at one end. They may break up and reassemble in another part of the cell
 Their number increases during mitosis and meiosis. 
   Cholchicine inhibits assembly of microtubules, thereby preventing the formation of spindle during cell division. 

Functions: The microtubules have a variety of functions:

( i) Form and Support: The microtubules form a part of cytoskeleton that maintains the shape of the cell and also provides support to the cell and its processes.

(ii) Movement: The microtubules form the mootile elements of cilia and flagella .

(iii) Formation of Mitotic Apparatus: The microtubules form spindle in cell division.

(iv) Chromosome Movement: Chromosomal fibres of spindle bring about movement of chromosomes or chromatids to its opposite poles in the anaphase.

(v) Distribution of Pigment: The microtubules cause movement of pigment granules in the chromatophores.

(vi) Components of Centrioles and Basal Bodies: The microtubules are components of centrioles and basal bodies.

 (vii)  Intracellular Transport: The microtubules may also serve as tracks for the oriented transport of macromolecules and organelles such as lysosomes, mitochondria and vesicles from ER and Golgi complex, in the cell, thus forming microcirulatory system. Microtubules proteins use energy from ATP to move the organelles and macromolecules along them.

(viii) Cell Differentiation: The microtubules play a role in cell differentiation. 

(ix) Cell Polarity: The microtubules play a role in the determination of polarity of the cell.

(x) : Orientation of Microfibrils: Microtubules control orientation of the cellulose microfibrils in the plant cell wall.

(xi) They participate  in cell movements  alongwith microfilaments. 

(xii) Microtubules are found in many sensory cells, e.g., cels of inner ear, rods and cones of retina. They converte stimuli into nerve impulses,  i.e., sensory transduction. 

(xiii) They control  the space of wall_ less cells and nuclei.





Microfilaments:

The are ultramicroscopic long, narrow cylindrical rods or protein filaments which occur in Eukaryotic plant and animal cells. They are made up of actin ( also present in muscle myofibrils). They show a periodic  beaded appearance due to close helical arrangement  of otherwise globular actin molecules. They often associate to form hexagonal  bundles and may also occur in parallel  bundles or loos network. They generally  lie at so__ gel_ inter_ phase as well as below plasma membrane. Microfilaments are connected with spindle fibres, endoplasmic reticulum, chloroplasts, etc.


 


The microfilaments are visible with electron microscope only.

Discovery: The microfilaments were discovered by Paleviz and his coworkers in 1974.

Location: The microfilaments are found in almost all eukaryotic cells. The form an extensive network in the cytoplasm of nonmuscle cells and may be associated with plasma membrane. They extend into the core of the Microvilli when the latter are present. The microfilaments are most prominent in the muscle cells. Here they are called myofilaments. The myofilaments occur in bundles. The microfilaments are absent in the prokaryotic cells.

         Structure: The microfilaments are solid, unbranded, rod_ like fibrils of indefinite length and about 50__ 60 A° in diameter. They are composed mainly of a globular protein actin but have some filamentous protein myosin also. Microfilaments are also called actin filaments after their components proteins. A microfilament is a helical polymer of dumbbell __ shaped subunits that are monomers unlike the tubulin subunits.  As with microtubules, free actin molecules in the cytoplasm can assemble into microfilaments when needed and later disassemble into actin subunits which can be reused. 



Functions: The microfilaments serve a number of functions. 

(i) Support: The microfilaments form a part of cytoskeleton to support the relatively fluid matrix.

(ii) Intracellular Movement: The microfilaments bring about directed movements of particles and organelles along them in the cell.

(iii) Streaming Movement: The microfilaments also produce streaming movements of cytoplasm. 

(iv)  Cleavage :  The microfilaments also cause cleavage of animal cells which is brought about by contraction of a ring of microfilaments. 

(v) Locomotion: The microfilaments also participate in gliding amoeboid motion shown by amoebae, leacocytes and macrophages. 

(vi) Change in Form: The microfilaments are also responsible for the change in cell shape during development, motility and division. 

(vii) Contraction: Myofilaments bring about muscle contraction. 

(viii) Movement of Villi: The microfilaments cause movements of villi to quicken absorption of food.

(ix) Movement of Plasma Membrane: The microfilaments are responsible for the movement of cell membrane during endocytosis and exocytosis. 

(x) Membrane Undulations: Microfilaments cause plasma membrane undulations that enable the fibroblasts to move.

(xi) Formation of Spindle: Microfilaments form spindle in some primitive organisms.

(xii) Microfilaments play an important part in change of cell form during development  and differentiation.

(xiv) Various cell components  e.g., pigments  granules, chloroplasts and other cell organelles change their position  inside the cytosol by means of microfilaments. 

Intermediate of Filaments ( IFs):





The intermediate filaments were first described in 1968.

Locarion: They are supportive elements in the cytoplasm of the eukaryotic cells, except the plant cells.They are  missing in mammalian RBCs and in the prokaryotes

Structure: The IFs are somewhat larger than the microfilaments and are about 10 nm thick. They are solid, unbranhed and composed of nonmotile structural proteins, such as Keratin, desmin, vimentin. Each IF is formed of 8 protofilaments. 





            Types: The IFs are of 5 types regarding their component proteins ___ 

(i) The keratin __ containing IFs occur in the epidermal cells. They become increasingly cross__ linked as the cells differentiate, leading to the formation of stratum corners of dead cells. The tonofibrils of the desmosomes are bundles of keratin __ containing IFs.

(ii) Desmin__ containing IFs are present in muscle cells. They serve to spatially integrate the various components of a muscle into a functional unit capable of shortening.

( iii) The vimentin__ containing IFs are the most wide__ spread. They often interconnect the nuclear envelope with the cell membrane and seem to support the nucleus and maintain its position in the cell.

(iv) The neurofibrils of nerve cells are bundles of IFs formed by copolymerization of three distinct proteins, NFi,NFm and NFh.

( v) The glial cells that surround the neurons have IFs composed of glial fibrillary acid protein ( GFAP).

Functions: The IFs serve a variety of functions __ 

 (i) They form a part of cytoskeleton that support the fluid cytosol and maintains the shape of the cell. 

 (ii) They stabilize the epithelia by binding to the spot Desmosomes. 

 (iii) They form major structural proteins of skin and hair.

( iv) They integrate the muscle cell components into a functional unit.

( v) They provide strength to the axons.

( vi) They keep nucleus and other organelles in place.

( vii) Cardiac muscle cells are interconnected by sport desmosomes.  Desmin filaments interconnect these desmosomes, allowing the stress and strain of the contractile force of one muscle to be transmitted to the other.





















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