CYTOSKELETON

The cytoskeleton is a network of filaments and tubules that extends throughout the cytoplasm, which is all of the natural within a cell except for the nucleus.

It is found in all cells through the proteins.

The cytoskeleton supports the cell, gives its shape organises and tethers the organelles and has a role in molecule transport, cell division and cell signalling.

In eukaryotes, it is composed of three main components, microfilaments, intermediate filaments and microtubules and these are all capable of rapid growth or disassembly dependent on the cell’s requirements.

Cytoskeleton of prokaryotic cells was originally thought not to exist. It was not discovered until early 1990s.

Cytoskeleton of prokaryotes are less complex, with no true nucleus or organelle except ribosomes.

       FUNCTIONS OF CYTOSKELETON

1. STRUCTURAL SUPPORT:

Microtubules provide rigidity and shape to the cell. They act as tracks for intracellular transport, guiding organelles and vesicles to their destinations.

Microfilaments contribute to the cell’s shape and help in cell movement. They are involved in the formation of cellular extensions like pseudopodia and microvilli.

• CELLULAR MOVEMENT:

Microtubules also play a role in chromosome segregation during cell division.

Microfilaments participate in muscle contraction, amoeboid movement and the formation of contractile rings during cell division.

• INTRACELLULAR TRANSPORT:

Microtubules serves as tracks for motor proteins, facilitating the movement of vesicles and organelles within the cell.

• CELL DIVISION:

Microtubules form the mitotic spindle during cell division, ensuring accurate separation of chromosomes into daughter cells.

• CELLULAR ORGANISATION AND SHAPE:

Microfilaments and intermediate filaments contributes to maintaining cell shape and provide structural stability.

• CELLULAR SIGNALING:

Microfilaments are involved in signal transduction pathways by interacting with proteins that regulate gene expression and cell growth.

• CELLULAR SUPPORT AND ANCHORAGE:

Intermediate filaments provide mechanical support and anchor organelles like the nucleus within the cell.

     EUKARYOTIC CYTOPLASMA

Eukaryotic cells contain three main kinds of cytoskeleton filament : microfilaments, microtubules and intermediate filaments.

Each type is formed by the polymerisation of a distinct type of protein subunit and has its own characteristic shape and intracellular distribution.

Microfilaments are polymers of protein actin and are 7nm in diameter. Microtubules are composed of tubulin and are 25nm in diameter. Intermediate filaments are composed of various proteins, depending on the type of the cell in which they are found, they are normally 8-12 nm in diameter.

The cytoskeleton provides the cell within structure and shape and by excluding macromolecules crowding in this compartment. Cytoskeletal elements interact with cellular membranes.

Accessory proteins including motor proteins regulate and link the filaments to other cell compounds and each other and are essential for controlled assembly of cytoskeletal filaments in particular locations.

A small molecule cytoskeletal drugs have been discovered that interact with actin and microtubules.

These compounds have proven useful in studying the cytoskeleton and several have clinical applications.

                    MICROFILAMENTS

Microfilaments, also known as actin filament are composed of linear polymers of G-actin proteins and generate force when the growing end of the filament pushes against a barrier, such as the cell membrane.

They also act as tracks for the movement of myosin molecule that affix to the microfilament and walk along them.

The major component or protein of microfilaments are actin. The G-actin monomer combines to form a polymer which continue to form the microfilament.

Myosin motoring along F-actin filaments generate contractile forces in so called actomyosin fibres, both in muscle as well as most non muscle cell type.

FUNCTIONS INCLUDE:

• Muscle contraction
• Cell movement
• Intracellular transport
• Maintenance of eukaryotic cell shape
• Cytokinesis
• Cytoplasmic streaming

INTERMEDIATE FILAMENTS

Intermediate filaments are a part of the cytoskeleton of many eukaryotic cells. These filaments arranging 10nm in diameter are more stable than microfilaments and heterogenous constituents of the cytoskeleton.

Like actin filaments, they function in the maintenance of cell shape by bearing tension during mitosis and during the positioning of centrosome.

Intermediate filaments are most commonly known as the support system or scaffolding for the cell and the nucleus while also playing a role in some cell function.

In combination with proteins and desmosomes, the intermediate filaments from cell-cell connections and anchor the cell matrix junctions that are used in messaging between cells as well as vital functions of the cell. These connections allow the cell to communicate through the desmosomes of multiple cells to adjust structures of the tissue based on signals from the cell environment.

Mutations in the IF proteins have been shown to cause serious medical issues such as premature ageing, alxender disease.

              MICROTUBULES

Microtubules are hollow cylinders about 21nm in diameter most commonly comprising 13 protofilaments that are polymers of alpha and beta tubulin. They have a very dynamic behaviour, binding GTP for polymerisation. They are commonly organised by the centrosome.

In nine triplet set, they form the centriole and in doublet oriented about two additional microtubules they form cilia and flagella. The latter formation is commonly referred to as 9+2 arrangement where in each doublet is connected to another by the protein dynein. As both flagella and cilia are structural components of the cell and are maintained by microtubules. They can be considered part of the cytoskeleton. There are two types of cilia: motile and non-motile cilia.

Cilia are short and more numerous than flagella. The motile cilia have a rhythmic waving motion compared to the non-motile cilia processing signals from the other cells or fluid surrounding it.

They play important role in:

• Intracellular transport
• The axoneme of cilia/flagella
• The mitotic spindle
• Synthesis of cell wall in plants