The cytoskeleton is composed of three main types of filaments: microfilaments, intermediate filaments, and microtubules. Microfilaments are the thinnest at 7nm in diameter and are composed of actin subunits. They help determine cell shape and enable cell movement and contraction. Intermediate filaments are 10nm thick and composed of proteins like keratin and desmin. They contribute to cell shape and nuclear anchoring. Microtubules are the most rigid at 25nm in diameter, composed of tubulin subunits. They radiate from the centrosome and are involved in organelle movement within cells.

1. The Cytoskeleton
• By definition, it is the cytoplasmic filamentous
network associated with cell shape and
movement.
• There are three classes of the cytoskeletal
filaments, starting from the thinnest:
1. microfilaments/actin filaments
2. Intermediate filaments
3. Microtubule

2. Structure, composition and function
of the cytoskeleton

3. Microfilaments
• They have a diameter of 7nm and are abundant in all eukaryotic cells as they were first
discovered in the skeletal muscle and are made up of the protein subunit g-actin(globular
actin)which assemble into a polymer of two twisting chains known as F-actin(filamentous)
• They compose of the contractile protein cutin, which makes up a major portion of the
cytoskeleton in all cells where they slide along filaments of another protein called myosin
to make the cells contract
• In none muscle cells actin filaments are less organised and much less prominent
• Like microtubules, actin filaments have a plus a minus ends with more ATP-powered
growth occuring at a filament’s plus end
• They can assemble and disassemble rapidly
• In many types of cells, networks of actin filaments are found beneath the cell cortex
,which is the meshwork of membrane-associated proteins that supports strengthens the
plasma membrane
• Such networks allow cells to hold- and- move specialised shapes ,such as the brush order
of microvilli
• They are also involved in cytokinesis and cell movement
• In short the main functions of the microfilaments are:
• In determining cell shape
• Ability of movement by the amoeboid like movements
• Cell division
• Muscle cell contraction

4. • Intermediate filaments
• They are composed of twisted strands of several
different proteins including keratin, desmin,and
lamin
• They have a diameter of 10nm
• Filaments also contribute to cell shape and help
anchor the nucleus
• They provide considerable strength to cells and
consequently are most extensively developed in
the region of cell subject to mechanical
stress(e.g in association with desmosomes)
• Once they are assembled they less readily
disassemble

5. • Microtubules
• these are hollow tubes of about 25nm in diameter
• They compose of the protein subunits tubulin
• Tubulin contains two polypeptide subunits string together to make long strands
called protofilaments,13 protofilaments then come together to form the hollow,
straw-shaped filaments of microtubules
• They are the most rigid of the cytoskeletal filaments and are present in the long
processes of neurons, where they provide the framework that maintains the
processes’ cylindrical shape
• Microtubules are ever changing ,with reactions constantly adding and subtracting
tubulin dimers at both ends of the filaments
• The rates of change at either end are not balanced, one end grows more rapidly
and is called the plus end and the other the minus end
• In cells, the minus ends of microtubules are anchored in structures called
microtubule organising centres(MTOCs)
• The primary MTOC in a cell is called the centrosome and it is usually located
adjacent to the nucleus
• Microtubules radiate from a region of the cell known as the centrosome which
sorrounds two cylindrical bodies called centrioles composed of nine sets of fused
microtubules
• Microtubules and actin filaments have also been implicated in the movement of
organelles within the cytoplasm