Cytoskeleton is a three dimensional network of filamentous proteins that extend throughout the cytosol (= cytoplasmic matrix) of all eukaryotic cells.
The cytoskeleton is closely involved in many processes including cell division, growth, maintenance of cell shape, differentiation, wall deposition, movement of organelles etc.
Cytoskeleton consists of three types of elements: microtubules, microfilaments and intermediate filaments. They are visible only with the help of electron microscope. They are absent in prokaryotic cells. Microtubules and microfilaments are dynamic structures, constantly forming and disassembling.
I. Microtubules:
History:
De Robertis and Franchi (1953) first discovered microtubules in the axoplasm of nerve fibers.
Location:
They are found in the cytoplasmic matrix of all eukaryotic cells. They are also present in structures like centrioles, basal bodies, cilia or flagella, sensory hair, spindle apparatus, chromosome fibres, nerve processes, sperm tail etc. They are absent in prokaryotic cells.
Structure:
Microtubules are unbranched, hollow cylinders of about 0.2 – 25, wm long. Their outer diameter is 25 nm with a core of 15 nm and a wall of 5 nm thick. The wall consists of 13 parallel proto-filaments. Each proto filament is a polymer of tubulin dimers (a – and β-tubulin). The ends of microtubules are designated as’+’ and’-‘.
The polymerization (assembly) is more rapid in + end. The site of microtubule form and growth is called microtubule organizing center (MTOC). Satellites of centrioles, kinetochores of chromosomes and basal bodies act as MTOC. At places microtubules may have lateral projections. The half life of individual microtubule is 10 minutes in non-dividing animal cells and just 20 sec. in dividing animal cells.
Functions:
1. They maintain cell shape.
2. They are the structural components of centrioles, basal bodies, spindle apparatus, chromosomal fibers, cilia and flagella.
3. In term of Chromosomal fibers, they bring about anaphasic movement of Chromosome.
4. They determine the orientation of cellulose micro fibrils in cell wall.
5. They are associated with intracellular transport of organelles, vesicles and macromolecules.
6. In chromatophores they cause distribution of pigment granules.
II. Microfilaments (Actin Filaments):
Discovery:
They were discos
Location:
They occur in almost all eukaryotic plant and animal cells. They form sheets or bundles below the plasma membrane. They are abundant in muscle cells.
Structure:
Microfilaments are long solid fibers of 7 nm in diameter. Each microfilament consists of two helically coiled protein chains called F-actin. Each F-actin is a polymer of G- actin or globular actin.
Functions:
1. They are involved in cyclosis or cytoplasmic streaming by which coordinated movement of organelles, occur through cytosol.
2. They cause growth of the pollen tube down the style towards the embryo sac.
3. They guide the exocytosis and endocytosis of vesicle at appropriate sites. Such as site of cell wall formation.
4. They are responsible for cleavage at the time of cytokinesis of animal cells.
5. They are responsible for cellular movements such as contraction, crawling, gliding and movement of microvilli.
III. Intermediate Filaments (IFs):
They are most stable element of cytoskeleton. IFs are hollow filaments of 8-10 nm in diameter, intermediate in size between microtubule and microfilament, (that is why they are called intermediate filaments). Its wall consists of overlapping tetramers of proteins.
Its are of four types:
(a) Keratin Filaments: They form keratin of skin;
(b) Neuro-filaments: They form a lattice with bundles of microtubules in axons and dendrons of nerve cells;
(c) Glial Filaments: they are found in astrocytes;
(d) Heterogeneous Filaments: They occur in muscles as basket around nucleus and connected to centriole.
Intermediate filaments are believed to play mechanical role in positioning various cell organelles, provide site for the attachment of microfilaments, function as cytoskeleton, structural proteins of skin and hair, cell shape etc.