The following points highlight the three main types of cytoskeletal structures of eukaryotic cell. The types are: 1. Microfilaments 2. Intermediate Filaments 3. Microtubules.
Cytoskeletal Structure of Eukaryotic Cell: Type # 1. Microfilaments:
They are ultramicroscopic long, narrow cylindrical rods or protein filaments which occur in eukaryotic plant and animal cells. Microfilaments are made up of actin (also present in muscle myofibrils) constituting 10—15% of total cell protein.
They are 6-8 nm in thickness and show periodic beaded appearance due to close helical arrangement of otherwise globular actin molecules (Fig. 8.43). Microfilaments often associate to form Fig. 8.43.
Microfilaments often associate to form hexagonal bundles. They may also occur in parallel bundles or loose network. Microfilaments generally lie at sol-gel interphase as well as below plasma membrane. Microfilaments are also connected with spindle fibres, endoplasmic reticulum, chloroplast, etc.
In some primitive organisms spindle apparatus seems to be made of microfilaments. During mitosis of animal cells, they have been found associated with cleavage furrows. Stabilisation of membrane proteins has recently been found to be related to their association with microfilaments.
Microfilaments are contractile. Association with myosin protein seems to be essential for contraction of microfilaments. Myofibrils of muscle fibres also contain microfilaments.
Microfilaments form the contractile machinery of the cell which aids in motility, like formation and retraction of pseudopodia and plasma membrane undulations, formation of microvilli, endocytosis, cytoplasmic streaming and movement of other cell organelles.
Microvilli are thread-like protoplasmic projections which are formed on the free surface of absorptive cells like those of intestine. Each microvillus is covered by an extension of plasma lemma. Its core contains a number of microfilaments. The microfilaments are attached to the plasma lemma extension.
Functions:
1. Cytoplasmic Streaming:
Cyclosls is caused by the activity of microfilaments.
2. Membrane Proteins:
They help in stabilisation of membrane proteins.
3. Support:
They are components of cytoskeleton of cell that is required to support otherwise fluid cytoplasmic matrix.
4. Change in Form:
Microfilaments play an important part in change of cell form during development and differentiation.
5. Myofibrils:
Myofibrils are contractile elements of muscles. They have microfilaments.
6. Microvilli:
Microvilli are maintained through the support provided by microfilaments.
7. Movement of Microvilli:
Microvilli show microfilament mediated movements. This aids in quicker absorption of materials.
8. Membrane Undulations:
Fibroblasts are able to move due to plasma membrane undulations caused by microfilaments.
9. Pseudopodia:
Microfilaments help in the formation and retraction of pseudopodia.
10. Endocytosis and Exocytosis:
Microfilaments are responsible for changes in plasma membrane during endocytosis and exocytosis.
11. Spindle Apparatus:
The spindle apparatus of few organisms is composed of microfilaments.
12. Cleavage:
Microfilaments are associated with cleavage furrow at the time of cytokinesis.
13. Movement of Cell Components:
Pigment granules, chloroplasts and other cell organelles are able to change their position inside the cytosol by means of microfilaments.
Cytoskeletal Structure of Eukaryotic Cell: Type # 2. Intermediate Filaments:
They are nearly solid un-branched filaments of about l0nm thickness which are formed by a variety of proteins and often form a network.
Intermediate filaments are of four types:
(a) Keratin Filaments:
They form tonofibrils of desmosomes and keratin of skin.
(b) Neuro-filaments:
Filaments form a lattice with bundles of microtubules in axons and Dendron’s of nerve cells.
(c) Glial Filaments:
They are intermediate filaments found in astrocytes.
(d) Heterogeneous Filaments:
They are intermediate filaments found in muscles (Z-lines, M-lines), as basket around nucleus and connected to centriole, etc.
Heterogeneous filaments are of three types— synemin filaments, vimentin filaments and desmin filaments. Inter- mediate filaments do not occur in unicellular eukaryotes. They evolved in multicellular eukaryotes. The filaments are cross linked with one another as well as various cellular structures including plasma lemma by means of IF associated proteins, e.g., plakins, plectins.
Functions:
1. Nuclear Matrix:
It is mainly formed of intermediate filaments.
2. Support to Membranes:
IF provide support to all bio membranes including plasma lemma and nuclear membranes.
3. Cytoplasm:
IFs constitute scaffold or supporting array for cytoplasm.
4. Muscles:
A lattice of desmin filaments not only surrounds each Z-disc but is also connected to sarcolemma. This provides support to contractile units or sarcomeres.
5. Desmosomes:
Desmosomes are supported by intermediate filaments called tonofibrils.
6. Epithelial Tissues:
IFs maintain the integrity of epithelial tissues.
7. Keratin:
Keratin deposited in the skin cells provides protection against abrasions.
8. Nervous Tissue:
Intermediate filaments provide mechanical strength to axons and dendrons of nerve cells (as neurofilaments) and astrocytes (as glial filaments).
Cytoskeletal Structure of Eukaryotic Cell: Type # 3. Microtubules:
Microtubules are un-branched hollow submicroscopic tubules of protein tubulin which develop on specific nucleating regions and can undergo quick growth or dissolution at their ends by assembly or disassembly of monomers.
Colchicine prevents assembly of microtubules. It, therefore, prevents spindle formation during cell division. With the exception of Slime Moulds and Amoebae, microtubules occur widely in eukaryotic cells.
They are present in the cytoplasm as well as in specialized structures like centrioles, basal bodies, cilia or flagella, sensory hair, equatorial ring of thrombocytes, spindle apparatus, chromosome fibres, nerve processes, sperm tails, axostyle of parasitic flagellates, fibre system of Stentor, cyto-pharyngeal basket of Nassula, etc. Microtubules are of indefinite length.
Their diameter is 25 nm with a core of 15 nm and wall of 5 nm thickness. The wall is formed of 13 laterally associated and helically arranged longitudinal strands called protofilaments. These strands are made of alternate spirals (Fig. 8.45) of two related proteins called a- and p-tubulins.
The surface of a microtubule may also possess arms lateral projections of 100-400 A length and 20-50 A thickness. They may help in forming cross-bridges among themselves and various types of cellular structures like plasma lemma, endoplasmic reticulum, nuclear envelope and other organelles. The arms seem to be involved in movement of cytosol in the area of microtubules.
Functions:
1. Structural Components:
Microtubules are constituents of spindle fibres, chromosome fibres, centrioles, basal bodies, flagella and cilia.
2. Cytoskeleton:
Microtubules function as cytoskeleton. They provide rigidity and shape to some cell parts like pseudopodia of some protistans and axons of nerve cells.
3. Intracellular Transport:
They are believed to function either as microcirculatory system or directing movement of vesicles to a particular part with the help of their arms.
4. Orientation of Micro fibrils:
In plant cells the microtubules control orientation of cellulose micro fibrils of the wall.
5. Shape:
Distribution of microtubules control the shape of wall-less cells and nuclei.
6. Nuclear Movements:
They help in the movement of nuclei during division.
7. Movement of Chromosomes:
As chromosome or tractile fibrils, the microtubules take part in the anaphasic movement of chromosomes.
8. Cell Plate:
Place of future cell plate formation has been found to be determined by a micro tubular band.
9. Pushing of Food:
In protistans, microtubules help in driving the food in the gullet.
10. Cell Differentiation:
They are believed to play a vital role during differentiation.
11. Cell Polarity:
Distribution of microtubules determines cell polarity.
12. Movements of Cilia and Flagella:
Being capable of sliding past one another, microtubules help in the movement of flagella and cilia.
13. Cell Movements:
Along with microfilaments they take part in cell movements.