The cell. 7. Cytosol.
|« Cytosol||Actin filaments »|
Cytoskeleton is composed of filaments of proteins with a number of functions: cell integrity, motility, organization, division, etcetera.
There are three types of filaments:
• Microfilaments or actin filaments
• Intermediate filaments
The nucleus and the rest of the organelles are not randomly scattered through the interior of eukaryotic cells. Indeed, there are an internal organization mainly ruled by several types of proteins arranged in filaments, jointly known as cytoskeleton. These filaments form a dynamic scaffolding distributed through the cytosol, although some of them are found inside the nucleus. Cytoskeleton is particularly important in animal cells because plasma membrane, which basically is a sheet of lipids, would break easily without the anchoring to cytoskeleton filaments. Plant cells, however, are protected by the cell wall. The word cytoskeleton may lead to misunderstand the function of these filaments because they do not only work as a structural scaffold for the cell and organelles, but they are also the "muscles" of the cells. In this regard, cytoskeleton allows cell movement and sets the shape of the cells, determines the internal organization of the organelles, allows the communication between organelles (vesicular traffic), exocytosis and endocytosis, and cell division (both mitosis and meiosis). At the same time, it withstands mechanical loads and cell shape deformations, and determines the cell size of animal cells. Cytoskeleton was invented by eukaryotic cells during evolution, although homologous proteins have been found in prokaryotic cells.
Organization of the three main components of the cytoskeleton in animal cells. Actin filaments are found near the plasma membrane, microtubules are organized radially from the centrosome, and intermediate filaments are anchored to cell junctions and some of them being found inside the nucleus. The general organization of the cytoskeleton may change depending on the cell type and physiological state. In plant cells, the organization of cytoskeleton is completely different than in animal cells.
Cytoskeleton is composed of three types of filaments: actin filaments or microfilaments, microtubules, and intermediate filaments. Actin filaments, polymers of repeated units of actin protein, are in charge of cell movements, endocytosis, phagocytosis, cytokinesis, and more. They also are part of the molecular machinery needed for muscle contraction, and contribute to form some cell junctions (adherent junctions and tight junctions). They are named as microfilaments because their diameter is lower than those of the other cytoskeleton components. Microtubules, as the name suggests, are tubules made up of dimers of α- and βubulin. Microtubules are needed for the intracellular movement of organelles and vesicles, constitute the skeleton of cilia and flagella, drive the chromosome segregation during cell division, etcetera. Actin filaments and microtubules are used by motor proteins, which are able to carry cargoes using these filaments as rails. Cargoes may be chromosomes, organelles, or macromolecular complexes. Intermediate filaments are responsible for the cell integrity, since they function as intracellular strong cables anchored to cell junctions such as desmosomes and hemidesmosomes. They make possible the adhesion between contiguous cells and cell-extracellular matrix, contributing to the cohesion of tissues. They are specialists in withstand mechanical forces. Unlike the other components of the cytoskeleton, intermediate filaments are polymers that can be made up of different families of proteins, such as keratins, vimentins, laminas, etcectera.
|« Cytosol||Actin filaments|
Updated: 28-01-2018. 15:16
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo