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The cytoskeleton of a cell plays a vital role in controlling the movements of the cell. 3 types of fibres form this structure; microtubules, microfilaments and intermediate filaments. Microtubules are large hollow fibres, microfilaments are the smallest at only ‘6nm in diameter’ (1) and intermediate filaments are in between.
Microtubules are formed from the subunits α – tubulin and β – tubulin, which form dimers. Each tubulin dimer contains 1 α – tubulin subunit and 1 β – tubulin subunit. These then polymerise to form long protofilaments, which join together into sheets. Once a sheet of 13 protofilaments has been formed, they then fold over to produce a hollow microtubule, which continues to elongate from the +ve end where only β – tubulin subunits are showing.
Microfilaments are made of a protein subunit called actin. There are two types; G – actin and F – actin. G – actin is turned in to F – actin by the hydrolysis of ATP, forming a fibrous filament. The actin filament continues to elongate from the -ve barbed end.
There are several types of Intermediate filaments, each made of different proteins depending on the function of the cell in which they are contained. Type I Keratins acidic and Type II Keratins basic are found in epithelial cells and in the hair and nails. Nuclear lamins are found in the nuclear lamina and type III vimentin/desmin/peripherin are found in muscle cells, some neurones and stem cells (2). The monomers form dimers by coiling. Tetramers are then formed from dimers arranged in staggered formation with opposite domains together. These then arrange together forming strong coiled filaments.
‘Many cells move by crawling over surfaces’ (3). Of the three fibrous structures, explained previously that make up a cell, the microfilaments play the largest part in the movement and migration of a cell. This means that actin is the vital component for cell movement. A fibroblast crawls by a leading protrusion, called a lamellipodium, which is where most of the cell’s actin is located. The actin is extremely flexible due its twisted, linked structure. The filaments clump / bundle together near the cell membrane, particularly at the protrusions such as the microvilli in an intestine lining epithelial cell. The actin penetrates in to the cytoplasm, ‘ where they become cross – linked in to a three dimensional meshwork, governing the shape and mechanical properties of the plasma membrane and the cell surface.’ (4) . This means that the actin provides structural support, as well as movement potential for the cell. Filopodia are very similar to lamellipodia. They also protrude out from the main body of the cells like lamellipodia, and act like fingers for the cell to ‘feel’ the environment around it and also to aid the detection of where the cell is supposed to migrate to. Filopodia have almost the exact same structure as lemellipodia, however have smaller protrusions and therefore contain less actin. ‘They are about 0.1 µm wide and between 5 – 10 µm long, and each contains a loose bundle of 10 – 20 actin filaments, orientated with their positive ends pointing outward'(5), exactly like the orientation of the actin filaments in lamellipodia. However, filopodia are present around the whole circumference of the cell, not just protruding from the lamellipodium, which creates an even larger surface area for the cell to detect its surroundings. The protrusions ‘grow’ by the growth of the actin filaments, where dimers are added to the positive terminals of the filaments. Although dimers are removed from both terminals, particularly the negative end, this is outweighed by the more rapid addition of actin at the positive end. The growth is a very quick process, which therefore allows the cells to move around the body at a fairly fast pace. Once the lamellipodium is protruded from the main structural shape of the cell, the newly produced bottom section of the cell then adheres to the surface it is moving along, hence pulling the rest of the cell along with it. At the same time, ‘contraction occurs at the rear of the cell, then draws the body of the cell forward, in the direction the lamellipodium is protruding, in a process called traction.’ (6)
Cell migration is extremely important to an organism’s survival. It is essential for the movement of cells in the immune system, an example of which is a macrophage, which finds destructive cells and ‘eats’ them. This is because it is vital for cells to be able to travel to the site of an infection in order to fight it and clear it up or to communicate with other cells, to let them know that there is a problem, which they can then sort out. Cells that can do this are called ‘fibroblasts, which migrate through connective tissue, remodelling them where necessary and helping to rebuild damaged structures.’ (7). If this was not possible, the organism would be in serious trouble and could die or be badly damaged by very superficial wounds and mild diseases which are overlooked as non-serious in humans due to cells being able to migrate. The cell migration mechanism is vital to a macrophage, as its job is to move around the body, detecting and destroying harmful cells. If movement was not possible, the macrophage would only be able to detect destructive cells that were situated in the immediate vicinity, which means to successfully remove all harmful pathogens would be impossible. This would mean the organism would have very little protection against disease.
Cells are also able to migrate by means other than by lamellipodium protrusions. Cilia are one example, as well as a male human’s sperm, which moves by a tail like structure called a flagellum. The sperm is able to beat the flagellum, which is ‘designed to move the entire cell, and instead of generating a current, they propagate regular waves along their length that propel and drive the cell through liquid’ (8). Unlike most other migrating cells in the human body as described above, the main component for movement in flagella is tubulin. Microtubules span the whole length of the flagellum in an axoneme, ‘which contains two central microtubules that are surrounded by an outer ring of nine pairs of microtubules. ‘ (9). The movement is enabled by ‘molecules of ciliary dyenin that form bridges between neighbouring microtubules around the circumference of the axoneme.’ (10) The end tail of one molecule attaches to a microtubule, while its other end, the head of the molecule attaches to another microtubule. This promotes a sliding mechanism similar to that of actin in the migration of cells with lamellipodium protrusions.
Bacteria and cilia also have flagella, made of flagellin and dynein. The bacterial flagellum has a similar structure to a microtubule in the way that it is a hollow, tube – like shape. ‘Ciliary beating can either propel single cells through a fluid or can move fluid over the surface of a group of cells in a tissue’ (11). The second is apparent in the human respiratory system, where ciliated respiratory epithelium cells in the trachea prevent any foreign, potentially harmful particles such as dust and bacteria in the air from entering the bronchioles and lungs. They do this by acting like tiny hairs and by beating the saliva containing the harmful particles back up the trachea to exit the nasal cavities by coughing. If the cilia are unable to beat, it causes problems such as Kartagener’s syndrome or primary ciliary dyskinesia. Although this syndrome is extremely rare, it is a genetic disorder, meaning it is hereditary. Due to the respiratory system having little to no defence against dust and pathogens which enter the nasal cavities and then travel down the trachea and bronchi, harmful particles may enter the lungs. This causes infection and disease of the lungs, such as pneumonia or bronchitis.
In conclusion, the presence of cell migration mechanisms in organisms as small as bacteria to the large, multi – cellular organisms such as humans is extremely vital to their individual survival. Without such an important ability, cells would not be able to detect or fight disease, from minor superficial impediments to very serious illnesses. Reproduction in humans would not be possible and bacteria would find it extremely difficult to invade host cells for reproduction. Without cell migration, the whole human immune system would not be able to function correctly. Although the movement of cells is quite complex, it is only the beginning of a massive sequence of mechanisms in which cells can communicate with each other to orchestrate the correct workings of the human body.
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