Eukaryotic cytoskeleton shown using fluoresecence microscopy Actin filaments are shown in red, microtubules are green and the blue is the nucleus. Picture from
If one consults any of the standard developmental biology textbooks, the cytoskeleton will be briefly presented as the support structure of the cell and about as interesting to most biologists as the floor struts in a ballet school would be to an artistic director seeking the next prima ballerina. However, to think of cytoskeleton only as the structure on which cells do their remarkable things, leaves out a good part of the story of how cells work.
We prefer to think of the cytoskeleton as a troupe of acrobats. They run about in the cell, come together, stack to form some remarkable structure and then as rapidly as it appears, the structure falls apart and/or moves and the acrobats appear elsewhere on the stage. These acrobats are the individual proteins that make up the cytoskeleton, and the many proteins that bind to cytoskeleton, transiently hold it together, or move along it. The surface area available on the cytoskeleton filaments for molecules to electrostatically and chemically bind is huge, far exceeding that of all cell membranes in an organism. Ignoring the cytoskeleton while doing molecular biology would be like trying to choreograph a classical ballet on the same stage that has a troupe from Cirque de Soleil rehearsing.
This unfortunate attitude towards the cytoskeleton is beginning to change. For example, several of the pharmaceutical scientific supply companies have recently begun manufacturing kits designed specifically to detect the phenomenon of “cytoskeletal rearrangements” because all kinds of interesting other stuff seems to happen precisely when these rearrangements occur and therefore the rearrangements can be used to time such events. We find it ironic that only a few people seem to actually think this is more than a convenient coincidence.
The cytoskeleton is, as the name implies, the skeleton of the cell giving it shape and form. The concept goes back to Nikolai K. Koltsov in 1903. Koltsov formulated the idea that the deviation of the shape of a cell from the simple ball, that one would expect if it were a liquid drop, is caused by a stiff but elastic cytoskeleton within the cell. Cytoskeleton is nowadays more precisely defined as the various structures that are filamentous polymers of a single class of protein. These filamentous polymers have long-range order within the cell. They should have been called metapolymers, because the proteins they are made of are themselves polymers of amino acids. These cytoskeletal polymers are commonly classified as supramolecular structures. They are polymers of polymers, roughly topologically linear in their structure.
There are three types of cytoskeletal filaments, arranged in order of decreasing diameter as seen by transmission electron microscopy: microtubules (MTs, 25 nm), intermediate filaments (IFs, 11 nm), and microfilaments (MFs, 7 nm) (nm = nanometer = 10‑9 meter = 10‑3 µm). For things this small, we find it convenient to think in terms of the size of the hydrogen atom, whose diameter is 0.1 nm = 1 Å (Angstrom), so a microtubule, for instance, is 250 hydrogen atoms wide. For something so thin, microtubules can be incredibly long, with reported lengths up to 68 µm (micrometers). In Drosophila (fruit fly) species, the length of the sperm can be 5.8 cm (58000 µm), implying equally long microtubules. Such microtubules are therefore over 2 million times longer than their diameter. If we imagine a microtubule as a piece of cooked spaghetti with a width of 3 mm then the length of the spaghetti would be as long as 7 kilometers! The three cytoskeletal filaments are found throughout the cell. They are connected to each other at many points, as well as to the inner surface of the cell membrane , to the outer nuclear membrane and to the inner surface of the nucleus via specialized attachment proteins.
There is a growing awareness of other proteins that may well also be cytoskeletal in nature and deserving of addition to this triumvirate. The cytoskeleton, as a concept, can also go beyond just filaments to include structural sheets, patches and meshes. For example, septins, which are found in animals, are an example of these other cytoskeletal-like proteins. Septins use a protein called anillin to attach to microfilament rings and thereby form rings themselves. These rings line the inner surface of the cell membrane, increasing the membrane’s rigidity. Another example is CTP synthase, which forms filaments in bacteria, yeast and animals. We will nevertheless concentrate on the three best known, best characterized and universally accepted cytoskeletal components. However, the reader should keep in mind that there are likely to be more types of cytoskeletal-like structures that need to be considered in embryogenesis. We will also focus on the mechanical properties of cytoskeleton. There are poorly understood electrical and related potential memory/epigenetic properties of cytoskeleton. These properties may also become important in explaining aspects of embryogenesis.
The cytoskeleton is an extremely dynamic structure that can depolymerize, move and repolymerize within the cytoplasm in a matter of minutes. The three components are often interlinked in a meshwork that is required for cell movements, maintaining or altering cell shape, and for organizing and powering mitosis and meiosis. The cytoskeleton is required for transporting and sequestering proteins to specific regions of the cell. Cytoskeletal associated proteins act as levers and motors pulling cargo such as the vesicles involved in nerve transmission, chloroplasts and whole nuclei.
The cytoskeleton also serves as the host for a variety of enzymes. The cytoskeleton is directly involved in protein functioning of these enzymes by binding to (and thereby changing the functioning of) associated enzymes. The cytoskeleton can therefore direct a response to external physical or chemical signals in the form of movement of the cell towards or away from the signal by polymerizing or depolymerizing during cytoskeletal rearrangements. Due to the cytoskeleton, external signals can even travel down to and into the nucleus and trigger changes in gene expression.
The cytoskeleton found in the nucleus is also made of actin, microtubules and microfilaments but it requires special consideration. Bundles of microtubules ring the inner membrane of some nuclei. The nucleus contains additional unique protein attachments that connect it to the cytoskeleton. At the inner surface of the double nuclear membrane is a sheet of intermediate filaments called lamins, whose functions may include maintenance or change of nuclear shape, protection of the nucleus from mechanical shocks, and intranuclear rotation (rotation of the contents of the nucleus). Intranuclear rotation includes both nuclear membranes, but without causing the cytoplasm to rotate. Therefore there may be a layer just outside the outer nuclear membrane that has low viscosity, like the cell cortex. If this is the case, the motive force is occurring on the outside surface of the outer nuclear membrane. In some cells nuclear rotation and cell membrane rotation are coupled.
The cytoskeletal elements in the nucleus combine with a variety of nuclear proteins forming a “nuclear matrix” which acts like a scaffold on which the DNA is attached. For this reason the whole lot is referred to as the “nucleoskeleton”. The nucleoskeleton itself is then connected to the cytoskeleton that is outside the nucleus via proteins that bridge the two nuclear membranes.
These three filaments, tubulin, intermediate filaments and actin were once thought to be unique to eukaryotes. Many old textbooks included a table of differences between eukaryotes and prokaryotes and the presence of cytoskeletal elements figured prominently as a defining characteristic of the eukaryotic cell. This has proven to be incorrect, and as we showed in Chapter 1, many characteristics of eukaryotes are found in, if not inherited from, prokaryotes. Homologs of all three cytoskeletal proteins have been found in bacteria, albeit usually having different functions. There is a form of tubulin found in bacteria called FtsZ that is also required for cell division in most prokaryotes (but not all), which may represent the evolutionary precursor of all of the tubulins
.This book excerpt is introduction to Chapter 5, “The Cytoskeleton” in “Embryogenesis Explained”.