Hey y'all. A lot of people think that eukaryotic cells are just kind of these
amorphous, squishy, disorganized, bags of goo,
but I'm here to tell you nothing could be further from the truth. In fact most
eukaryotic cells contain a complex network of tubes and filaments that
provide the structural scaffolding on which your organelles, macromolecular
complexes, vesicles, etc. are hung and organized and able to move around inside
the cell, and we call this collection of tubes and filaments the cytoskeleton.
There are three essential components of the cytoskeleton. The first of these are
microtubules. Microtubules are heterodimers, meaning two different
monomers alpha tubulin and beta tubulin, assemble into a dimer which then further
polymerizes to give us our microtubule. Because of the asymmetry that we see
here on this face of the beta tubulin molecule, as the dimer stack up on top of
each other they're going to form this kind of spiral staircase or helix, and in
fact it takes 13 heterodimers to make a single turn of the helix. If you were to
look over here at the single microtubule that we see in the structure, which we're
going to talk about in a second, you would see that there are 13 individual
heterodimers poking out in this cross-section. Microtubules, while the
largest of the components of the cytoskeleton are still only 24
nanometers across, so in the world of ultrastructure
that's actually kind of big. There are two main assemblages of microtubules
that you will see in eukaryotic cells. The first of these, the one i've been
drawing on in this ultram...in this transmission electron micrograph on the
left, it's called an axoneme. The axoneme is a collection of microtubules in a very
particular pattern which we call the 9 plus 2 arrangement. We call it this, not
surprisingly, because there are nine micro tubular doublets arranged in the
outer circle, plus two singlet microtubules in the middle. The axoneme
forms the main core structure of the motile organelles ,flagella and cilia.
if you look at a spermatozoon, that is the swimming mature sperm cell. You
will find an axoneme team running down the length of that tail, that's what allows
it to move. If you look in the cilia found say in your upper respiratory
tract or in the female reproductive system, you would find axonemes inside
those cilia, which allow them to move, and impart current in fluids.
The other main microtubular arrangement that we find in the eukaryotic cell is
the centriole. While similar to the axoneme it does have some distinct differences.
The centriole has what we call a 9 + 0 arrangement. So there are nine sets of
microtubules in the ring around here. The other thing that's different is that
if you look carefully there are three microtubules in each set of microtubules.
So it's nine microtubular triplets instead of doublets in the centriole. You
probably know centrioles from cell division, specifically from mitosis or
nuclear division, and that's because the centrioles form what are called
microtubular organizing centers or MTOCS. These are going to organize the
microtubules that become the spindle fibers that are actually going to
separate chromosomes during mitosis, and facilitate equal splitting of DNA
between the two daughter cells. Microtubules are the largest of
cytoskeletal component. The smallest are microfilaments. Unlike microtubules,
microfilaments are mono polymers of the protein actin.
They are microfilaments, they are the smallest component because they are 5 to
7 nanometers wide compare that to 24 in a microtubule. Microfilaments
are going to do all sorts of important things for our body not limited to, but
including: (1) muscle contraction, microfilaments form the thin filaments
inside muscle cells, (2) cytoplasmic streaming, if you look and see organelles
vesicles especially other small particles being moved inside a cell
that's typically facilitated by microfilaments, (3) the formation of an
endocytosis vesicle is caused by the assembly and disassembly of microfilaments at the
plasma membrane, and while microtubules are responsible for separating
chromosomes during mitosis, it's acts
the act and microfilaments that divide the parent cell into the two daughter
cells during cytokinesis. Microtubules are the largest cell...are the largest
filament in the cell, microfilaments are the smallest in the cell and in between
them we have the cryptically named intermediate filament. Intermediate
filaments come in about two dozen varieties for our sake the most
important are keratins, which you know quite well because these are the
filamentous proteins that give your hair and nails their structure, desmond which
is really important in muscle tissue it's also important in certain cell to
cell junctions, so how we hold two cells together, and finally the one we're
looking at here at the end of the blue arrow neurofilaments. Neurofilaments
provide structure to neural appendages such as the dendrites on these Purkinje
cells in the image on the right. Now intermediate filament come intermediate
filaments come in a number of varieties, and we'll cross those bridges when we
get there, but for now just know that any cytoskeletal structure that is
intermediate in size between a micro filament and a microtubule is going to
be classified as an intermediate filament. Well that's all for now. Have
fun. Study hard. See you later.