Hey guys, Dr. Brown here with another fascinating video about the human body
and histology, and in this case we're going to be talking about the protein
collagen. Now collagen, in addition to being the most abundant fiber protein in
the extracellular matrix of connective tissues, is also the most abundant
protein in the human body. You are in fact about 30 percent dry weight
collagen alone. So if we were to like take all the water out of you 30 percent
of what's left would be collagen. Collagen is actually not a single
individual protein it is a family of proteins, and we're going to be talking
about several of the members of this family of proteins, and collagen scum in
three varieties fiber forming collagen, fiber associated collagen, and network
forming collagen. And these fiber forming collagen are the ones we're going to
spend the most time on, because these are some of the strongest biological
structures in nature. In fact the collagen from animal tendons,
what we call sinew, is so strong that it was used to form the torsion springs in
ancient siege weapons like catapults, which mean we can admit our the inferior
siege weapon. The most important fibril forming collagen and indeed the most
abundant of all the collagen is what's known as type 1 collagen. Now type 1
collagen actually starts out as three individual monomers, which then form a
heterodimer known as troppo collagen. This is about a
90 nanometer long protein which will spontaneously assemble, integrating in a
six wide fibril called the collagen fibril. So as a note just when you see
fibril note that that means that something small that's then gonna be
assembled or bundled together to make a larger fiber structure known as a fiber.
You can notice some things about these fibers. There's a gap in between
individual troppo collagen trimers. My thing is acting up really bad today. Okay
here we go. So here you can see there's a little gap right there. Now because these
gaps will occasionally occur on both edges of the collagen fibril you'll
notice the fibril kind of dips in. Right. The
fireballs coming along like this and it dips in and comes back up and the same
thing happens over here. And when viewed in the electron microscope you can see
these gaps in the collagen fibrils here, and here,
etc. So you get this very distinct striated appearance in the electron
microscope. Now collagen fibers can contain varying
amounts of collagen fibrils depending on the thickness of fiber that's needed in
any given tissue. Let's look at a few other collagen of note before we wrap
up this video. Type 2 collagen is found primarily in cartilage, and type 2
collagen is going to form a much less dense Network. It's gonna form these
looser fibers, which are going to then have space filled in with substances
that we normally associate with the ground substance, like these proteoglycan
aggregates and fibronectin. And this space is all going to be filled
in here by these highly glycosylated compounds, which are then going to
attract water, and the combination of the loose type 2 collagen fibers, and the
extra water and the space between those fibers is what gives cartilage it's kind
of special springiness. Now there are some other fibril forming collagen that
aren't the bulk of the fiber, but instead are going to help integrate with the
type 1 and type 2 colleges to stabilize the network, and those are types 5 & 6. So
type 5 collagen integrates with type 1... and sorry that's not a 6 it's an 11...
..type 11 collagen is gonna integrate with collagen type 2. Couple other halogens of
note that are really interesting and important for us as we continue to study
connective tissues are the type 3 collagen, which form reticular fibers. And
those are these dark fibers that we can see in here. These fibers appear dark in
this light micrograph, because it's been stained with a silver containing dye. So
type 3 reticular fibers of what we call argyrophilic, meaning they have a fond
they have an affinity for silver. These reticular
fibers are going to be found primarily in expansible tissues like say the
walls of an artery, and these along with elastic fibers, which we're going to
learn about in a little bit, and type one collagen are going to give the artery
wall a mixture of strength and elasticity, so that when that artery is
pressurized by the contraction of the ventricles of the heart it can expand,
but then once that pressure goes away when the ventricles relax the elasticity
imparted by these three different components of the extracellular matrix
will allow the artery wall to return to its original size. And then finally we
have type 4 or network forming collagen, and I don't have a good picture
of that, it's not visible in the light microscope, and even in the EM it's not
very impressive, but what's interesting about type 4 or network forming
collagen is that this is the collagen that makes up the primary components of
the basal lamina. And so as you might recall the basal lamina is secreted not
by fibroblasts down in the in the reticular layer, but the basal lamina is
secreted by the epithelial cells above. So this is the only collagen that is
abundantly secreted by epithelial tissue instead of connective tissue, like all
the rest. So that's our little overview of collagen. I hope you had a good time.
Study hard. Have fun. See you later.