Hey guys. Dr. Brown here with another exciting histology video. I thought for a
fun change of pace I'd let you look at my face, since actually dressed up to
come to school today. Today we're gonna be talking about
Kandra Genesis. That is how do we make cartilage tissue from its precursor
tissue mesenchyme. Also at the end of the video we're gonna briefly go over once
we make a cartilage what are the two different ways that that cartilage
tissue can then grow and expand. So let's jump right into it shall we.
Cartilage, like all true connective tissues begins as mesenchyme, and these
mesenchymal stem cells are going to be in kind of an amorphous mess, often
within some kind of membrane, and they're gonna undergo two processes on their way
to becoming cartilage tissue. So they're gonna first proliferate. Proliferation
meaning that the cells are going to multiply, we're going to get more of them.
They're also going to differentiate, meaning they are going to shift from
being a mesenchyme cell, which is not specialized, to become a specialized
cartilage matrix secreting cell or a condor blast.
So the mesenchymal stem cells will undergo both proliferation and
differentiation to become early chondroblasts. So you can see that we
have more cells, and the cells have changed their shape, and remember that
suffix "blast". We saw that with fibroblasts in connective tissues proper.
That suffix blast indicates a cell that is very metabolically active, and a cell
that is rapidly secreting. Speaking of which these early chondroblasts are
going to begin secreting the extracellular matrix characteristic of
cartilage tissue, and you see the matrix starting to fill in between the
chondroblasts. Now as these contra blasts continue to secrete this extracellular
matrix, the build-up of matrix in between the cells is going to push the cells
apart. So as they build matrix up, as the cells produce matrix and the place in
between them fills up with matrix, the cells are going to be kind of pushed
apart. So what we end up with in is mature cartilage cells are chondrocytes
completely surrounded with matrix. Now when this happens their
metabolism is going to slow way down. Like if we just pick a random cell, let's
say this guy right here. So here's the chondrocyte, remember as this metabolism
slows in its growth rate slows it goes from being a contra blast to a
chondrocyte, and this chondrocyte is in it's empty a little pocket within the
matrix called the lacunae. So you probably remember the lacunae, but not to
be confused with hakuna, the latter is timon and pumba, the former cartilage
cells. Sorry...dad joke. Anyway remember that this cell in order to get oxygen
and to get nutrients, those...those gases and nutrients have to diffuse through
the cartilage matrix to get through the cell. So because it has limited access to
resources the cell is going to slow down in its growth and metabolism, and that's
why it's called a chondracyte. These are not however hibernating, these are not
dormant or anything like that, they're still alive they're still active, it's
just that the activity is a little bit slower, but eventually this cell will
eventually undergo division, and you get something like this. So here you can see
the formation of two daughter cells from mitosis in the parent chondracyte that
eventually...initially started out in his spot. So now
that we've made a piece of cartilage, how does it get bigger, and this is chondrogenesis.
This is making a cartilage, but it doesn't describe how that cartilage
gets bigger once it's made. So for that there are actually two kinds of
cartilage growth. The first is appositional,, and it's the word is where it
is for a reason, it's going to be towards the edge of a piece of cartilage. And
then we have interstitial growth, which occurs deeper down within the cartilage.
So most cartilages are surrounded by a layer of connective tissue known as the
perichondrium. So in this image the perichondrium is this layer of stuff
right here, and that perichondrium is going to contain a number of semi
differentiated mesenchyme cells known as chondroprogenitor cells. And these
progenitor cells will eventually complete their differentiation and
become chondroblasts that see here. These chondroblasts will
rapidly secrete cartilage matrix, and the cartilage will continue to grow. So if
the chondroblasts are secreting matrix this way the whole cartilage is going to
be growing that way. So it's appositional growth. It's from the perichondrium it's
facilitated by chondroblasts which makes it very rapid. So appositional growth is
rapid from the edge of the tissue, the perichondrium, and it's fast. Okay.
Interstitial growth, what's happening to lower down in the cartilage, this is
coming from the interior, and it is much slower, because it's being carried out by
chondrocytes. So even though this chondrocyte is completely surrounded by matrix,
and getting nutrients in and waste out is difficult, it is still very slowly
able...(inaudible)...it's very slowly able to secrete small amounts of matrix.
Okay. Now we mentioned in the last picture that these cells will eventually
grow enough that they undergo mitosis, and we can see here that one daughter
cell has divided...or one parent cell has divided into two daughter cells. Over
here we can see where cell division has taken to...taking place twice in the same
lacunae, and we call these groups of daughter cells, these clones of cells
that resulted from the single parent cell, Isogenous groups. Iso- meaning the
same, -genous meaning beginning. So genous means coming from the same
beginning. So these groups come from the same beginning. So we start with mesenchyme,
we proliferate and differentiate into
early chondroblasts, the chondroblasts secrete cartilage matrix. They do so, so
quickly that they eventually get surrounded by matrix their metabolism
slows way down, and they become chondrocytes. From that point on the
cartilage can either grow from the edge rapidly, via appositional growth carried
out by chondroblasts, or from deep within, but much more slowly carried out by
chondrobl...chondrocytes. Almost said the wrong word there. Alright, well that's
it for now. I hope you had fun. Study hard.