Hey y'all. So far we've learned about things on the apical surface of our
epithelial cells. We learned about microvilli and cilia. We've learned about cell-
cell junctions on the lateral surfaces of our epithelial cells, desmosomes, and
other cell-cell connections like gap junctions, and tight junctions, etc. And so
we're gonna wrap up by talking about what's on the bottom of an epithelial
cell, and that is the basement membrane. So let's walk through a little
terminology here real quick. You can see our epithelial cells appear
at the top, and then the fibers underneath our connective tissue fibers.
Primarily collagen type 3, but there are also some other intermediate filaments
in here, and all these fibers are secreted by these cells down here called
fibroblasts. So this is a fibroblast down here, and immediately underneath the
epithelial cells we see what's known as the basal lamina. And this little "em"
under here indicates that the basal lamina is only visible in the electron
microscope. This layer of tissue is not something that we can see in the white
microscope, but in the electron microscope it's gonna appear as two
distinct layers: a lamina lucida that's right in here lucid meaning clear and
this one's not always present, but then there's gonna be the lamina densa, which
is the main part of the basal lamina here, and that's going to contain mostly
collagen type 4 in an intermediate filament known as lebanon. And this part
is actually secreted by the epithelial cells. So the basal lamina here is
secreted by the epithelial cells, and then this forms the connection to the
underlying connective tissue down here, secreted by these fibroblasts, which
we're going to learn more about in the next unit.
So the basal lamina is here these connective tissue fibers are sometimes
referred to as the reticular lamina. You're gonna see that word quite a lot,
and, you know, you know, about the endoplasmic reticulum. Ri-​ˈti-​kyə-​lə.
Pardon the handwriting. I'm tell you right now it's hard to write on this thing, but
that just means an interconnected meshwork, you know, you can think of
network, or a meshwork of stuff. And that's all
this stuff down here. So that's called the reticular lamina. So all of these
things together combined...comprise what we know as the basement membrane, and
basically basement membrane just means the stuff that you can see in the light
microscope. And it's usually formed either by having two basal lamina next
to each other; so if you have two of these things like if you had another
epithelium right under here directly, so if there were another basal lamina right
under here the two of them together might be visible in the light microscope,
but more often than not the basement membrane is the basal lamina from
epithelial cells, and some bit of this reticular lamina coming from the
underlying connective tissue. And this is visualized in the light microscope using
a stain known as the periodic acid shift stain. Shift being someone's named.
Finally what does this thing do? Well two main functions: (1) one support you will
often see the this basement membrane referred to as the lamina propria, the
layer that props up, and it in this case it can be doing that, (2) the other thing
that the basal lamina can be doing is regulation. So the basal lamina can
control the access of macromolecules to the epithelial cells above. Interestingly
the basal lamina is also going to often have the binding sites for growth
factors. So as the epithelial tissue is developing the growth factors that tell
it how to develop it more importantly in which direction to and develop are going
to be found on the basal surface of the cell, as part of that basal lamina. And
finally the basal lamina is going to participate in certain cell-cell
interactions, particularly in nerve reinnervation. So if a nerve is severed
as the the nerve fibers from the cell try to go back down into the nerve
sheath a lot of that's going to be facilitated by the basement membrane. So
just a little quickie lesson on the basement membrane. Study hard. Have fun.
See you later. Bye.