Epithelial tissue is one of the four types
of tissue in the human body, and we find it everywhere from the inside of our hearts to
every inch of your skin. In this video, I’ll teach you how to look
at the histology of the different types of epithelial cells so you can see how microscopic
anatomy influences anatomy we can see with the naked eye. If you’re new to the channel, welcome, my
name is Patrick and this channel is all about anatomy and how we learn about it. As always, I have the accompanying notes for
this video linked in the description if you want to check those out. Otherwise, let’s get started. Most of our body mass is made of muscle and
connective tissue like bone, fat, and liquids. Meanwhile, epithelial tissue, or epithelium,
doesn’t make up a ton of your body mass but still shows up in crucial spots around
the body and does a handful of important jobs. Epithelium’s two main jobs are number one,
forming layers of cells that cover internal and external surfaces like the lining of your
blood vessels and skin. And number two, secreting different substances
either within the body or outside the body. In some cases they’re responsible for forming
the functional bulk of certain organs, what’s called parenchyma. For instance, the liver is 80% epithelial
liver cell, or hepatocyte, by mass. Plus, epithelium can come in all kinds of
different specialized tissue types that lets them do other unique jobs depending on where
we find them, but most of the time we care about them as protective layers and secreting
cells. So when we look at image of epithelial tissue,
we have a big challenge: Epithelium is usually mixed in with all kinds of different tissue
types, so our first job is to get our bearings and identify the tissue of interest. The cool thing about epithelial cells though
is that they’re polar — they have distinct top and bottom poles that are oriented around
a basement membrane which separates it from the structures around it. We’d call the side that touches the basement
membrane the basal pole while the opposite side, the apical pole, faces the lumen, or
the inside of a tube like a blood vessel or bronchi. We can put all kinds of fun stuff on the apical
surfaces, like cilia, little hairlike things that wiggle back and forth — the lungs are
full of them. Underneath the basement membrane is the lamina
propria, a layer of connective tissue that has all kinds of blood vessels that feeds
the cells above it. inally, epithelial cells have lateral faces
on their sides, which lets them communicate between neighboring cells. That might be through gap junctions which
let really tiny particles like ions through, tight junctions which let a few specific proteins
through, or desmosomes which help anchor cells together. This organization of basement membrane, lamina
propria, then epithelium is a big deal because epithelial tissue itself is avascular, and
it gets its oxygen and nutrients by diffusion from the capillaries in that layer, as well
as a supply of stem cells to replace old epithelial cells. Our skin’s epithelium does this really quickly,
which is why it grows back so fast if we scrape off a layer. Let’s see what that looks like on a real
slide though. This is a slide of tissue from the respiratory
system. We can see our epithelial cells on top. They’re those bright pink cells with the
cilia sticking out of their apical side. Underneath that is their basement membrane
— it’s really thin. And then underneath that is the lamina propria. We see all those blood vessels, we see some
connective tissue, and it’s much thicker than the epithelium on top of it. The fact that we have a straightforward organization
gives us our classification system for epithelial cells. We name the layer style followed by the cell
shape. First up are squamous cells which are squished
flat like pancakes. Because of that, they have a squishy, oval
shaped nucleus and not a whole lot of organelles within them. We also have cuboidal, or cubed shaped epithelium,
which typically have large, round nuclei and plenty of organelles. They can also be columnar, or column shaped. A lot of those specialized epithelial cells
like mucus secreting cells are columnar. Once we know the shape of the cell, we need
to arrange them on the basement membrane. That part of their organization makes up the
first part of their name. If it’s organized into a single sheet, it’s
called simple epithelium. So you could have a simple squamous epithelium
which lines blood vessels. It’s thinness makes it ideal for when you
need to pass /some/ substances like oxygen through but still want it to keep blood contained. You can have simple cuboidal for secretion
of substances like in the seminiferous tubules in the testis, or you can have simple columnar
like in the walls of the gastrointestinal tract that can also secrete and absorb things. Either way, when we say simple epithelium
we’re talking about a single layer. The caveat is pseudostratified epithelium,
which is technically a type of simple columnar tissue, but it looks like it’s stacked into
multiple layers. The analogy I use is a group of 10 year olds
lining up after recess. They’re different heights, so it looks like
some of their heads are in different rows, but all of their feet are on the same ground. Just like how all of the columnar cells attach
to the same basement membrane, but they’re different heights, so they don’t all poke
up to the surface. Because of that, they’re pseudostratified. Our last organization, stratified epithelium,
happens anytime you have two or more layers of epithelial cells. You can have stratified simple, stratified
cuboidal, and stratified columnar. Now, remember how I said the epithelium gets
its nutrients from diffusion from those blood vessels deep under it? Well, the more cells you stack on the basement
membrane, the farther away they get from that source of nutrients. So we find a special type of stratified epithelium
called keratinized epithelium, which is what it sounds like. Those epithelial cells are far away from the
lamina propria, they die, they lose their nucleus and get filled with the tough protein
keratin. If you’ve heard that your top layer of skin
is all dead cells, it’s true, because it’s this stuff. The last type of stratified epithelium is
transitional epithelium — stacks of different types of epithelial cells that transition
from one cell type to another. We mostly see this in anatomy that needs to
stretch — like this is bladder tissue, what sometimes gets called urothelium. You can see a clear layer of transitional
epithelium here since the bladder is an organ that needs to stretch. So quick summary. When you have a histology slide in front of
you, remember to first get your bearings. Find the basement membrane and find the apical
side, the edge of tissue that’s closest to empty space, and you can start picking
out cell shape and layer organization. But epithelium has a different job that we
still need to talk about. Remember how these epithelial cells make up
the lining of blood vessels and hollow tubes? Well that means they’re constantly bumping
into the dissolved particles in the blood, so they’re great targets to either pump
things into the bloodstream or receive messages from the bloodstream. We call these kinds of cells glandular epithelium,
and when we get a bunch of them together, they can form full organs like the thyroid
gland. We divide glands further based on how and
where they release their message. They might send and receive messages outside
the body, what are called exocrine glands, or within the body, what are called endocrine
glands. If that name sounds familiar, it’s because
the endocrine system is responsible for hormones. When endocrine glands secrete hormones, they
travel through the bloodstream, until they arrive at its target tissue, then they communicate
their message. And these glands can show up in a lot of places. Like here’s the pineal gland in the brain. It’s home to nervous tissue like astrocytes,
but also to epithelial cells called pinealocytes that pump out the hormone melatonin. You also have endocrine cells alongside other
tissues. For instance, the cells responsible for making
sperm are in the testes, but so are different cells called Leydig cells that secrete testosterone
from the testes into the bloodstream. But some glands don’t pump things into the
body at all, exocrine glands pump them outside the body or into different cavities within
the body. For instance, goblet cells in the GI tract
secrete a protective layer of mucus out into the intestine, which is technically outside
of our bodies. Or salivary glands — they secrete a bunch
of enzymes and proteins into the saliva. Those enzymes aren’t communicating anything
like a hormone would, so it’s an exocrine gland. Another example are apocrine glands, a subtype
of exocrine gland that gives off odorants. Obviously, something that carries a smell
is more useful if it travels outside the body to a nose, so it’s an exocrine gland, not
endocrine. One of the main places we see epithelial cells
is in the digestive system, and I’ll have a video right here all about digestive histology. Otherwise, I’m building a whole histology
playlist that you can find here with lots of helpful videos. And hey, if you found this video helpful,
consider helping me pay my bills with Patreon or by sharing this video with an anatomy student
in need. Have fun, be good, thanks for watching.
