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.