- [Voiceover] There are four different types
of animal tissue that are all made up of eukaryotic cells.
Epithelial tissue, connective tissue,
muscle tissue and nervous tissue.
In this video we're gonna talk about
epithelial tissue and connective tissue.
When you think about epithelial tissue
you can think about it as a lining.
Both an inner lining and an outer lining.
So for example, epithelial tissue makes up
the outer layer of our skin.
It makes up the outer layer of organs.
It lines organs so the lumen of organs
will be lined with epithelial tissue
and it also lines the inside of cavities,
inside of the cavities of the organism.
Epithelial tissue also make up glands
so that would include both exocrine glands
and endocrine glands.
And just to remind you,
exocrine glands will release their substances
directly to the target organ.
Whereas endocrine glands usually release hormones
but into the bloodstream,
not to the target organ directly.
And epithelial tissue comes in two forms.
It can be simple
and that means it's one layer thick.
Or it can be stratified which means
it can have two or more layers.
And where will you expect to find simple epithelium
versus stratified epithelium.
Well, you'll find simple epithelium
in places where substances need to diffuse
from two different places.
For example the alveoli of the lungs
are lined with simple epithelium
because carbon dioxide and oxygen
need to diffuse from the alveoli
into the bloodstream and vice versa.
And of course that will be pretty difficult
if you had a thick layer of cells.
And you'd expect to find stratified epithelium
in places that need to resist
chemical or a mechanical stress.
For example, the esophagus
is lined with stratified epithelium.
That's because the esophagus will have food
coming through it.
The food might be sharp, it might be hot
and we want a thick layer of cells
to protect the underlying tissue of the esophagus.
A stratified layer epithelium
acts as that protective layer.
Let's take a look at a section of simple epithelium
and epithelial cells are attached
to something known as the basement membrane.
The basement membrane is not made up of cells
but rather it's made up of different types of fibers.
For example one fiber
that can be found in the basement membrane is collagen.
And the basement membrane is semipermeable
to certain substances and that's pretty important
because epithelial tissue is avascular.
That means that epithelial cells have no blood vessels
which then makes us ask the question of
how do they get nutrients?
They get nutrients from the underlying tissue.
What happens is that nutrients will diffuse
from the underlying tissue through the basement membrane
to the epithelial cells.
And that's how epithelial cells get their nutrients.
Let's just recap some of the places
that you'd expect to find epithelial cells.
We already mentioned the outer layer of the skin,
the tissue lining the mouth, esophagus and GI tract
and of course this is not an exhaust of list.
In the tissue lining the kidney tubules,
and the tissue lining blood and lymphatic vessels.
And in fact the tissue that lines
blood vessels and lymphatic vessels has a special name.
It's known as endothelium.
Let's talk about connective tissue.
Connective tissue supports tissues, connects tissues
and separates different types of tissues from each other
and then there are different types of connective tissue
that don't necessarily fall into these neat categories.
What are some examples of connective tissue?
Bones, cartilage, blood,
lymph, adipose tissue which is fat.
The membranes covering the brain and the spinal cord
and other types of tissues.
What does connective tissue look like?
What are some characteristics of connective tissue?
Basically it has three components.
It has cells.
It has what's known as a ground substance
and then it has fibers.
The ground substance and the fibers together
make up a matrix.
Let's see what this looks like.
Here we have the ground substance
which is usually a viscous type of fluid.
Then interspersed in the ground substance are fibers
and then we have cells,
and these cells are usually what is producing the matrix.
Let's look at some connective tissue in more detail.
The first we'll talk about is areolar tissue
which is this tissue right over here.
Areolar tissue is a very common type of connective tissue.
It binds together different types of tissue
and it provides flexibility and cushioning,
and we can actually see the structure
pretty clearly in this picture.
You can see the cells over there, those little dots.
There's no cell over here.
You can see the fibers running through it.
Over here and over here.
And then the ground substance
is the kind of background yellow,
viscous liquid that you're seeing.
Then we have adipose tissue.
Adipose tissue is basically fat, tissue is a fat.
it provides cushioning for the body,
it stored energy
and it actually is an exception to the rule.
It does not have fibers like most other connective tissue.
Then we have what's called fibrous connective tissue.
Fibrous connective tissue is pretty strong.
It provides support and shock absorption
for bones and organs,
and you find it in the dermis
which is the middle layer of the skin,
tendons and ligaments.
Here are some more types of connective tissue.
We have blood.
Blood is also an exception like adipose tissue
and that it does not contain fibers.
And the matrix of blood is the plasma
and you can see the matrix, this yellowish liquid
in which the blood cells are suspended.
Then we have osseous tissue or bone tissue.
These cells in osseous tissue are known as osteocytes.
Those are those brown cells
that are kind of forming a pattern
and the matrix in osseous tissue
is what's known as bone mineral or hydroxyapatite,
which is basically collagen fibers
with different minerals like phosphates,
magnesium, calcium, et cetera.
And then we have hyaline cartilage.
The cells in hyaline cartilage are chondrocytes.
You can see them over here,
there are a bunch of these cells.
And they're found in surfaces of joints.
These are all examples
of different types of connective tissue
and many of them provide some form or another
of support for tissues and organs.