So, here we have a very basic diagram of a cell.

And I'm just going to walk you through this.

And show you four key features that we can then go on and

look for in our slide under the virtual microscope.

The first point to note is the outer membrane of the cell.

It's called the Cell membrane.

It tells us where the boundaries of that cell are.

Note that it's very easy to see this cell membrane on our little diagram here.

But, you'll find out when we look under the real microscope,

this actually can become much more challenging when we're looking at real

tissues under the microscope.

Inside the cell membrane, we have the Cytoplasm.

The Cytoplasm will stain a shade of pink.

That shade will depend on the nature of that cell.

But, within the cytoplasm,

there are lots of elements that help with the function of that particular cell.

Then, we have our Nucleus.

The Nucleus is a round or an oval structure, again,

depending on the cell and depending on the way the section has been cut.

It will stain a blue or purple kind of color.

And, it is a circular or oval structure as I see.

And then, an important point to note is that,

sitting within that Nucleus, you will often see one or more nucleoli.

So, in my diagram here, we're looking at once single Nucleolus.

That is a small,

a very small, dark blue staining structure sitting within the nucleus.

Sometimes, it can be a point of confusion for novices.

That they see a Nucleolus and

they think, that's the Nucleus and then the bit surrounding it is the Cytoplasm.

So, just watch out for that when we move on and

look at our real histology section under our virtual microscope.

So, just to summarize a few key points then, before we go on and

look at our section.

The main stain that we use for looking, at least in the first instance at

histological sections, is called Haematoxylin and Eosin or H&E.

It stains tissues various shades of pink and purple.

Some things don't stain with H&E, for example, fat.

Epithelial cells are the types of cells that perform the function of

organs in the body.

And, connective tissues act like a scaffold to support

these epithelial cells.

Okay, so what we're looking at now then is a section of skin.

And this is skin taken from a cat.

And just to orientate you just a little bit, this white area here,

this clear area, this would be the outside of the cat.

And then, we come to the surface of the skin here, the epithelial cells.

I'm going to focus in on a bit more in a minute,

this is called epidermis of the skin.

And then, we move underneath from the epidermis into a part of the skin called

the dermis.

I'll have a look at both these areas in a little more detail in a minute.

The other structures that you can see, if you're just wondering what these are.

These round circular structures,

with lots of small circles within them, these are hair follicles.

And in the cats, you have lots of hair follicles grouped together like this.

And so, that's simply what we're looking at there is a cross-section through some

hair follicles.

So, also a very important point to remember that,

when we're looking at a section like this,

this is a two-dimensional representation of a three-dimensional structure.

So, in real-life,

of course, the skin would actually be operating in all dimensions.

It would be coming through, from the screen towards us,

as well as it's going up and down the screen.

And that is sometimes why looking at these histological sections can be a little bit

challenging.

But, we're going to go up on higher power.

I'm going to point out some individual features of the cells.

So, now we're on higher power.

We can start to see, in more detail,

the individual nuclei that we've just been describing in a more schematic way.

So, what we're looking at on this power,

you can see this dark blue dot in the middle of the cell here.

That is the nucleolus, and

surrounding that is this more pale staining oval structure.

It's still a sort of delicate purple color.

This is the nucleus, and then surrounding that in pink is the cytoplasm.

Now, what's happening here is that, because these cells are so

closely packed together in the epidermis, we can't actually see

the border between one individual cell and the other in this case.

So, the cytoplasm of this cell is merging with the cytoplasm of this cell and

so on and so forth.

If we move down a little bit here, and I just move up the objectives a little bit,

we can start to see some cells here.

And this cell here has two nucleoli, okay?

It's got two of these darker blobs in it.

And then we've got the nucleus surrounding it there.

In this particular cell, we can see the cytoplasm here.

And we can also see this very delicate white area here,

which represents the junction between this cell here, and the next cell here.

So, in some instances,

because of a little bit of shrinkage in the tissues when it's processed,

we can actually see that delicate junction between one cell and another.

So, these are the individual epithelial cells that make up

the epidermis of the skin.

In this particular example, we're looking at an epidermis that is sort of two

to five cells thick, depending on the area that we look at.

It's important to bear in mind that, depending on where you are in the body,

this epidermis may very markedly in thickness.

The best example of this would be in the foot part of dogs and cats.

These areas would be very,

very thick in terms of how many cells would be in the epidermis.

If you're wondering what this dark pink purple staining

material is here on the surface, this is called keratin.

So, if you were to scratch your own skin and

see some sort of white flakes appearing, that's keratin.

This is the mature epidermal cells that have matured and are now dying,

and they're falling off.

And these will be replaced by new cells that divide from this basal layer here.

They divide, and they move out, and then they fall off.

And then the whole process starts again.

So, the skin is continually regenerating and

replacing the dead cells that fall off the surface.

Sitting underneath the epidermal cells,

these epithelial cells within the skin, are these big wavy lines of collagen.

This is called dermal collagen, and this is a connective

tissue structural material that keeps our skin strong, yet pliable.

And these are wavy sheets of collagen that intersperse between each other and

give us a structurally sound skin.

So, these are the main, sort of, cell and tissue elements.

If you like, the epidermis here, the dermis here.

And one other interesting point here is that, the skin obviously performs a very

important mechanical function in protecting us, in protecting the body.

It's also immunologically a very active site.

These are not just cells that are just sitting here performing a mechanical

protective function.

They are cells of the immune system active here.

They are wandering around in this area, watching out for

any pathogens, anything that shouldn't be there.

And then these cells can allow the body to develop an immune response

appropriate to whatever the invading agent it may be.

So, that gives you a little bit of an insight into the staining qualities and

characteristics that we'll see on histological sections when we look

under the microscope.

So, just to summarize then from this section.

Cell boundaries can be difficult to see clearly in a Haematoxylin and

Eosin stained section.

The outermost layer of the skin is called the epidermis.

The epidermis varies in thickness, depending on the location in the body.

So, for example, the foot pad is very thick.

The epidermis is supported by connective tissue called the dermis.

And this is composed mainly of a connective tissue called collagen.

Histology in Action – the Skin

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