First layer we're going to be looking at is the epidermis. Again, referring back to that little section of the skin, your epidermis is the uppermost part of your dermis or the most superficial part of your dermis. I like both of these figures for different reasons. I think the figure on the left here, which is an actual histological slice, shows the layers of the epidermis very well, and the figure on the right shows the cells of the epidermis very well.
In this video, we're going to be covering the cells of the epidermis. So the epidermis is the outermost layer of skin. It is avascular, it is strong, flexible, and waterproof for the most part.
Four major cells are anchored here. All are protective for different reasons, so I want you to think about as we go through the different cells, what type of protection is this giving to the body? So most of the cells that you see in this picture are called keratinocytes. Down at the bottom most layer, which is called the stratum basale, by the way, at the bottom most layer, the cells are living, and they're actually called pre-keratinocytes. They have kind of underdeveloped levels of keratin inside of them, and as they start to grow outwards, they become more keratinized, and they actually begin to die off.
So most of the cells, the ones that are kind of in a tan color or orange color here, are keratinocytes. Now the root site, C-Y-T-E, means cell. And so that is a root that you're going to be coming across quite a bit in A&P. And keratin is a protein that gives, it's a fibrous structural protein.
It basically gives structure or a toughness to anything that it's a part of. So you'll find nail polish, shampoos, things like that that contain keratin. So here we have naturally occurring keratin in the body in the form of keratinocytes, or keratin-containing cells.
The next cell that we're going to be looking at down here is called a tactile cell. This used to be called a Merkel cell. It's a spiky little cell that is associated with the sensory nerve ending. Together, these are called a Merkel disk.
At this point, this is only called a tactile cell. It is no longer named a Merkel cell, at least in most textbooks. And the purpose of the Merkel cell is sensation. It is sensitive to touch, and it is very sensitive to pain. Next we have the dendritic cell.
You're going to go up a little bit further. This purple guy right here is the dendritic cell. The dendritic cell used to be called a Langerhans cell. It's actually a macrophage. A macrophage is a cell that's capable of consuming things like pathogens, bacteria or viruses that somehow penetrate through the skin and get to this area.
will usually encounter a dendritic cell, and they in turn will consume the pathogen and then activate the immune system and say that we're under attack, and it's possible that we may need to be mounting an attack against this particular bacterium, for example. Now keep in mind, just a side note here, that if this particular figure shows up on the test, you will not have the ability to actually see the colors. The tests are photocopied in black and white, and so you will not be able to see a purple cell, for example. So you need to get to know them by shape.
The last of the cell we're going to cover is this one right here. It is called a melanocyte, a melanin-containing cell. Melanin is a pigment. There's a couple of different types of melanin, but usually it's thought of as a dark brown or a dark black color. The cell itself is a spider-shaped cell.
It produces this melanin pigment, which accumulates. on the keratinocyte nucleus for protection. If you look closely at this figure, you'll actually be able to see little spots inside of the keratinocytes, especially as they're deeper inside of the epidermis like this. You'll see little spots and they're always on the sunny side of the nucleus. So there will be a nucleus on one side and then on the superficial side of that, on the sunny side, you'll see little spots.
Those are actual melanin pigments that have been deposited. by a melanocyte. So with sun exposure, your melanin output increases. It's important to note, it's important to note that melanocytes are relatively the same in number in different people.
So differences in skin color come not from the number of melanocytes, but rather from the type and amount of melanin that is output by each melanocyte. So again, a person who has very dark skin or a person who has very light skin. Everyone has just about the same number of melanocytes, but for some people their melanocytes are much more active and they will have darker skin.