In this chapter, for now just focus on the epithelial portion of the skin. That means, focus on the epidermis, and just skim over the dermis and the hypodermis (a.k.a. subcutaneous layer) for now.
Skin = epidermis + dermis
The hypodermis is below that, so it is not part of the skin... it just lies underneath it. Here's the order in which they lie (but keep in mind that epidermis and dermis are the only parts of the skin).
Types of skin:
The epidermis of skin is keratinized:
Because skin needs to be tough and protective, its epithelial surface contains keratinized cells. That means that the epithelial cells, shortly after being born, begin to make a protein called keratin that they store inside of them. This protein is really tough. As the cells get older and older, more and more keratin is found within them. This continues until they get so full of keratin that they can't survive any more (there's another reason that they can't survive, based on obtaining nutrients, described a bit below here). When they die, they remain as packets of keratin that form a tough protective barrier that we appreciate for our skin.
Keratinization of the cells in epidermis is important. You see, skin is not the only place we find stratified squamous epithelium, but it is the place where we find keratinized stratified squamous epithelium. The stratified squamous epithelium on our tongues and that lines the insides of our cheeks is not keratinized; does that make sense?
The oldest cells of the epidermis are the most apical cells:
Epithelial cells are born all the time. You already learned this when you learned that epithelial tissue can recover well from injury. However, the epithelial cells are born at the most basal level of the epithelium, from cells sitting directly on the basement membrane. The basal cells continue to divide and make more and more cells. As they do so, the ones that were born shortly before the newest ones get pushed farther and farther up toward the apical level. Therefore, the oldest, and consequently, the dead cells, are at the apical surface.
5 layers of the epidermis can be described.
When you look at the epidermis, the cells that lie along the basement membrane look very different from those that lie at the apical edge. In fact, in thick skin, one can see 5 different appearances to the cells from the basal edge to the apical edge. These 5 different appearances have been given names that reflect their appearance, and are considered layers of the skin. The 5 different layers are indicated in this figure from your textbook (Figure 6.3 on page 163). Hairy skin typically only has 4 layers (the stratum lucidum is not present).
These same 5 layers can be seen in the Slide 3 photomicrograph that you can see from a histology web page series after you click on the "slide 3" written in the left frame.
Remember from the intro to epithelial tissue that one characteristic of all epithelial tissue is that it is not vacularized? Well, the epidermis is no exception. The problem is that the cells of our bodies receive their nutrients from the blood. Since the blood doesn't enter the epithelium, it has to get to the epithelial cells by diffusion. That's fine when epithelium is thin. But epidermis (the epithelium found in skin) is thick-- even in thin/hairy skin. It has many layers-- 4 or 5! What is the consequence of this?
The cells that lie closer to the basement membrane (closer to the underlying dermis which is vascularized) are closer to the nutrient source. They use the nutrients that are diffusing into the epidermis. Almost all the nutrients are used up by the cells in the stratum basale and stratum spinosum. Therefore, once a cell has been alive so long that it has gotten pushed up into the stratum granulosum, it stops receiving any nutrients and it has no choice but to begin to die.
These are large cells that sit on the basement membrane. These cells have elaborate extensions, kind of spidery in appearance, into the more apical stratum spinosum and even into the stratum granulosum. These cells make a pigment (pigments are proteins that provide color) called melanin. These cells make even more melanin when exposed to sunlight. The melanin courses throughout their cytoplasm, into all their spidery extensions. These extensions pop off little pieces of themselves that contain melanin which then get picked up by adjacent epidermal cells via endocytosis. This transfer of melanin from the melanocyte to other epidermal cells is called cytocrine secretion.
This is made of connective tissue, which you haven't learned about yet. You should be able to pick up the notion that this contains blood vessels (for nutrition of skin cells) and nerves (for sense of touch and pain and temperature). It is also responsible for our fingerprints (I'll discuss that in lab later).
This is made up of a different type of connective tissue (just like there are different kinds of epithelial tissues, there are different types of connective tissues) than the dermis was made up of. But one thing about this layer is that it has a lot of fat in it. This helps to insulate our bodies.
© 2011 STCC Foundation Press