Skin is the biggest organ in our body. It protects us from injury and invasion from bacteria and viruses, and protects us from losing body heat and water through evaporation. It regenerates itself about every 28 days, and faster in some areas and after an injury. If you stretched out your skin on a flat surface, it would cover the floor in a walk-in closet.
The innermost layer is called the hypodermis which means “under the skin.” Small arteries and veins feed nutrients and oxygen to the skin through here, and most of the layer is made up of subcutaneous fat, which helps us maintain our body temperature.
Above that, the dermis is a fairly thick layer with all sorts of stuff in it. Here’s where you find the base of hair shafts, sweat glands, nerve endings for temperature, pain and touch, white blood cells that protect you from invasion and teeny little muscles that make your hair stand on end when you’re cold. There are also even smaller blood vessels here that expand when you’re hot or embarrassed, making your skin look red because more blood flows through them when they swell up. They shut down when you’re cold to keep heat in which makes you look pale when you get really cold. All of this sits in a mixture of collagen and elastic fibers that help your skin stretch and be flexible.
The dermis is also where the majority of collagen resides in your skin. Collagen is a protein that holds the skin structure together. One type of epidermolysis bullosa (EB), which is similar to but a lot more severe than ichthyosis, is a disorder where collagen doesn’t hold the skin together very well. So any sort of force on the skin, like wearing clothes or picking up an affected baby, means that pretty much their entire skin rips off, leaving deep injuries that don’t heal easily.
On the top is a layer called the epidermis, the “outside skin”. “Layer” is a loose term, though, as the epidermis alone has 4 separate layers, called strata. (basale, spinosum, granulosum, corneum) In the picture below, the dermis is light pink on the bottom and the epidermis is the purple area. The outside of the skin is on top. All forms of ichthyosis and most forms of EB are problems with the epidermis.
On the bottom is the stratum basale, the “basement membrane.” See the winding line of dark spotty cells at the pink/purple border? It is basically a bunch of stem cells that keep dividing and making new skin, which gets pushed up and out.
Right above that is the stratum spinosum, the “spiny cell layer.” These cells do cell things–they make proteins and stick fat molecules into little boxes called lamellar bodies and hang them on the outside of the cells. Enzymes outside the cell pop the lamellar bodies and let the contents leak out around the cells. The fat molecule works like cement, holding the cells together from the outside of the cell.
The main protein the spiny cells make is a twisty rope called keratin, and a bunch of enzymes are needed to make it fold and shape right. Each enzyme does a different job like men working an assembly line to build a car. One of the important things that happens here is that the end of the keratin protein gets shoved out of the cell so it can attach to its neighbors. The linking end is called a desmosome and that’s primarily what holds the cells together. The part inside the cell is the skeleton that gives it shape. The keratin cells layer together like a brick wall, held together by the fat molecules and the desmosome links.
As the cells get shoved further to the outside, the keratin finishes cooking, and all the other stuff in the cell (the cell’s brain and working parts, etc.) starts to die off. In the picture, you can see the thick purple line that’s almost black–that’s the stratum granulosum, the “grainy layer”. Keratin is in its almost finished form, a version called keratohyalin, and pretty much everything else is dead. The cells flatten as they lose their organs and squish together like stacks of paper. Enzymes on the outside start to break up the desmosomes so skin can flake off at the same rate that it is made, leaving us with a nice, constant layer.
Finally, the top layer is the stratum corneum, which is the “horny layer.” It’s basically a wax coating that protects you from losing water through your skin, stops you from getting injured when you run into the table, and keeps the nasty viruses and bacteria out. After a few days, the cells reach the top, the desmosomes have completely dissolved, and you leave a trail of invisible flakes everywhere you go.
At least, that’s the way it’s supposed to work. I’ll talk about what goes wrong in future entries.