Your skin is composed of three layers - the epidermis, the dermis, and the hypodermis or subcutis. Our main focus will be the epidermis - the outer layer of our skin - but we don't want to ignore the other layers because they are important, too!
Just taking a quick look at this picture, you can see the hair follicle, sebaceous glands, veins and arteries, and nerves. We will be going into greater detail about hair follicles and sebaceous glands in the next few weeks.
I find it interesting that the upper level of our skin does not contain any veins or arteries, and is nourished by a concept called diffusion, in which there is a movement of molecules from a region of higher concentration to a region of lower concentration. In the case of our skin, the living cells of the epidermis receive what it needs from the underlying dermal capillaries. It can also move nutrients from the outer layer inwards, which is how our creations work to moisturize our skin!
The epidermis consists of five layers or strata - the stratum basale (at the bottom), the stratum spinosum, the stratum granulosum, the stratum lucidum, and the stratum corneum.
Skin cells are formed through mitosis in the stratum basale. Their shape and composition change as they move up to the stratum corneum. As they move up, they release cytoplasm and take on keratin. When they reach the top layer, they are sloughed off. (This process is called keratinization). This top layer of skin is responsible for creating the skin barrier - keeping water in and keeping the world out! There are somewhere between 10 to 30 layers of these dead cells on the top layer of your skin.
When the cells reach the stratum corneum, these corneocytes are considered dead cells as they contain no organelles inside. They are completely flattened and are now filled with keratin and lipids, fatty acids, and ceramides. Keratin is a protein that keeps our skin hydrated by preventing water loss or absorbing water from the atmosphere. It is responsible for the "spring back" or elasticity of our skin.
The lovely fluids inside the corneocyte is called our skin's natural moisturizing factor (or NMF). When we absorb water from the atmosphere (or lotions!), this water dissolves these molecules and they act as humectants in our skin drawing water from the atmosphere. And the major components of this NMF is sodium lactate, urea, and pyrrolidone carboxylic acid (or sodium PCA), all great humectants.
The stratum corneum is sometimes called the "horny layer" (yes, you may laugh now...) because the corneocytes are hornified with an outer layer of proteins to keep them solid. Many discussions about the stratum corneum likens the cell configuration to a brick wall, with the corneocytes acting as bricks and the lipid barrier structure as the mortar. (If you look at the picture to the left, you can see the individual cells of the stratum corneum. Think of those as bricks and the spaces around them filled with the lipid barrier structure to create a space-less layer.)
The lipidic barrier structure consists of various fatty acids and their salts that have low solubility in water but are capable of forming a very stable layer. Their job is to keep irritants out and prevent trans-epidermal water loss. (That diffusion I mentioned before...that's how we lose water!)
The thickness of the stratum corneum varies on your body. It is thinner, and therefore more permeable, on your face, forehead, and scrotum (if you have one!), and less permeable on your torso, arms, legs, hands, and feet. Your palms are particularly impermeable to most things - except water, which is why you get that exciting pruning on your hands after a long bath!
This is the ideal situation for our skin. Cells form in the lower layers and migrate to the upper layers. They contain NMF inside the cell, which is now coated in proteins to make it more solid. A lipid barrier structure surrounds the corneocytes to keep our skin healthy. This all works to keep horrible things out and good things in - yay, skin!
Join me tomorrow for the process of desquamation!