Sunday, March 4, 2012

Chemistry of our skin: An updated overview

I've updated this post with interesting things I've learned in the past two years. The original post can be found here

Your skin is composed of three layers - the epidermis, the dermis, and the hypodermis or subcutis. Our main focus will be the epidermis or the outer layer of our skin.

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 (also called the corneoepidermial junction)
• the stratum corneum (at the top)

Skin cells are formed through mitosis in the stratum basale (the bottom layer). 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. (This process is called desquamation, and I go into more detail about it in this post.

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 attract water to our skin from the atmosphere or apply lotions containing water to our skin, the 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 water levels of the epidermis are high - about 70% - but these levels drop sharply at the junction of the stratum granulosum and stratum corneum from this high of 60% to about 15% to 30%. It is believed this is where the barrier of water loss is located, in the corneoepidermal junction. "It is widely accepted that the barrier function to paracellular influx or loss of water and solutes is primarily provided by intercellular lipids filling the space between cells of the uppermost living cell layer and the SC. However, recent studies suggest that tight junction structures exist in the human epidermis and are also required for the control of skin permeability. Tight junctions (TJ) are intercellular structures that control paracellular permeability (i.e., the diffusion of water and solutes across intercellular spaces). They consist of transmembrane proteins (e.g., claudins, occludin, and junctional adhesion molecules) and of plaque proteins (e.g., zonula occludens proteins ZO-1, ZO-2, and ZO-3; Fig. 3).50 Studies in animal models have suggested the importance of TJ for control of skin permeability. Furuse et al. reported that mice lacking claudin-1 die because of significant body dehydration." (Page 79, this document). In other words, although we've long through that the barrier to water loss was thanks to the intercellular lipids, like our natural moisturizing factors and stratum corneum lipids, it turns out that there's another structure - the tight junction - that can help prevent moisture loss.

Prevention of water loss is an essential part of why we make our products! Learn more by reading this post on transepidermal water loss

The stratum corneum is sometimes called the "horny layer" (did I hear a giggle there?) 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. 

So here's the ideal cycle for our skin...Cells form in the lower layers and migrate to the upper layers. They contain NMF, 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!

If you really want to get into learning more about our skin, please read this document Skin hydration: A review of its molecule mechanisms. It's simply fascinating and really in-depth. And I suggest that you read the posts below for more information. It seems silly for me to re-post them all when there's not a ton of new information to add!

Join me tomorrow for fun with aquaglyceroporins!

Related posts:
Chemistry of our skin: Desquamation
Chemistry of our skin: Natural moisturizing factor (NMF)
Chemistry of our skin: Stratum corneum lipids
Chemistry of our skin: Transepidermal water loss
Dry skin type


Anonymous said...

Yay! I didn't expect to find a post this morning! I was just cooming back to do more research.. I thought you were taking some much needed time off?! hmmmm.. miss Susan ;-)
Thank you, thank you, thank you for your contiued efforts to inspire us all. The information you provide is in-valuable to us.


geri said...

It's a great article again! I learn so many things from your work! Thank you!