If we think of the corneocytes in the stratum corneum as "bricks", then the stratum corneum lipids are the mortar. (The water soluble natural moisturizing factor is found inside the corneocytes; the lipid soluble stratum corneum lipids are found outside the corneocytes.)
They make up about 15% of the dry weight of the stratum corneum, and contain about 40% to 50% ceramides, 20% to 25% cholesterol, 15% to 25% fatty acids (those with C16 to C30 chain lengths, with C24 to C28 being the most common), and 5% to 10% cholesterol sulfate.
The lipids are arranged in a highly organized lamellar arrangement (fine layers alternating between different materials) with small amounts of water present. This is considered to be a very effective barrier to trans-epidermal water loss (TEWL). Water trying to escape the through the stratum corneum would have to navigate a complicated maze through the bilayer and the corneocytes to get to the surface of your skin - so the lipids and corneocytes make this a much harder task!
The ceramides in our skin are a major component of the structural organization of the lamellar bilayers. There are nine different types offers something to the organization of our stratum corneum and cohesion of the skin barrier. There is a subclass of ceramides called acylceramides, which are ester linked to hydroxy fatty acids, specifically linoleic acid - hence the concept that an oil high in linoleic or gamma-linoleic acid can help improve our skin's barrier properties. In the winter, the ratio of the EOS-oleate increases which means the EOS-linoleate decreases; this happens with dry skin as well. This has a dramatic effect on lipid organization. (The EOS means it is linked to sphingosine.)
We need to treat the stratum corneum lipids well because without them, we end up with really dry, perturbed skin. Particular solvents (hexane, toluene) can damage the lipids and leave our skin defenceless. Harsh cleansers can also mess up the lamellar structure, leading to serious changes in the health of our skin.
So again, mild cleansers for our skin with moisturizers and film formers are always a great idea. Following up cleansing with some kind of lotion is also a good idea. Limit your use of hand sanitizers - the alcohol is very dehydrating. If you really want to use anti-bacterial stuff, try the new ones with benzalkonium chloride or use a really nice cold process soap purchased from your local soap-maker. And follow up with a lovely lotion.
Oils with linoleic or gamma-linoleic acid are always welcomed by our skin (like sunflower, sesame seed, borage, evening primrose, rice bran, and others), especially in the winter. And any oil that creates an occlusive barrier is welcome! Keep your hands out of the toluene - not really that hard to do! - and try to stay well hydrated by drinking lots of water and other fluids.
Join me tomorrow for mechanisms of trans-epidermal water loss!