Oils are lipids. The lipids present in our skin, the sebum and the epidermal lipids1 come from two very different sources.
The sebum is produced and secreted by the sebaceous glands onto the surface of our skin. The sebaceous glands are located in the dermis where it is usually found in association with hair follicles, forming the pilosebaceous unit. Sebum is a complex mixture of lipids: 57.5% tryglycerides and their hydrolysis products, 26.0% wax esters, 12.0% squalene, 3.0% cholesterol esters and 1.5% cholesterol2. Among these, squalene and wax esters are unique to human sebum and not found anywhere else in the body nor among the epidermal lipids3-4-5-6.
The epidermal lipids are comprised of 50% ceramides, 25% cholesterol, 15% of free fatty acids7 as well as smaller amounts of cholesterol esters and cholesterol sulfate8. Epidermal lipids are secreted by the cells of the epidermis called keratinocytes. As keratinocytes journey from the basal compartment of the epidermis (where they are born) towards the outer layers of the skin to die and shed off as scales, they must differentiate and undergo extensive physical and chemical changes. In their last step of differentiation in the outermost layer of the skin (stratum corneum) they release lipids (lamellar granules) into the spaces between the cells. These lipid packages fill in these spaces between the cells to act like mortar or cement, ensuring the skin barrier function9-10-11. This layer of lipids is our environmental shield barrier, protecting against the external elements and maintaining the internal environment intact. In contrast to free-flowing oils that make up sebum, the epidermal lipids are in a solid state at room temperature.
Oils have been shown to help maintain and repair the integrity of the lipid barrier, by inhibiting oxidative damage, known to be one of the primary cause of wrinkles12-13. Facial oils can be suitable for all skin types and are known to provide hydration in moisture depleted mature skin.