Author: Van Smeden J, Janssens M, Gooris GS, et al.
skin protects the body from unwanted influences from the environment as well as excessive water loss, barrier function of the skin is located in the stratum corneum (SC),SC consists of corneocytes embedded in a lipid matrix. This lipid matrix is crucial for the lipid skin barrier function, relation between lipid modulations and the impaired skin barrier function, relation between lipid modulations and the impaired skin barrier function, lipid composition, lipid organization and skin lipid barrier, skin consists of the epidermis and dermis as well as the subcutaneous fat tissue [1]. The epidermis is the outermost layer of the skin and consists of four distinctive layers. Each layer displays one of the sequential differentiation stages of the keratinocytes, the major cell type in the epidermis. The layers include the superficial stratum corneum (SC), stratum granulosum (SG), stratum spinosum (SS), and the inner most stratum basale (SB). The SG, SS and SB are part of the viable epidermis (thickness: 50100 ?m), whereas the SC (thickness: 1020 ?m) is part of the non-viable epidermis and the final differentiation product. The SB contains the proliferating keratinocytes. After the keratinocytes escape from the SB, they transiently migrate towards the SC, after which they are finally released from the skin surface, a process called desquamation. During this migration the keratinocytes differentiate: They flatten out and finally adopt the dimensions which are characteristic for the dead cells of the SC, the corneocytes, keratinocytes in the SG contain a high number of membrane-coating granules referred to as the lamellar bodies (LBs) in which lipids are stored, such as glucosylceramides (GlcCERs), sphingomyelin and phospholipids, these are precursors of the SC lipids, and are enzymatically processed into their final constituents: ceramides (CERs) and free fatty acids (FFAs), CERs and FFAs are, together with cholesterol (CHOL), the main lipid classes in the SC. By means of exocytose, the lipid content of the LBs is released together with hydrolytic enzymes into the intercellular space at the SG/SC interface, human SC contains 10 to 25 corneocyte layers that are oriented approximately parallel to the skin surface and are embedded in a lipid matrix , structure of the SC is often referred to as a bricks in mortar structure, in which the corneocytes are the bricks and the lipids are the mortar, corneocytes are filled with water and microfibrillar keratin that is surrounded by a cornified envelope which consists of a densely crosslinked layer of proteins such as filaggrin, loricrin and involucrin , monolayer of non-polar lipids (?-hydroxylated CERs and FFAs) referred to as the lipid envelope is esterified to the cornified envelope, mainly to glutamate residues of involucrin, lipid envelope is suggested to form a template for the formation of the intercellular lipid layers , cornified envelope, together with the lipid envelope, minimizes the uptake of most substances into the corneocytes and allows proper formation of the lipid matrix. Indeed, a deficient lipid envelope results in a defective skin permeability function and an irregular lipid matrix, lipid matrix acts as the main barrier for diffusion of substances through the skin
