Stratum corneum
The stratum corneum is typically 10-20 µm thick and is composed by 10–25 layers of dead mature keratinocytes, denominated as corneocytes, which are obtained from terminally differentiated keratinocytes during the passage from granular layers to SC . This process is characterised by loss of nuclei and cytoplasmic organelles, flattening and elongation of the keratinocytes. The continuous shedding of corneocytes from the skin surface is called desquamation (Ng and Lau, 2015) and it balances proliferating keratinocytes that constitute thestratum basale . The cells migration through the epidermis to the surface takes approximately fourteen days (Ovaere et al., 2009). Morphological dimension of the corneocytes are about 0.2 µm thick and 40–60 µm wide (Ng and Lau, 2015, Agache, 2004). Their membrane is 0.015 to 0.020 µm thick and includes involucrin and keratolinin as structural proteins (Pouillot et al., 2008).
The composition of SC is generally 20% of lipids and 70% of insoluble keratins (Walters and Roberts, 2002) and the organization of the layer is usually compared with the ”bricks and mortar” model (Figure 2A). The bricks are the corneocytes and the extracellular matrix analogous is compared to the mortar in a brick wall, which is composed of lipids (stacked lipid bilayers that surround the corneocytes) like the cement holding the bricks together (Prausnitz et al., 2012, Uchida and Park, 2016, Menton and Eisen, 1971).