Figure 1Structure of human skin. A) Schematic representation; B) Characteristics of the three skin layers.
Epidermis
Epidermis constitutes the outermost barrier between the organism and the external environment and is itself organized in four layers thestratum basale, stratum spinosum, stratum granulosum andstratum corneum (SC ) (Figure 1A). A complementary layer,stratum lucidum can be found on the palm and sole of the foot, which is responsible for the thickened skin. Nevertheless, thestratum lucidum is often considered the lower part of theSC but not an individual epidermal layer. There are also hair follicles and sweat ducts that cross different skin layers (Ng and Lau, 2015).
The most intrinsic layer of the epidermis, the stratum basale is composed by different types of cells: keratinocytes, melanocytes, Merkel cells and Langerhans cells, which participate in the immune response. The following layer, the stratum spinosum , is composed by keratinocytes. The third layer, stratum granulosum , consisting of several rows of keratinocytes and is a reservoir of ceramides which are relevant lipids for the constitution of the outermost layer, thestratum corneum (Sofia A. Costa Lima., 2018, Lee et al., 2006). Together, the three most intrinsic layers of the epidermis form the denominated “viable epidermis” (Ng and Lau, 2015). The viable epidermis is generally 50–100 µm thick and contains no blood vessels and sensory nerve endings (Ng and Lau, 2015, Prausnitz et al., 2012). Keratinocytes originate from the stratum basale and expose oneself to progressive differentiation while migrating to the SCthey are necessary to maintain the SC by replacing lost corneocytes and SC lipids during desquamation (Ng and Lau, 2015). The viable epidermis is considerably hydrophilic (> 50%) in the opposite to SC which is lipophilic and contains at ca. 10% of water (Forster et al., 2009).
Due to the importance of the SC for the permeation of drugs and the protection of the skin, the structure and functions of this layer will be more extensively discussed in this review in the following subsection.