The skin is composed of three primary layers: epidermis, dermis, and subcutaneous tissue. Each layer possesses specific characteristics and functions. Although research regarding skin layers continues, much is already known about the structure of each component. New discoveries about these components have already led to prenatal diagnoses of many inherited diseases and to improved therapies. In the future, study of these components will likely lead to an enhanced understanding of skin aging and the effects of topical products on the biologic function of the skin.
The epidermis is the most superficial layer of the skin. It is very important from a cosmetic standpoint, because it is this layer that gives the skin its texture and moisture, and contributes to skin color. If the surface of the epidermis is dry or rough, the skin appears aged. Knowledge of the basic structure of the epidermis best enables a practitioner to improve the appearance of patients’ skin.
Keratinocytes, also known as corneocytes, are the cells that comprise the majority of the epidermis. Keratin filaments are major components of the keratinocytes, and provide structural support. There are two types of keratin filaments: acidic (type I, K10–20) and basic (type II, K 1–10). They both must be expressed for a keratin filament to develop.1 In other words, an acidic type and a basic type are always expressed together and they form a keratin filament together. Keratinocytes are “born” at the base of the epidermis at the dermal–epidermal junction (DEJ). They are produced by stem cells, which are also called basal cells because they reside at the base, basal layer, of the epidermis. When the stem cells divide, they create “daughter cells,” which slowly migrate to the top of the epidermis. This process of daughter cells maturing and moving to the top is called keratinization.
As these cells progress through the epidermis and mature, they develop different characteristics. The layers of the epidermis are named for these characteristic traits. For example, as mentioned, the first layer is the basal layer because it is located at the base of the epidermis. Basal cells are cuboidal in shape. The next layer is referred to as the spinous layer because the cells in this layer have prominent, spiny attachments called desmosomes. Desmosomes are complex structures composed of adhesion molecules and other proteins and are integral in cell adhesion and cell transport. The next layer is the granular layer, named so because these cells contain visible keratohyaline granules. The last, outermost layer is the stratum corneum (SC), a condensed mass of cells that have lost their nuclei and granules (Figs. 1-1 and Fig. 1-2). The SC is covered by a protein material called the cell envelope, which aids in providing a barrier to water loss and absorption of unwanted materials.
The layers of the epidermis.