The epidermis is the outermost layer of the skin (Figure 1). It forms a protective barrier over the body's surface which prevents pathogens from entering (Figure 3). It is also responsible for retaining water in the body and absorbing nutrients.
The epidermis is the outermost layer and helps the skin regulate body temperature. It does not contain any blood vessels, and cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis.
The epidermis can be further subdivided into five strata or layers:
- Stratum corneum - corneocytes are surrounded by a protein envelope (cornified envelope proteins), filled with water-retaining keratin proteins, attached together through corneodesmosomes and surrounded in the extracellular space by stacked layers of lipids.
- Stratum lucidum (only on palms and soles of feet).
- Stratum granulosum - in this layer, keratinocytes lose their nuclei and their cytoplasm appears granular. Lipids, contained in these keratinocytes within lamellar bodies, are released into the extracellular space through exocytosis to form a lipid barrier.
- Stratum spinosum - Langerhans cells, immunologically-active cells, are located in the middle of this layer.
- Stratum germinativum (basale)- composed mainly of proliferating and non-proliferating keratinocytes, attached to the basement membrane by hemidesmosomes. Melanocytes are present, connected to numerous keratinocytes in this and other strata through dendrites. Merkel cells are also found in the stratum basale (Figure 2).
The epidermis consists of stratified squamous keratinizing epithelium, composed of proliferating basal and differentiated suprabasal keratinocytee, with an underlying basement membrane. Keratinocytes are the major cells, constituting 95% of the epidermis, while Merkel cells, melanocytes, and Langerhans cells are also present.
Keratinocytes in the stratum basale proliferate during mitosis and the daughter cells move up the strata, changing shape and composition as they undergo multiple stages of cell differentiation to eventually become anucleated. During that process keratinocytes will become highly organized, forming cellular junctions (desmosomes) between each other and secreting keratin proteins that aid in protection, and lipids which contribute to the formation of an extracellular matrix which serves to provide mechanical strength to the skin.
Keratinocytes from the stratum corneum are eventually shed from the surface (desquamation). This process is called keratinization and takes place within about 30 days.