As the outermost layer of skin, the epidermis forms the boundary between the body and the outside world. It serves primarily as a protective shield against invading, disease-causing organisms.
What is the epidermis?
Schematic diagram showing the anatomy and structure of the epidermis. Click to enlarge. The term epidermis is derived from the Greek word epi (over) and dermis (skin), and refers to the outermost layer of skin in vertebrates. The excretory ducts of the sweat and sebaceous glands end in this superficial layer of the skin. Their secretion is responsible for moisturizing and lubricating the skin. The epidermis does not contain nerves and vessels, which is why injuries in this skin layer neither hurt nor bleed. Nutrients are supplied by the fine blood vessels of the underlying skin layer, the dermis (leather skin). The epidermis is the skin layer that can be influenced by cosmetic products. The effect of individual products can be circulation-enhancing, plumping or cell-protecting, depending on their properties.
Anatomy and structure
Anatomically, the epidermis is divided into the following five layers from the inside out:
- Basal layer (stratum basale)
- Prickle cell layer (stratum spinosum)
- Granular layer (stratum granolosum)
- Glossy layer (stratum lucidum)
- Horny layer (stratum corneum)
For the most part (about 90%), the epidermis consists of so-called keratinocytes – horn-forming cells. This cell type produces keratin and differentiates during the cornification process from basal cells in the deepest to flat, nucleated horn cells in the outermost layer of the epidermis. This process takes about 4 weeks – thus the epidermis renews itself every month. Once in the uppermost layer, the cells are then gradually destroyed again and reed off from the skin as fine skin flakes by touch or washing. During wound closure, new skin cells are formed starting from the basal layer, which then slowly migrate over the healing wound. The cohesion between the individual keratinocytes is formed by the desmoses (cellular adhesion structures). They provide stabilization of the cellular bond against shear and tensile forces.
Function and tasks
As the outermost layer of skin, the epidermis forms the immediate protective covering against the environment. Due to the dense association of cells in this layer, microorganisms face a usually insurmountable barrier. Embedded in the epidermis are also the melanocytes. These cells produce the pigment melanin, which gives our skin color and prevents penetration into deeper layers of the skin when exposed to dangerous UV radiation. This protects the cells from burning and altering DNA. In addition, the epidermis has the following function: Due to a more pronounced cornification, it can adapt to stronger mechanical loads. This results in a highly variable thickness of the epidermis. In the area of the sole of the foot, for example, the epidermis is up to 2 mm thick, while in the area of the eyelids it measures only about 0.05 mm. In the plant world, leaves also have an epidermis. Here, too, they form the closing tissue to the outside and are also responsible for protecting the underlying plant tissue.
Diseases and ailments
The term epidermolysis bullosa circumscribes a group of hereditary skin diseases that are essentially characterized by fragility of the epidermis. Due to a weakness of the connective tissue between the epidermis and the underlying dermis, blisters form on the skin in response to minor mechanical stress. These can be extremely painful and can also occur on the mucous membranes inside the body (for example, around the mouth). Depending on the type, the effects of the disease vary from minor impairment to severe disability or even death of the child. Impetigo contagiosa (Latin impetere=attack, contagiosus=contagious) is a highly inflammatory, purulent inflammation of the epidermis. This disease is most common in newborns and children. Synonyms for this term are “pus lichen”, “grind lichen” or “drag rash”. Basically, a distinction is made between a small-blistered and a large-blistered variant – both forms have their beginning mainly on the face. Red spots form here, which quickly turn into blisters filled with watery fluid.After drying, characteristic yellow crusts are formed. Therapy is with local antibiotic application.
