Understanding the Epidermis

The Anatomy and Function of the Skin's Outermost Layer

photo of a mans abdomen showing his epidermis
What are the layers which make up the epidermis of the skin and what are their functions?. Joe Raedle / Staff / Getty Images

The epidermis is the outermost layer of the skin. The thickness of the epidermis varies depending on where on the body it is located. It is at its thinnest on the eyelids, measuring just half a millimeter, and at its thickest on the palms and soles at 1.5 millimeters.

The Anatomy of Skin

The skin's anatomy is composed of three layers: the epidermis, the dermis, and subcutaneous tissue. These layers are home to sweat glands, oil glandshair follicles, blood vessel, and certain vital immune cells.

Functions of the Epidermis

The epidermis acts as a barrier that protects the body from ultraviolet (UV) radiation, harmful chemicals, and pathogens such as bacteria, viruses, and fungi.

Historically, it was thought that the function of the epidermis was to regulate fluid and protect the body from mechanical injury. In recent years, we've come to understand that it is a complex system that plays a key role in how the immune system communicates and target defense.

Within the epidermis are several distinct layers, consisting of (from bottom to top):

  • Stratum basale, also known as the basal cell layer, is the innermost layer of the epidermis. This layer contains column-shaped basal cells that are constantly dividing and being pushed toward the surface. The stratum basale is also home to melanocytes that produce melanin (the pigment responsible for skin color). When exposed to the sunlight, melanocytes produce more melanin to better protect the skin from UV exposure. Abnormalities in the development of these cells can lead to melanoma, the most deadly type of skin cancer.
  • Stratum spinosum, also referred to as the squamous cell layer, is the thickest layer of the epidermis located just above the basal layer. These are composed of basal cells that have matured into squamous cells, known as keratinocytes. Keratinocytes are responsible for producing keratin, a protective protein that makes up skin, nails, and hair. The squamous layer is also home to Langerhans cells which attach themselves to foreign substances as they infiltrate the skin. It is also responsible for synthesizing cytokines, a type of protein that helps regulate the immune response.
  • Stratum granulosum is made up of keratinocytes that have moved up from the squamous layer. As these cells move closer toward the skin's surface, they begin to flatten and stick together, eventually drying and dying out. 
  • Stratum corneum is the outermost layer of the epidermis. It consists of 10 to 30 layers of dead keratinocytes that are constantly being shed.  Shedding of these cells slows significantly with age. The complete cell turnover, from basal cell to stratum corneum, takes around four to six weeks for young adults and about a month and a half for older adults.
  • Stratum lucidum only exists on the palms of the hands and soles of the feet. It consists of four layers rather than the typical four.

Conditions Involving the Epidermis

The epidermis can be impacted by more than just injury. This outermost layer is subject to both genetics and external forces that contribute to the aging of this skin. These factors include smoking, alcohol, and excessive UV exposure, all of which contribute to the development of wrinkles, sunspots, and the uneven thickening or thinning of skin.

The epidermis is also where rashes and blisters appear, caused by everything from infections and allergies to diseases and toxins.

It is also the origin of both non-melanoma and melanoma skin cancers, and where certain diseases like diabetes and lupus can manifest with an array dermatological symptoms.

Penetration of the epidermis can cause infections that they body can otherwise defend against. These include diseases caused by insect or animal bites, as well as those pathogens that enter the body through open sores, cuts, abrasions, or needlestick injury. 

Source:

Tan, S.; Roediger, B.; and Weninger, W. "The Role of Chemokines in Cutaneous Immunosurveillance."Immunology and Cell Biology. 2015; 93(4):337-46.

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