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 the type of skin. For example, it's thinnest on the eyelids at just half a millimeter, and thickest on the palms of the hands and soles of the feet at 1.5 millimeters.

The Three Layers of the Skin

The skin's anatomy is composed of three layers: the epidermis, the dermis and subcutaneous tissue. These layers are home to sweat and oil glands, hair follicles, blood vessels and melanocytes: cells that produce the pigment melanin and can cause melanoma.

Epidermis

The epidermis acts as a barrier that protects the body from threatening pathogens (bacteria, viruses, and fungi), ultraviolet light from the sun, and harmful chemicals. It's also acts as a waterproof barrier and is responsible for skin tone. The epidermis is constantly regenerating. Within the epidermis are either four or five more layers, depending on the part of the body. From bottom to top:

  • Stratum basale. Stratum basale, also known as the basal cell layer, in the innermost layer of the epidermis. This layer contains column-shaped basal cells that are constantly dividing. New cells push older cells toward the surface of the skin to shed. It also has melanocytes that produce melanin, or skin pigment. When exposed to the sun, melanocytes produce more melanin in order to protect the skin from UV rays, which results in a suntan. Age spots, freckles and birthmarks are caused by patches of melanin. When melanocytes do not properly divide, it causes melanoma, the most deadly type of skin cancer.
  • Stratum spinosum. Strautum spinosum, referred to as the squamous cell layer, is located above the basal layer and is the thickest layer of the epidermis. These cells formed in the basal layer and have matured into squamous cells, known as keratinocytes. They produce keratin, which is a protective protein that makes up skin, nails and hair. The squamous layer is responsible for absorbing certain substances and getting rid of others. It's also home to Langerhans cells that attach to foreign substances that break through the skin and alert the immune system. This layer of the skin is also responsible for synthesizing cytokines, messenger proteins that communicate with the immune system and help to regulate the immune response.
  • Stratum granulosum. Keratinocytes from the squamous layer move into the stratum granulosum. As they move closer toward the skin's surface, the keratinocytes grow larger, flatter and stick together, and finally dry out and die. They combine to create more layers that continue to move closer to the surface of the skin.
  • Stratum lucidum. This layer only exists on the palms of the hands and soles of the feet. It's referred to as thick skin because it has five layers, not four.
  • Stratum corneum. The stratum corneum is the outermost layer of the epidermis. It consists of 10 to 30 constantly-shedding layers of dead keratinocytes. When cells on the outermost layer die and shed, they're replaced with new layers of cells. Shedding slows with age. The complete cell turnover process, from basal cell to stratum corneum, takes about 4 to 6 weeks for young adults and about a month and a half for older adults.

The appearance of these layers is depicted in this illustration of the epidermal layers.

Dermis

The dermis is the thickest layer of skin and has two parts: the papillary layer and the reticular layer.

The dermis contains a lot of the body's water supply, and its key functions are to regulate temperature and supply the epidermis with blood. This layer of the skin also contains:

  • Blood vessels
  • Lymph vessels
  • Hair follicles
  • Sweat glands
  • Sebaceous glands
  • Nerve endings
  • Collagen and elastin

Subcutaneous Tissue

Also referred to as the subcutis or hypodermis, subcutaneous tissue is the innermost layer of the skin. It's made up of fat and collagen cells that help to regulate temperature, absorb shock, protect organs and act as an energy reserve. Blood and lymph vessels, hair follicles and nerve endings also pass through this layer. The thickness of subcutaneous tissue varies from person to person.

Medical Conditions Involving the Epidermis

The epidermis can be involved medically in many ways, not just through injuries such as abrasions and lacerations (cuts.) This layer is responsible for the changes (wrinkles) associated with aging, and with hair loss. It is the location of rashes and blisters. It is the origin of both non-melanoma and melanoma skin cancers, and it is also involved in other conditions such as diabetes (diabetic skin problems) and lupus.

Overall Function of the Epidermis

Historically it was thought that the purpose of the epidermis was primarily to regulate fluid and protect the body from mechanical injury. In recent years we've learned that, instead, it is actually a complex organ with complex functions, including important roles in immune system communication signals.

Sources:

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

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