Composed of two primary layers: the epidermis, and the dermis.

The third layer of the skin is the subcutaneous layer.

The largest organ in the body.

Covers more than 3000 square inches in the average adult.

Accounts for approximately 7% of body weight.

The epidermis is composed of the outermost layers of the skin, forming a protective barrier over the body’s surface, responsible for keeping water in the body and preventing pathogens from entering, and is a stratified squamous epithelium, composed of proliferating basal and differentiated suprabasal keratinocytes.

Epidermis helps the skin regulate body temperature.

Keratinocytes are the major cells, constituting 95% of the epidermis.

Epidermis consists mostly of dead keratin cells.

The skin epithelium is replaced once a month.

Merkel cells, melanocytes and Langerhans cells are also present in the epidermis.

The skin can be further subdivided into the following strata or layers.

Stratum corneum

Stratum lucidum present in palms and soles

Stratum granulosum

Stratum spinosum

Stratum germinativum ( stratum basale)

Keratinocytes in the stratum basale proliferate through 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.

Keratinocytes become organized, forming cellular junctions, known as desmosomes between cells and secreting keratin proteins and lipids which contribute to the formation of an extracellular matrix and provide mechanical strength to the skin.

Keratinocytes from the stratum corneum are shed from the surface by desquamation.

The epidermis contains no blood vessels.

Cells in the deepest layers of the skin are nourished by diffusion from blood capillaries extending to the upper layers of the dermis.

An important function of the skin is to retain water.

A superficial layer of complex lipids on the skin helps retain water, and this epidermal water barrier can repair itself.

Aging impairs the ability to repair the skin reducing epidermal barrier, resulting in xerosis, dry skin.

Immunosenescence refers to a pro-inflammatory state of the skin which occurs with aging and contributes to a high frequency frequency of eczema and other inflammatory skin reactions in the elderly.

The epidermis and dermis are separated by the basement membrane that controls the traffic of cells and molecules between them.

The basement membrane also binds cytokines and growth factors, and serves as a reservoir for their release during remodeling or repair processes of the skin.


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