Cells of the Epidermis

The epidermis contains keratinocytes, melanocytes, dendritic cells, and Merkel cells. Keratinocytes form the majority of the epidermis and produce keratin, a protective fibrous protein. Melanocytes produce melanin pigment that protects against UV radiation. Dendritic cells ingest microorganisms and activate immune responses, while Merkel cells function as sensory receptors for touch.

Newly produced keratinocytes are continuously pushed toward the skin surface by epidermal growth factors, displacing older cells. As keratinocytes reach the surface, they die due to lack of dermal blood supply and are shed. This process renews the entire epidermis every 25 to 45 days, maintaining a protective barrier.

Melanin is a pigment produced by melanocytes and transferred to nearby keratinocytes through cellular processes. It protects the nucleus of keratinocytes from damaging ultraviolet radiation, preventing DNA damage and reducing skin cancer risk. Melanin also gives skin and hair their color.

The stratum basale is a single layer of cuboidal basal cells that serve as stem cell precursors for all epidermal keratinocytes. These cells constantly undergo mitosis to produce new cells that are pushed superficially upward. Merkel cells and melanocytes are also dispersed among basal cells in this layer.

In the stratum spinosum, keratinocytes begin synthesizing keratin and release water-repelling glycolipids that help prevent water loss. As cells move to the stratum granulosum, they flatten and their membranes thicken, accumulating keratin and keratohyalin as lamellar granules. These proteins give the layer its characteristic grainy appearance.

The stratum lucidum consists of dead, flattened keratinocytes densely packed with eleidin, a clear protein derived from keratohyalin, giving them a transparent appearance. The stratum corneum is composed of dead keratinocytes that are regularly shed and replaced by cells from deeper layers. The entire stratum lucidum is replaced approximately every four weeks.

Dendritic cells, also called Langerhans cells, are star-shaped cells dispersed throughout the epidermis that function as macrophages. They ingest bacteria, foreign particles, and damaged cells, then activate immune responses to protect against pathogens. These cells are especially abundant in the stratum spinosum layer.