The epidermis is the outermost layer of the skin, serving as the primary shield against environmental hazards, pathogens, and moisture loss. Unlike the underlying dermis, this layer is entirely avascular, meaning it contains no blood vessels of its own. Instead, it relies on diffusion from the dermal blood supply for its nourishment and oxygen. Histologically, it is composed of stratified squamous epithelium, packed tightly together to provide mechanical strength. Its thickness varies across different parts of the human body, being thinnest on the eyelids and thickest on the palms of the hands and soles of the feet where friction is highest.
From the deepest layer to the surface, the epidermis is organized into distinct strata, each representing a different stage of cell maturation.
Keratinocytes constitute about ninety percent of all cells in the epidermis. As these cells migrate upward from the stratum basale toward the surface, they undergo a biochemical transformation known as keratinization. During this journey, they synthesize large amounts of the fibrous protein keratin, which gives skin its strength and water-resistant properties. By the time they reach the stratum granulosum, the cells flatten, their organelles disintegrate, and they fill completely with keratin before eventually dying and forming the tough outer shield of the stratum corneum.
Besides keratinocytes, the epidermis houses specialized cells with unique defensive and sensory functions.
Skin color is determined primarily by the amount and type of melanin produced by melanocytes during a process called melanogenesis. Melanocytes package melanin into vesicles called melanosomes and transfer them to surrounding keratinocytes, forming a protective cap over the cell nucleus to shield genetic material from ultraviolet radiation damage. Differences in skin tone do not result from varying numbers of melanocytes, which remain relatively constant across all human populations, but rather from the rate of melanin production, the size of the melanosomes, and how long the pigment is retained.
The epidermis acts as a dynamic barrier that prevents excessive water loss from the body while blocking the entry of harmful microorganisms and toxins. This protective shield relies on a continuous cycle known as epidermal turnover. New cells generated in the stratum basale travel upward, mature, shed from the surface, and are replaced from below. This entire journey takes approximately four weeks in a healthy adult. Disruptions to this cycle, such as accelerated cell shedding seen in conditions like psoriasis, compromise the skin's barrier integrity and clinical appearance.