Epidermis

The epidermis is the outermost layer of the skin, serving as the body's primary barrier against the external environment. It is a stratified squamous keratinized epithelium composed predominantly of keratinocytes, which undergo a tightly regulated process of proliferation, differentiation, and programmed cell death as they migrate from the basal layer to the surface.

Structure and Layers

The epidermis is organized into distinct layers (from deep to superficial):

• Stratum basale (basal layer) — A single layer of columnar to cuboidal keratinocytes resting on the basement membrane. These are the proliferative stem cells that continuously replenish the epidermis.

• Stratum spinosum (spinous/prickle cell layer) — Multiple layers of polyhedral keratinocytes connected by desmosomes, giving them a "spiny" appearance on histology.

• Stratum granulosum (granular layer) — Cells containing keratohyalin granules and lamellar bodies, which release lipids that form the intercellular lipid barrier critical for waterproofing.

• Stratum lucidum — A thin, translucent layer found only in thick skin (palms and soles).

• Stratum corneum (cornified layer) — The outermost layer composed of flattened, anucleate, dead keratinocytes (corneocytes) embedded in a lipid matrix, forming the principal physical and permeability barrier.

Cellular Composition

Beyond keratinocytes (~90% of epidermal cells), the epidermis contains several other important cell types:

• Melanocytes — Neural crest-derived cells located in the basal layer that produce melanin, providing photoprotection against ultraviolet radiation.

• Langerhans cells — Bone marrow-derived dendritic antigen-presenting cells that play a key role in cutaneous immune surveillance.

• Merkel cells — Mechanoreceptors associated with nerve endings, concentrated in areas of high tactile sensitivity (e.g., fingertips, lips).

Key Functions

• Barrier function — Protection against water loss, mechanical injury, chemical penetration, and microbial invasion, largely attributable to the stratum corneum and its intercellular lipid lamellae.

• Immune defense — The epidermis functions as an active immunological organ through Langerhans cells, keratinocyte-derived cytokines, and antimicrobial peptides (e.g., defensins, cathelicidins).

• Photoprotection — Melanin produced by melanocytes absorbs and scatters ultraviolet radiation, protecting underlying structures from DNA damage.

• Sensation — Merkel cells and free nerve endings within the epidermis mediate touch, temperature, and pain perception.

• Thermoregulation and vitamin D synthesis — Keratinocytes in the epidermis convert 7-dehydrocholesterol to previtamin D₃ upon UVB exposure.

Thickness and Turnover

The epidermis varies in thickness from approximately 0.05 mm (eyelids) to 1.5 mm (palms and soles). Epidermal turnover — the time for a basal keratinocyte to reach the surface and desquamate — is approximately 28 to 30 days under normal conditions, though this is accelerated in hyperproliferative conditions such as psoriasis.

The epidermis is avascular; it receives nutrients by diffusion from the underlying dermis through the dermoepidermal junction (basement membrane zone), a complex structure critical for adhesion and signaling between the two layers.