epidermis

The epidermis is the outermost layer of the skin, consisting of keratinized stratified squamous epithelium that provides a protective barrier between the body and the environment. It is avascular and relies on diffusion from the underlying dermis for nutrients and oxygen.

The human epidermis comprises four main layers in thin skin and an additional stratum lucidum in thick skin. From the basement membrane upward: stratum basale (germinativum), stratum spinosum, stratum granulosum, stratum lucidum (only in thick skin), and stratum corneum.

Key cell types: keratinocytes form the majority; melanocytes in the basal layer synthesize melanin; Langerhans cells act as antigen-presenting dendritic cells; Merkel cells are mechanoreceptors associated with nerve endings.

Functions: the epidermis acts as a barrier to water loss and to physical and chemical threats; melanin protects against ultraviolet radiation; it participates in immune defense and sensation; keratinization forms corneocytes; desmosomes and tight junctions help cohesion and barrier integrity.

Turnover and renewal: keratinocytes proliferate in the basal layer and migrate upward, undergoing maturation as they move toward the surface. Complete turnover is about 28 days in young adults, with rates varying by age and body site.

Structure and junctions: the epidermis rests on the basement membrane, which includes a complex of proteins at the epidermal-dermal junction that anchors the two layers and maintains tissue integrity.

Clinical relevance: diseases such as melanoma arise from melanocytes; non-melanoma skin cancers include basal cell carcinoma and squamous cell carcinoma; inflammatory conditions like dermatitis and psoriasis affect epidermal function and turnover.

Development and variation: the epidermis is derived from ectoderm and interacts with the dermis during development and repair; hair follicles and sweat glands originate from epidermal invaginations. Thickness and pigmentation vary by site and exposure.