epidermises

Epidermises, a rarely used plural in modern anatomy, refers to the epidermis, the outermost layer of the skin in vertebrates. The epidermis is avascular and lies above the dermis, receiving nutrients by diffusion. It forms a protective interface with the environment and participates in immune defense and sensory functions. The tissue is primarily made up of keratinocytes, with smaller populations of melanocytes, Langerhans cells, and Merkel cells.

Structure of the epidermis is organized into distinct layers. From the base upward, the stratum basale (germinativum) contains proliferative keratinocytes and stem cells. Above it lie the stratum spinosum and the stratum granulosum, where keratinocytes begin to differentiate. The stratum lucidum is present only in thick skin, such as the palms and soles. The outermost layer is the stratum corneum, composed of flattened, anucleate cells that form a tough, protective barrier.

Development and renewal: The epidermis renews itself continuously. Basal keratinocytes divide and supply new cells that migrate outward, undergoing keratinization as they move toward the surface and are eventually shed as corneocytes. In humans, the turnover time is typically a few weeks.

Functions: The epidermis provides a protective barrier against mechanical injury, pathogens, and chemical exposure, and minimizes water loss. Melanin in melanocytes protects against ultraviolet radiation. Langerhans cells contribute to immune surveillance, and Merkel cells are involved in gentle touch sensation. The epidermis also participates in vitamin D synthesis upon UV exposure.

Clinical relevance: Disorders of the epidermis include psoriasis, atopic dermatitis, and epidermolysis bullosa, which affect barrier integrity or epidermal-dermal adhesion. Burns and infections can damage or disrupt epidermal layers.