Epidermis

The epidermis is the outermost layer of animal skin. In humans, it forms a renewing barrier that limits water loss and blocks many environmental threats.

Keratinocyte: A keratin-producing cell that makes up most of the epidermis. Keratinocytes build the epidermal layers and strengthen its protective surface.
Basal cell: A dividing cell in the deepest epidermal layer that produces new keratinocytes. Basal cells replenish the epidermis as surface cells are shed.
Psoriasis: A chronic inflammatory disease that causes sharply demarcated, thickened, scaly skin plaques. Accelerated epidermal growth and altered maturation produce its characteristic plaques.
Dermis: The connective-tissue layer beneath the epidermis, containing blood vessels, nerves, and structural fibers. The dermis supports and nourishes the epidermis but has a different tissue composition.
Epidermal development: The embryonic formation and differentiation of the epidermis from surface ectoderm. Development establishes the epidermal layers and their specialized cell populations.
Stratum corneum: The outermost epidermal layer, made chiefly of flattened dead cells embedded in lipids. This layer forms the epidermis’s principal barrier against water loss and external substances.
Melanocyte: A pigment-producing cell that synthesizes melanin and transfers it to neighboring cells. Melanocytes in the epidermis help protect cell nuclei from ultraviolet radiation.
Atopic dermatitis: A chronic inflammatory skin disease marked by itchy, relapsing eczema and impaired barrier function. Barrier disruption in the epidermis contributes to dryness and heightened sensitivity.
Hypodermis: The layer of loose connective tissue and fat beneath the dermis. It lies deeper than the epidermis and provides insulation, cushioning, and attachment.
Ectoderm: The embryonic germ layer that gives rise to the epidermis and several nervous-system structures. The epidermis originates from ectoderm during embryonic development.

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