Skin, the largest organ in the human body, has a variety of functions, but it primarily serves as a physical barrier for the retention of water and as a form of innate immunity. The epidermis is the stratified, outermost epithelial layer of human skin. It is principally composed of two elements: the adnexa and the interfollicular epidermis. The adnexa is the collective term used to refer to the hair follicles and associated sebaceous glands and arrector pili muscles, as well as the sweat glands and nails. The interfollicular epidermis is mostly made of keratinocytes, which make up 95 percent of the cells of the epidermis. These cells are generated in the stratum basale, become terminally differentiated, and migrate to the skin surface, forming multiple layers.

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