The epidermis is made of four or five layers of epithelial cells, depending on its location in the body. From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.
The cells in all these layers except the stratum basale are called keratinocytes, a type of cell that manufactures and stores the protein keratin. The keratinocytes in the stratum corneum are dead and regularly slough away, being replaced by cells from the deeper layers.
The stratum basale is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All keratinocytes are produced from this single layer of cells, constantly going through mitosis to produce new cells. As new cells are formed, the existing cells are pushed superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell, which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch. These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte, a cell that produces the pigment melanin. Melanin gives hair and skin their color and helps protect the living cells of the epidermis from ultraviolet (UV) radiation damage.
The stratum spinosum is composed of eight to ten layers of keratinocytes formed due to cell division in the stratum basale. Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell, which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells in this layer. The keratinocytes in the stratum spinosum begin the synthesis of keratin and release a water-repelling glycolipid that helps prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes are produced atop the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum.
The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum. The cells (three to five layers deep) become flatter, and their cell membranes thicken, generating large amounts of the proteins keratin, which is fibrous, and keratohyalin, which accumulates as lamellar granules within the cells. These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the next two layers of the epidermis – the stratum lucidum, and the stratum corneum.
The keratinocytes that compose the stratum lucidum are dead and flattened. These cells are densely packed with eleidin, a clear protein derived from keratohyalin, giving these cells a transparent (i.e., lucid) appearance.
Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum (or stratum lucidum in the case of the palms and soles of feet). The entire layer is replaced in about four weeks. Cosmetic procedures, such as microdermabrasion, help remove some dry upper layers and keep the skin looking "fresh" and healthy.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 5.1: Layers of the Skin
