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# Epidermal Strata (a) Photomicrograph and (b) diagram compare the order and relationship of the epidermal strata in thick skin. ## Stratum Basale The deepest epidermal layer is the **stratum basale** (strat'ŭm bah-sā'lē), also known as the **stratum germinativum**, or basal layer. This single laye...

# Epidermal Strata (a) Photomicrograph and (b) diagram compare the order and relationship of the epidermal strata in thick skin. ## Stratum Basale The deepest epidermal layer is the **stratum basale** (strat'ŭm bah-sā'lē), also known as the **stratum germinativum**, or basal layer. This single layer of cuboidal to low columnar cells is tightly attached by hemidesmosomes (see section 4.6d) to an underlying basement membrane that separates the epidermis from the connective tissue of the dermis. Three types of cells occupy the stratum basale (figure 6.2b): - **Keratinocytes** (ke-ra'ti-nō-sīt; keras = horn) are the most abundant cell type in the epidermis and are found throughout all epidermal strata. The stratum basale is dominated by large keratinocyte stem cells, which divide to generate new keratinocytes that replace dead keratinocytes shed from the surface. Their name is derived from their synthesis of **keratin**, a protein that strengthens the epidermis considerably. Keratin is one of a family of fibrous structural proteins that are both tough and insoluble. Fibrous keratin molecules can twist and intertwine around each other to form helical intermediate filaments of the cytoskeleton (see section 4.6b). The specific type of keratin proteins found in keratinocytes are called **cytokeratins**. Their structure in these keratinocytes gives skin its strength and makes the epidermis water resistant. - **Melanocytes** (mel'ă-nō-sīt; melano = black) have long, branching processes and are scattered among the keratinocytes of the stratum basale. They produce and store the pigment **melanin** (mel'ă-nin) in response to ultraviolet light exposure. Their cytoplasmic processes transfer melanin pigment within granules called **melanosomes** (mel'ă-nō-somes) to the keratinocytes within the basal layer and sometimes in more superficial layers. This pigment (which includes the colors black, brown, tan, and yellow-brown) accumulates around the nucleus of the keratinocyte and shields the nuclear DNA from ultraviolet (UV) radiation. The darker tones of the skin result from melanin produced by the melanocytes. Thus, "tanning" is the result of the melanocytes producing melanin to block UV light from causing mutations in the DNA of your keratinocytes (in the epidermis) and fibroblasts (in the dermis). - **Tactile cells**, also called **Merkel cells**, are few in number and found scattered among the cells within the stratum basale. Tactile cells are sensitive to touch and, when compressed, they release chemicals that stimulate sensory nerve endings. These cells are more common in the stratum basale in sensitive areas of the skin, such as the fingertips (see section 16.2a). ## Stratum Spinosum Several layers of polygonal keratinocytes form the **stratum spinosum** (spī-nō'sum), or spiny layer. Each time a keratinocyte stem cell in the stratum basale divides, the new cell is pushed toward the external surface, while the other cell remains as a stem cell in the stratum basale. Once this new cell enters the stratum spinosum, it begins to differentiate into a nondividing, highly specialized keratinocyte. The keratinocytes in the stratum spinosum attach to their neighbors by many membrane junctions called **desmosomes**, which serve to provide structural support between cells of the epidermis (see section 4.6d). The process of preparing epidermal tissue for observation on a microscope slide shrinks the cytoplasm of the cells in the stratum spinosum. Because the cytoskeleton and desmosomes remain intact, the shrunken keratinocytes in the stratum spinosum resemble miniature porcupines that are attached to their neighbors. This spiny appearance accounts for the name of this layer. In addition to the keratinocytes, the stratum spinosum also contains the fourth epidermal cell type, called **epidermal dendritic (Langerhans) cells** (figure 6.2b). Epidermal dendritic cells are immune cells that help fight infection in the epidermis. These immune cells are often present in the stratum spinosum and stratum granulosum, but they are not identifiable in standard histologic preparations. Their phagocytic activity initiates an immune response to protect the body against pathogens that have penetrated the superficial epidermal layers as well as epidermal cancer cells (see section 22.3c).