Terminal differentiation of the epidermal cells, which begins with the cell divisions in the stratum basale, is considered a specialized form of apoptosis. Cells in the stratum granulosum exhibit typical apoptotic nuclear morphology, including fragmentation of their DNA. However, the cellular fragmentation associated with normal apoptosis does not occur; instead, the cells become filled with filaments of the intracellular protein keratin and are later sloughed from the skin surface.
The stratum basale provides for epidermal cell renewal.
The stratum basale is represented by a single layer of cells that rests on the basal lamina (Plate 42, page 515). It contains the stem cells from which new cells, the keratinocytes, arise by mitotic division. For this reason, the stratum basale is also called the stratum germinativum. The cells are small and cuboidal to low columnar. They have less cytoplasm than the
FIGURE 15.2 • Photomicrograph showing the layers of thick skin. This specimen obtained from the skin of the sole of the foot (human) shows epidermis (Epi) containing the extremely thick stratum corneum (SC). Remaining layers of the epidermis (except for the stratum lucidum, which is not present on this slide)—that is, the stratum basale (SB), the stratum spinosum (SS), and the stratum granulosum (SGr)—are clearly visible in this routine H&E preparation. The duct of a sweat gland (D) can be seen on the left as it traverses the dermis (Derm) and further spirals through the epidermis. At the sites where the ducts of the sweat gland enter the epidermis, note the epidermal downgrowths known as interpapillary pegs. The dermis contains papillae, protrusions of connective tissue that lie between the interpapillary pegs. Note also the greater cellularity of the papillary layer (PL) and that the collagen fibers of the reticular layer (RL) are thicker than those of the papillary layer. 65.
cells in the layer above; consequently, their nuclei are more closely spaced. The closely spaced nuclei, in combination with the basophilic cytoplasm of these cells, impart a noticeable basophilia to the stratum basale. The basal cells also contain various amounts of melanin (described later) in their cytoplasm that is transferred from neighboring melanocytes interspersed in this layer. Basal cells exhibit extensive cell junctions; they are connected to each other and to keratinocytes by desmosomes and to the underlying basal lamina by hemidesmosomes. As new keratinocytes arise in this layer by mitotic division, they move into the next layer, thus beginning their process of upward migration. This process terminates when the cell becomes a mature keratinized cell, which is eventually sloughed off at the skin surface.
FIGURE 15.3 • Photomicrograph of the stratum spinosum and stratum basale. The epidermis of thin skin is shown here at higher magnification. The one-cell-deep layer at the base of the epidermis just above the connective tissue (CT ) of the dermis is the stratum basale (SB). The cells of this layer rest on the basement membrane. A layer referred to as the stratum spinosum (SS) is located just above the stratum basale. It consists of cells with spinous processes on their surfaces. These processes are attached to spinous processes of neighboring cells by desmosomes and together appear as intercellular bridges. 640.
The cells of the stratum spinosum characteristically exhibit spinous processes.