Photomicrograph showing the layers of thin skin. This H&E-stained specimen from human skin shows the two chief layers of the skin, the epidermis (Epi) and dermis (Derm). The epidermis forms the surface; it consists of stratified squamous epithelium that is keratinized. The dermis consists of two layers: the papillary layer, which is the most superficial layer and is adjacent to the epidermis, and the more deeply positioned reticular layer. The boundary between these two layers is not conspicuous; the papillary layer is, however, more cellular than the reticular layer. In addition, the collagen fibers of the reticular layer are thick (clearly visible in the lower part of the figure); those of the papillary layer are thin. x45.
are cuboidal to low columnar. They have less cytoplasm than the 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, imparts 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.
The cells of the stratum spinosum characteristically exhibit spinous processes
The stratum spinosum is at least several cells thick. The cells are larger than those of the stratum basale. They exhibit numerous cytoplasmic processes or spines, which gives this layer its name (Fig. 14.3). The processes are attached to similar processes of adjacent cells by desmosomes. In the light microscope, the site of the desmosome appears as a slight thickening called the node of Bizzozero. The processes are usually very conspicuous, in part because the cells shrink during preparation and a resultant expanded intercellular space develops between the spines. Be-
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