AT, adipose tissue P, pigment SS, stratum spinosum

BV, blood vessels SB, stratum basale SW, sweat gland

D, duct of sweat glands SC, stratum corneum arrowhead (inset), granules in cell of stra-

De, dermis SGI, sebaceous gland turn granulosum

Ep, epidermis SGr, stratum granulosum arrows (inset), "intercellular bridges" HF, hair follicle

The epidermis contains four distinctive cell types: keratinocytes, melanocytes, Langerhans' cells, and Merkel's cells. Ker-atinocytes are the most numerous of these cells; they are generated in the stratum basale and move toward the surface. As they do so, they produce the intracellular protein keratin and the special extracellular lipid that serves as a water barrier in the upper layers of the epidermis. Histologically, the keratinocytes are the cells that show spinous processes in the stratum spinosum. The other three cell types are not readily identified in H&E-stained paraffin sections. The product of the melanocyte is, however, evident in H&E sections, and this is considered in the first two figures of this plate.

The skin contains a pigment, melanin, which protects the tissue against the harmful effects of ultraviolet light. It is formed by the melanocytes that then pass the pigment to the keratinocytes. More pigment is present in dark skin than in light skin; this can be seen by comparing light skin (Fig. 1) and dark skin (Fig. 2.). The epidermis and a small amount of the dermis are shown in each figure. Whereas the deep part of the dark skin contains considerable pigment, the amount of pigment in light skin is insufficient to be noticeable at this magnification. Cells for producing the pigment are present in both skin types and in approximately equal numbers. The difference is due to more rapid digestion of the pigment by lysosomes of keratinocytes in light skin. After prolonged exposure to sunlight, pigment is also produced in sufficient amounts to be seen in light skin.

Figure 1, light skin, human, H&E X300.

In routine H&E-stained paraffin sections of light skin, such as this sample, the melanocytes are among the cells that appear as small, rounded, clear cells (CC) mixed with the other cells of the stratum basale. However, not all clear cells of the epidermis are melanocytes. For example, Langerhans' cells may also appear as clear cells, but they are located more superficially in the stratum spinosum. Merkel's cells may also appear as clear cells, thus making it difficult to identify these three cell types with certainty.

Figure 2, dark skin, human, H&E X300.

In dark skin, most of the pigment is in the basal portion of the epidermis, but it is also present in cells progressing toward the surface and within the nonnucleated cells of the keratinized layer. The arrows indicate the melanin pigment

Figure 3, skin, human, H&E and elastin stain X200; inset X450.

This figure is included because it shows certain features of the dermis, the connective tissue layer of the skin. The dermis is divided into two layers: the papillary layer (PL) of loose connective tissue and the reticular layer (RL) of more dense connective tissue. The papillary layer is immediately under the epidermis. It includes the connective tissue papillae that project into the undersurface of the epidermis. The reticular layer is deep to the papillary layer. The boundary between these two layers is not demarcated by any specific structural feature except for the change in the histologic composition of the two layers.

This specimen was stained with H&E and also with a procedure to display elastic fibers (EF). They are relatively in keratinocytes of the stratum spinosum and in the stratum corneum. In light skin, the melanin is broken down before it leaves the upper part of the stratum spinosum. Thus, pigment is not seen in the upper layers of the epidermis.

thick and conspicuous in the reticular layer (see also inset), where they appear as the dark-blue profiles, some of which are elongate, whereas others are short. In the papillary layer, the elastic fibers are thinner and relatively sparse (arrows). The inset shows the typical eosinophilic staining of the thick collagenous fibers in the reticular layer. Although the collagenous fibers at the lower magnification of this figure are not as prominent, it is nevertheless possible to note that they are thicker in the reticular layer than in the papillary layer. The papillary layer is evidently more cellular than the reticular layer. Many of the small dark-blue profiles in the reticular layer represent oblique and cross sections through elastic fibers (see inset) and not nuclei of cells.

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