The layers of the epidermis of thin skin are shown here at higher magnification. The cell layer that occupies the deepest location is the stratum basale (SB). This is one cell deep. Just above this is a layer several cells in thickness, the stratum spinosum (SS). It consists of cells that have spinous processes on their surface. These processes meet with spinous processes of neighboring cells and, together, appear as intercellular bridges (arrows, inset). The next layer is the stratum granulosum (SGr), whose cells contain keratohyalin granules (arrowhead, inset). On the surface is the stratum corneum (SC). This consists of keratinized cells, i.e., cells that no longer possess nuclei. The keratinized cells are flat and generally adhere to other cells above and below without evidence of cell boundaries. In thick skin, a fifth layer, the stratum lucidum, is seen between the stratum granulosum and the stratum corneum. The pigment in the cells of the stratum basale is melanin; some of this pigment (P) is also present in connective tissue cells of the dermis.
KEY AT, adipose tissue BV, blood vessels D, duct of sweat glands De, dermis Ep, epidermis HF, hair follicle P, pigment SB, stratum basale SC, stratum corneum SGl, sebaceous gland SGr, stratum granulosum SS, stratum spinosum SW, sweat gland arrowhead, granules in cell of stratum granulosum arrows, intercellular bridges
The epidermis contains four distinctive cell types: keratinocytes, melanocytes, Langerhans’ cells, and Merkel’s cells. Keratinocytes 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 com-paring light skin (top figure) and dark skin (middle figure). 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 magni-fication. 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.
Light skin, human, H&E ×300. 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 mak-In routine H&E–stained paraffin sections of light skin, such ing it difficult to identify these three cell types with certainty.
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
Dark skin, human, H&E ×300. 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
In dark skin, most of the pigment is in the basal portion of the layers of the epidermis.