 FOLDER 15.2 Functional Considerations: Skin Color
The color of an individual’s skin is caused by several fac-tyrosinase, the transformation of tyrosine into 3,4 secretion because a small amount of cytoplasm surrounding the melanosome is also phagocytosed.
Taking into consideration the complexity of melanine biogenesis, protein trafficking, organelle movement, and cell-to-cell interactions within the epidermal-melanin unit, it is explainable that even minor changes in the cellular environment can affect melanosome structure and pigment donation process. Numerous intrinsic and extrinsic factors are also responsible for skin pigmentation, including age, ethnicity and gender differences, variable hormone levels and affinities of their receptors, genetic defects, ultraviolet radiation, climate and season changes, and chemical exposure to toxins and pollutants.
Langerhans’ cells are antigen-presenting cells in the epidermis.
Langerhans’ cells are dendritic-appearing, antigen-presenting cells in the epidermis. They originate from common lymphoid progenitor (CLP) cells in bone marrow, migrate via the bloodstream, and ultimately enter the epidermis where they differentiate into immunocompetent cells. Langerhans’ cells encounter and process antigens entering through the skin. Therefore, they constitute part of the mononuclear phagocytotic system (MPS; page 185). Once antigen is phagocytized, processed, and  displayed on the surface of the Langerhans’ cell, the cell migrates from the epidermis to a regional lymph node where it interacts with T lymphocytes.
Langerhans’ cells cannot be distinguished with certainty in routine H&E–stained paraffin sections. Like melanocytes, Langerhans’ cells do not form desmosomes with neighboring keratinocytes. The nucleus stains heavily with hematoxylin, and the cytoplasm is clear. With special techniques, such as gold chloride impregnation or immunostaining with antibody against CD1a molecules, Langerhans’ cells can be readily seen in the stratum spinosum. They possess dendritic processes resembling those of the melanocyte. The TEM reveals several distinctive features of a Langerhans’ cell (Fig. 15.9). Its nucleus is characteristically indented in many
FIGURE 15.9 • Electron micrograph of a Langerhans’ cell. The nucleus (N) of a Langerhans’ cell is characteristically indented in many places, and the cytoplasm contains distinctive rod-shaped bodies (arrows). Note the presence of tonofilaments (T ) in adjacent keratinocytes (K) but the absence of these filaments in the Langerhans’ cell. 19,000. Inset. Photomicrograph of the epidermis shows the distribution and dendritic nature of the Langerhans’ cells that were stained via immunostaining techniques with antibodies against CD1a surface antigen. 300. (Reprinted with permission from Urmacher CD. In: Sternberg SS, ed. Histology for Pathologists. Philadelphia: Lippincott-Raven, 1997.) places, so the nuclear profile is uneven. Also, it possesses characteristic, tennis racquet–shaped Birbeck granules. They represent relatively small vesicles, which appear as rods with a bulbous expansion at their end.