Figure 149

Formation of melanin pigment and secretion of pigment granules into keratinocytes. Melanocytes produce membrane-bounded structures that originate in the Golgi apparatus as premelanosomes ©. Within the early melanosomes, as maturation proceeds, melanin is produced from tyrosine by a series of enzymatic reactions®. Mature melanosomes and their melanin contents are transferred to neighboring keratinocytes by pigment donation, which involves the phagocytosis of the tips of the melanocyte In darker skin (on the right), the melanin is degraded slowly, and melanosomes remain discrete; in lighter skin (on the left), the melanin is degraded more rapidity (4) through the process of macroautophagy. (Based on Weiss L, Greep RO. Histology. New York: McGraw-Hill, 1977.)

boring keratinocytes. The nucleus stains heavily with hematoxylin, and the cytoplasm is clear. With special techniques, such as gold chloride impregnation or im-munostaining with antibody against CD la molecules, Langerhans' cells can be readily seen in the stratum spin-osum. They possess dendritic processes resembling those of the melanocyte. The TEM reveals several distinctive features of a Langerhans' cell (Fig. 14.10). Its nucleus is characteristically indented in many places, so the nuclear profile is uneven. Also, it possesses characteristic, tennis racket-shaped Birbeck granules. They represent relatively small vesicles, which appear as rods with a bulbous expansion at their end.

Like macrophages, Langerhans' cells express both MHC I and MHC II molecules, as well as Fc receptors for immunoglobulin G (IgG). Langerhans' cells also express complement C3b receptors as well as fluctuating quantities of CDla molecules. As an antigen-presenting cell, the Langerhans' cell is involved in delayed-type hypersensitivity reactions (e.g., contact allergic dermatitis and other cell-mediated immune responses in the skin) through the

CHAPTER 14 I hiletjumentniy System 409 BOX 14.1

Functional Considerations: Skin Color

The color of an individual's skin is due to a number of factors. The most significant is melanin content. Although the number of melanocytes is essentially the same in all races, the nature of the melanin that Is produced by the melanocytes differs. For example, due to the lysosomal activity of the keratinocytes, melanin is degraded more rapidly in individuals with light skin than in individuals with dark skin. In the former, melanosomes are more concentrated in the keratinocytes nearest the basal layer and are relatively sparse in the midregion of the stratum granulosum. In contrast, dark skin may exhibit melanosomes throughout the epidermis, including the stratum corneum.

In addition, melanin pigment comprises two distinct forms. One form, eumelanin, is a brownish black pigment. The other form, pheomelanin, is a reddish yellow pigment. Each is genetically determined. Coloration is most apparent in hair because of the concentration of melanin pigment granules, but it is also reflected in skin coloration.

Exposure to ultraviolet light, particularly the sun's rays, accelerates the rate of melanin production and thus protects against further radiation effects. Increased pigmentation of the skin may also result from hormonal imbalance, as, for example, in Addison's disease. Lack of pigmentation occurs in a condition known as albinism. In this condition, premelanosomes are produced by melanocytes, but because of the absence of tyrosinase, the transformation of tyrosine into DO PA and the subsequent transformation of DOPA into melanin fail to occur. Thus, there is no pigmentation in the skin or hair of these individuals.

The number of melanocytes decreases with age, resulting in decreasing availability of pigment donation to keratinocytes. Therefore, in older age the skin becomes lighter and the incidence of skin cancer increases. Other normal factors that affect skin coloration include the presence of oxyhemoglobin in the dermal vascular bed, which imparts a red hue; the presence of carotenes, an exogenous orange pigment taken up from foods and concentrated in tissues containing fat; and the presence of certain endogenous pigments. The latter include degradation products of hemoglobin, iron-containing hemosiderin and iron-free bilirubin, all of which impart color to the skin. Hemosiderin is a golden brown pigment, whereas bilirubin is a yellowish brown pigment. Bilirubin is normally removed from the bloodstream by the liver and eliminated via the bile. A yellowish skin color due to abnormal accumulation of bilirubin reflects liver dysfunction and is evidenced as jaundice.

uptake of antigen in the skin and its transport to the lymph nodes. Skin biopsy specimens from individuals with AIDS or AIDS-related complex reveal that Langerhans' cells contain HIV in their cytoplasm. Langerhans' cells appear to be more resistant than T cells to the deadly effects of the HIV and may, therefore, serve as a reservoir for the virus. Langerhans' cells are of mesenchymal origin and are derived from the CD34' stem cell in the bone marrow.

melanosome degradation

melanosome degradation

secretion

synthesis of melanin in early melanosome premelahosome — formation

Therefore, they constitute part of the mononuclear phago-cytotic system (MPS) (page 144).

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