Stacey Mills_Histology for Pathologists_9781496398949

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SECTION I : CutaneousTissue

plasma membrane of these melanocytes to the basal lamina. Laminin-5, a component of anchoring filaments, may be a ligand for melanocyte attachment to the basement mem- brane in vivo (49). In addition, melanocytes that are close to the basal lamina have structures resembling hemidesmo- somes of basal keratinocytes (50). Melanocytes produce and secrete melanin. Melanin can be red (pheomelanin) or yellow-black (eumelanin). The most important function of melanin is to protect against the injurious effects of nonionizing ultraviolet irradiation. Melanin is formed through a complex metabolic pro- cess in which tyrosinase is the main catabolic enzyme, using tyrosine as substrate. The synthesis of melanin takes place in melanosomes, lysosome-related organelles. In the early stages of development, melanosomes are membrane-limited vesicles, located in the Golgi-associated endoplasmic reticu- lum. The maturation of melanosomes undergoes four stages. Stage I melanosomes are round without melanin. These are seen in balloon cell melanoma. Stage II through stage IV melanosomes are ellipsoidal with numerous longitudinal fil- aments. Melanin deposits start at stage II. In stage III, mela- nin deposits are prominent. Stage IV melanosomes are fully packed, with melanin obscuring the internal structures. The developing melanosomes, with their content of melanin, are transferred to the neighboring basal keratino- cytes and hair follicular cells. The mechanism of melanin transfer is a complex process (51,52), with the end result being phagocytosis of the tip of melanocytic dendrites by the keratinocytes (Fig. 1.9) in a process called pigment donation (53). The number of melanocytes in normal skin is constant in all races, the ratio being 1 melanocyte for every 4 to 10 basal keratinocytes. Alteration of this ratio is important in the diagnosis of certain pigmented lesions such as malig- nant melanoma of the lentigo maligna type and etiologies of clinical hypopigmentation such as vitiligo.

FIGURE 1.7  Ultrastructure of basement membrane ( × 37,800) ( 1 , hemides- mosome; 2 , lamina lucida; 3 , lamina densa; 4 , lamina reticularis; 5 , melanin; 6 , tonofilaments).

the melanocytes are localized in the basal layer, and their dendritic processes extend in all directions. The dendritic nature of normal melanocytes is usually not seen in routine H&E-stained sections. In H&E preparations, melanocytes are composed of elongated or ovoid nuclei surrounded by a clear space (Fig. 1.8). They are usually smaller than the neighboring basal keratinocytes. Melanocytes do not con- tain tonofilaments and do not attach to basal cells with des- mosomes. However, anchoring filaments extend from the

FIGURE 1.8  Melanocytes in the basal layer, composed of ovoid nuclei within a clear space. Copyright © 2020 Wolters Kluwer Health, I c. Unauthorized reproduction of this content is prohibited. FIGURE 1.9  Electron micrograph showing membrane-bound phagocy- tized melanin in keratinocyte ( × 19,200).

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