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Disorders of Melanocytes

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Acquired Nevomelanocytic Nevi (NMN)

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  • NMN, commonly called moles, are very common, small (<1 cm), circumscribed, acquired pigmented macules, papules, or nodules.

  • Composed of groups of melanocytic nevus cells located in the epidermis, dermis, and, rarely, subcutaneous tissue.

  • They are benign, acquired tumors arising as nevus cell clusters at the dermal–epidermal junction (junctional NMN), invading the papillary dermis (compound NMN), and ending their life cycle as dermal NMN with nevus cells located exclusively in the dermis where, with progressive age, there will be fibrosis.

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Epidemiology and Etiology

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One of the most common acquired new growths in Caucasians (most adults have about 20 nevi), less common in blacks or pigmented persons, and sometimes absent in persons with red hair and marked freckling (skin phototype I).

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Race
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Blacks and Asians have more nevi on the palms, soles, and nail beds.

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Heredity
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Common acquired NMN occur in family clusters. Dysplastic melanocytic nevi (DN) (see Section 12), which are putative precursor lesions of malignant melanoma, are different from NMN and occur in virtually every patient with familial cutaneous melanoma and in 30–50% of patients with sporadic nonfamilial primary melanoma.

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Sun Exposure
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A factor in the induction of nevi on the exposed areas.

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Significance
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Risk of melanoma is related to the numbers of NMN and to DN. In the latter, even if only a few lesions are present.

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Clinical Manifestation

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Duration and Evolution of Lesions
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NMN appear in early childhood and reach a maximum in young adulthood even though some NMN may arise in adulthood. Later on there is a gradual involution and fibrosis of lesions, and most disappear after the age of 60. In contrast, DN continue to appear throughout life and are believed not to involute (see Section 12).

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Skin Symptoms
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NMN are asymptomatic. However, NMN grow and growth is often accompanied by itching. Itching per se is not a sign of malignancy, but if a lesion persistently itches or is tender, it should be followed carefully or excised, since persistent pruritus may be an early indication of malignant change.

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Classification

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NMN are multiple (Fig. 9-1A) and can be classified according to their state of evolution and thus according to the histologic level of the nevus cell clusters (Fig. 9-1B).

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  1. Junctional melanocytic NMN: These arise at the dermal–epidermal junction, on the epidermal side of the basement membrane; in other words, they are intraepidermal (Figs. 9-1B and 9-2).

  2. Compound NMN: Nevus cells invade the papillary dermis, and nevus cell nests are now found both intraepidermally and dermally (...

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