Nevi are lesions usually present at birth and are composed of mature or nearly mature structures. The term is more often applied in the literature to melanocytic nevi, which are benign proliferations of nevomelanocytes. Hamartoma is the appropriate term to describe nevi without nevus cells. j f

Melanocytic Nevi o

Melanocytic nevi are benign collections of nevus cells, which are identical to melano- §

cytes but arranged in clusters or nests. They present clinically as flat, pedunculated, dome-shaped, or papillomatous lesions with variable pigmentation. These tumors are extremely common and everyone has such blemishes, sometimes in profusion. They are commonly referred to as moles, usually develop during infancy and adolescence, and tend to disappear in middle age.

On histological examination, nevus cells have a variable appearance and sometimes the diagnosis is based on the observation of their gathering in clusters. Individual cells are cuboidal or oval, with homogeneous cytoplasm and a large or oval nucleus. Frequently they contain melanin. Deeper into the dermis, nevus cells resemble fibroblasts since they are elongated and posses a spindle-shaped nucleus.

Junctional nevi represent the earliest stage of development. The clusters of nevus cells are localized to the lower epidermis. On clinical examination they are either flat or slightly raised, and have a single color or are two-toned. In contrast to malignant melanoma, the pigment within the lesion is uniform. The surface is usually smooth and the skin creases preserved. The shape is oval or round, and size is variable but usually ranges from 6 to 10 mm.

Compound nevi represent a later stage of development with the nevus cells distributed in both the lower epidermis and superficial dermis. They are slightly to moderately raised, round, smooth or warty, sharply defined, and two-toned (Fig. 1). Some may grow coarse hairs on the surface.

Intradermal nevi tend to be flesh-colored because most cells containing pigment are situated deep in the dermis. Lesions are dome-shaped and may have telan-giectasias on the surface. On histological examination the junctional component is lost, which leaves an entirely intradermal lesion. Nevus cells become progressively

Figure 1 Compound melanocytic nevus of the lower midback. Lesion is small, slightly elevated, with well-demarcated borders and even pigmentation.
Figure 2 Giant bathing trunk nevus with extensive involvement of trunk and extremities. This variety of compund melanocytic nevus carries a significant risk for melanoma during infancy.

smaller with darkly staining nuclei and little cytoplasm. They may adopt a spindle cell form and frequently show what are termed neural features.

Atypical nevi or dysplastic nevi (3) are larger than their normal acquired melanocytic counterparts, ranging in size from 5 to 15 mm. They show various tones of tan, brown, black, and pink and characteristically there is a central palpable elevation. Atypical nevi continue to appear throughout life and are considered a marker and a risk factor for malignant melanoma. On histological examination these are compound nevi with atypical individual cells and nests.

Congenital melanocytic nevi (CMN) are present at birth. They affect approximately 1% of newborns and persist throughout life (4). They are usually larger than 1 cm, round to oval in shape, and elevated from surrounding skin. The surface may be smooth or warty and frequently hairy. Color ranges from light tan to dark brown with the darker portion located in the center of the lesion. The so-called giant bathing trunk nevus is a CMN covering one or several body segments (Fig. 2). On his- -g tological examination, CMN are either compound or intradermal nevi but extend deeper into the dermis and subcutaneous fat. The nevus cells involve sebaceous » glands, hair follicles, arrector pili muscles, and eccrine glands. In general, CMN smaller than 10 cm have a 1% chance of malignant transformation, whereas lesions ^

ci larger than 10 cm have a 6% chance. >3

Halo nevus (Sutton's nevus) is a melanocytic nevus surrounded by an area of |

complete depigmentation (5). It is thought to be an autoimmune phenomenon o because patients have detectable antibodies against melanocytes (Fig. 3). Lesions are single or multiple and are completely benign. There is an association with other

autoimmune disorders, particularly vitiligo. On histological examination, halo nevi represent compound melanocytic nevi. There is a marked inflammatory infiltrate surrounding the lesion with progressive destruction of pigmented cells.

Nevus spilus (6) is an acquired lesion characterized by a macular background -g with a raised and darker component within it. It has been associated with neurofi- |

bromatosis. On histological examination the lesion is characterized by epidermal len- »

tiginous hyperplasia in association with a junctional or compound nevus. c

Blue nevus (7) represents a failure of melanocytes migrating from the neural d crest to arrive at the dermal-epidermal junction. Lesions appear as round to oval >3

blue papules with a smooth surface. The melanocytes are found in the lower dermis |

and the refraction of light at this level results in the blue appearance (Tyndall's o effect). On histological examination there are heavily pigmented spindle-shaped mel-anocytes arranged in fascicles in the dermis. @

Figure 4 Nevus of Ota involving the left forehead, eyebrow, eyelid, and inner canthus. The lesion is blue, congenital, and permanent.

Mongolian blue spots (8) also represent a migratory arrest of melanocytes. They appear as extensive blue macules over the buttocks and sacrum. Asian, Latin American, and African-American newborns are more susceptible. Lesions tend to disappear with time and there is no risk of malignant transformation. There are elon- -g gated dendrytic melanocytes in the reticular dermis on histological examination.

Nevus of Ota and Ito (9) is a permanent blue macule indistinguishable from the »

Mongolian blue spot. Nevus of Ota occurs in the face on the area supplied by the ophthalmic and maxillary divisions of the trigeminal nerve (Fig. 4). Nevus of Ito ^

d affects the shoulder in an area supplied by the posterior supraclavicular and lateral brachial cutaneous nerves. Both are more frequent in Japanese patients. They are identical histologically to the Mongolian blue spot.

Spitz nevi (juvenile melanoma) are uncommon but important because they may create confusion with malignant melanoma (10). The lesions are usually solitary @

smooth-papules or nodules that often involve the face. They are dome-shaped and red to brown in color. Their size is variable but usually less than 1 cm. In contrast to melanoma, Spitz nevi typically affect children and adolescents. On histological examination the cells are epithelioid or, more commonly, spindle-shaped with abundant eosinophilic cytoplasm and large but typically uniform nuclei. Mononuclear and multinucleate giant cells may be present. The deeper aspect of the lesion shows evidence of maturation, as would be expected in a benign compound nevus. In approximately 60% of cases there are eosinophilic globules in the epidermis (Kamino bodies). In addition, the lack of atypical mitoses, and the evident maturation towards deeper parts of the tumor, help to differentiate a Spitz nevus from melanoma.

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