Common melanocytic nevi are tan-to-brown, uniformly pigmented, small papules (5 mm or less across) with well-defined, rounded borders (
Fig. 24.19A ). Early lesions are composed of round to oval cells that grow in “nests” along the dermoepidermal junction. Nuclei are uniform and round, and contain inconspicuous nucleoli with little or no mitotic activity. Such early-stage lesions are called junctional nevi. Eventually, most junctional nevi grow into the underlying dermis as nests or cords of cells (compound nevi), and in older lesions the epidermal nests may be lost entirely, creating intradermal nevi (
Fig. 24.19B
There are numerous types of melanocytic nevi, with varied appearances. Although these lesions usually are of only cosmetic concern, they may cause irritation or mimic melanoma, requiring their surgical removal. Compound and intradermal nevi often are more elevated than junctional nevi.
Dysplastic nevi may be sporadic or familial. The latter are important clinically because they identify individuals who
Fig. 24.18 Possible steps in the development of melanocytic nevi. (A) Normal skin shows only scattered melanocytes. (B) Junctional nevus. (C) Compound nevus. (D) Intradermal nevus. (E) Intradermal nevus with extensive cellular senescence.
Fig. 24.19 Melanocyticnevus.(A)Melanocyticneviarerelativelysmall,symmetric,anduniformlypigmented.(B)Anevuscomposedofmelanocytesthatlosepigmentationandbecomesmallerandmoredispersedastheyextendintothedermis—allsignsthatspeaktothebenignnatureoftheproliferation.
have an increased risk of developing melanoma. As with conventional melanocytic nevi, activating RAS or BRAF mutations are commonly found in dysplastic nevi and are believed to have a pathogenic role.
MORPHOLOGYDysplasticneviarelargerthanmostacquirednevi(oftenmorethan5mmacross)andmay number in the hundreds (
Fig.
24.20A ).Theyareflatmaculestoslightlyraisedplaques,witha“pebbly”surface.Theyusuallyhavevariablepigmentation(variegation)andirregularborders(
Fig.24.20A , inset).
Microscopically,dysplasticneviaremostlycompoundnevithatexhibitbotharchitecturalandcytologicevidenceofabnormalgrowth.Nevuscellnestswithintheepidermismaybeenlargedandexhibitabnormalfusionorcoalescencewithadjacentnests(bridging).Aspartofthisprocess,singlenevuscellsbegintoreplacethenormalbasalcelllayeralongthedermoepidermaljunction,producingso-called“lentiginous hyperplasia”(
Fig.24.20B ).Cytologicatypiaconsistingofirregular,oftenangulated,nuclearcontoursandhyperchromasiaisfrequentlyobserved(