A. These are benign nevi. Small brown flat to slightly raised nevi. They are usually less than a centimeter in diameter. B. LP micro appearance of a benign pigmented nevus. Small amounts of dark pigment are seen near the skin surface. The small blue nevus cells can extend into the dermis and around adnexal structures, but this is not invasion. C. In this junctional nevus, there are nevus cells in nests in the lower epidermis as well as nests appearing to "drop off" into the upper dermis. Unlike a melanoma, there is no significant atypia and no inflammation.
A, Melanocytic nevi are relatively small, symmetric, and uniformly pigmented. B. This dermal nevus shows rounded melanocytes extending into the dermis with loss of pigmentation and cells becoming smaller and more separated with depth-all reassuring signs of appropriate maturation.
Melanocytic nevus
Dysplastic nevus syndrome
Clinical appearance of dysplastic nevi in patient with the dysplastic nevus syndrome. These nevi are large, have an irregular outline, and feature a variegated appearance.A, Compound dysplastic nevi feature a central dermal component with an asymmetric "shoulder" of exclusively junctional melanocytes (left). The former correlates with the more pigmented and raised central zone (see C, inset), and the latter with the less pigmented flat peripheral rim. B, An important feature is the presence of cytologic atypia (irregular, dark-staining nuclei) at high magnification. The dermis shows peculiar but characteristic parallel bands of fibrosis often encountered in dysplastic nevi-part of the host response to these lesions. C, Numerous irregular nevi on the back of this individual suggest the dysplastic nevus syndrome; the clinical features are intermediate to those of benign nevi and melanoma. The lesions are usually greater than 5mm in diameter with irregular borders and variable pigmentation (inset).
Dysplastic nevus
There is dermal fibrosis, inflammation, and a proliferation of melanocytes at the dermo-epidermal junction, with bridging of rete ridges.
Dysplastic nevus
A, Normal skin shows only scattered melanocytes. Top row: b, Junctional nevus. c, Compound nevus. d, Intradermal nevus. e, Intradermal nevus with neurotization (extreme maturation). Bottom row: B, Lentiginous melanocytic hyperplasia. C, Lentiginous compound nevus with abnormal architecture and cytologic features (dysplastic nevus). D, Early or radial growth-phase melanoma (large dark cells in epidermis) arising in a nevus. E, Melanoma in vertical growth phase with metastatic potential. Note that no melanocytic nevus precursor is identified in most cases of melanoma. They are believed to arise de novo, perhaps using the same pathway.
Possible steps in development of melanocytic nevi and melanoma
A, Radial growth, showing irregular nested and single-cell spread of melanoma cells in the epidermis. B, Vertical growth showing nodular aggregates of malignant cells extending deeply within the dermis.
Melanoma
C, Melanoma cells have hyperchromatic nuclei of irregular size and shape with prominent nucleoli. Mitoses, including atypical forms (arrow), are often encountered. D, Lesions clinically tend to be larger than nevi, with irregular contours and pigmentation. Macular areas are early superficial (radial) growth, while elevated areas often indicate dermal invasion (vertical growth).
Melanoma