|
| Fig. 41.1 A, Junctional nevi are typically small, flat, and dark brown in color. B, An intradermal nevus also may be very exophytic or papillomatous, as shown here. C, Typical halo nevus of the back demonstrating a central brownish-red papule. D, Large congenital nevus with multiple smaller congenital nevi. These lesions present a surgical challenge and a significant cosmetic problem. |
Melanocytic nevus cells are derived from melanocytes and differ from normal epidermal melanocytes in a number of ways. They are no longer dendritic; they do not distribute melanin to surrounding keratinocytes; and they are less metabolically active. Melanocytic nevi are benign clonal proliferations of cells expressing the melanocytic phenotype, and are thought to be derived from precursor cells that acquire genetic mutations. These mutations are thought to activate proliferative pathways and/or or suppress apoptosis, allowing for the accumulation of melanocytic cells in the skin. The type of nevus that is formed is thought to be dependent upon the specific mutation, as well as local environmental factors. B-
Raf mutations are commonly seen in acquired melanocytic nevi. Acquired melanocytic nevi are thought to begin as a proliferation of nevus cells along the dermal–epidermal junction (forming a junctional nevus; Fig. 41-1A). With continued proliferation of nevus cells, they extend from the dermal–epidermal junction into the dermis (forming a compound nevus). The junctional component of the melanocytic nevus may resolve, leaving only an intradermal component (intradermal nevus; Fig. 41-1B). However, it should be stressed that there is debate regarding the direction of nevus growth.
Melanocytic nevi form naturally, possibly due to ultraviolet light exposure, from the ages of 6 months to 40 years and later. They may also resolve spontaneously. However, the appearance or disappearance of any melanocytic lesion should be brought to the attention of a physician.
Cane JF, Trainor PA: Neural crest stem and progenitor cells,
Annu Rev Cell Dev Biol 22:267–286, 2006.
Grichnik J: Melanoma, nevogenesis, and stem cell biology,
J Invest Dermatol 128:2365–2380, 2008.
