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| Fig. 41.2 Blue nevus on the lower leg. (Courtesy of Fitzsimons Army Medical Center teaching files.) |
Blue nevi and related melanocytic proliferations (i.e., congenital dermal melanocytoses including Mongolian spot, nevus of Ito and nevus of Ota) are a heterogenous group of congenital and acquired melanocytic lesions that have in common several clinical, histologic, and immunochemical features. They have been termed dermal dendritic melanocytic proliferations, because they are usually composed, at least in part, of dendritic melanocytes within the dermis. The deep dermal location of the pigment-producing cells, and therefore the pigment, causes the lesion to have its blue, black, or gray appearance due to the Tyndall effect (Fig. 41-2).
Blue nevi are usually acquired and have their onset most commonly in childhood and adolescence, but ,25% are congenital. In general, melanocytes disappear from the dermis during embryonic migration, but some cells do remain in the scalp, sacral region and dorsal aspect of the distal extremities. These sites correlate to the most common locations for blue nevi to occur. The three commonly identified varieties of blue nevi are the common blue nevus, cellular blue nevus, and combined blue nevus–melanocytic nevus.
