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Congenital Nevomelanocytic Nevus






Congenital nevomelanocytic nevi (CNN) are pigmented lesions of the skin usually present at birth; rare varieties of CNN can develop and become clinically apparent during infancy. CNN may be any size from very small to very large. CNN are benign neoplasms composed of cells called nevomelanocytes. which are derived from melanoblasts. All CNN, regardless of size, may be precursors of malignant melanoma.


Causes:

Congenital and acquired nevomelanocytic nevi are presumed to occur as the result of a developmental defect in neural crest-derived melanoblasts. This defect probably occurs after 10 weeks in utero but before the sixth uterine month; the occurrence of the "split" nevus of the eyelid is an indication that nevomelanocytes migrating from the neural crest were in place in this site before the eyelids split (24 weeks).

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