Skin mosaicisms [I mosaicismi cutanei]

Mosaicism is determined by a clone of mutated cells that is formed during the early stages of embryogenesis and is distributed in the organism as it develops. Several disorders, e.g. trisomy 21, can be present in the mosaic form. However, when the skin is involved, the mutated cell-line is immediately perceived, and it is for this reason that dermatologists are privileged observers of the mosaic condition. There are three main factors that determine the mosaic phenotype: 1) the type of mutation; 2) the stage of embryonic development during which the mutation occurs; 3) the position in the embryo and the destiny of the cell that underwent the mutation. As regards the mutation, it can be expressed solely in the skin or in other organ systems as well (e.g., the skin and CNS), thereby giving rise to purely dermatologic conditions or conditions involving also other organ systems. In the skin the mutation can be localized to the keratinocyte, the melanocyte, the dermis or may involve different cell types. We hypothesized that the different mosaic patterns observed (Blaschko lines, checkerboard, etc.) are due to the different type of cells involved: the keratinocytes follow the Blaschko lines and the melanocytes the checkerboard pattern, whereas other cells such as fibroblasts manifest a less distinct pattern of migration. The embryonic period during which the mutation occurs acts on the size of the lesion (the earlier the mutation the larger the lesion) and, except for rare cases, not on the pattern expressed. The position of the cell undergoing the mutation and the embryogenic destination of the cell, determines which body part or organ system will be involved.