Cutaneous mosaicism

Author: V.N. Hiromel de Silva, Dermatology Registrar, Waikato Hospital, New Zealand, 2008.

What is mosaicism?

Mosaicism describes an individual composed of two or more genetically different populations of cells existing side by side within the skin. The two cell lines develop very very early in the life of the embryo.

Many genetic skin disorders reflect cutaneous mosaicism.

There are two major genetic categories:

What are the different patterns of cutaneous mosaicism?

Five patterns of cutaneous mosaicism have been described.[1]

Type 1a Blaschko’s lines, narrow bands Incontinentia pigmenti
Type 1b Blaschko’s lines, broad bands McCune-Albright syndrome
Type 2 Checkerboard pattern Becker naevus / vascular malformation (portwine stain)
Type 3 Phylloid pattern (leaf-like) Mosaic trisomy13
Type 4 Large patches without midline separation Large congenital melanocytic naevi
Type 5 Lateralisation CHILD syndrome

Some conditions may occur in various mosaic patterns, for example, segmental vitiligo.

Proteus syndrome is an example of mosaicism in which the abnormal AKT1 gene in some cells produces a growth activating protein. Various forms of overgrowth and naevi occur and may progress throughout life.

What is the relationship between Blaschko lines and cutaneous mosaicism?

Cutaneous lesions following Blaschko lines are manifestation of cutaneous mosaicism. They represent lines of division of the two cell lines within the embryonic skin tissue.

But cutaneous mosaicism does not always follow Blaschko lines. This is thought to relate to the time the mosaicism arises during embryonic development.


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