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CHIMERISM
Block-Like Hyper- and Hypopigmentation Due to Chimerism
Chimerism differs from mosaicism in that the different clones originate from different organisms. Human chimeras can result from fusion of two zygotes (i.e. fused dizygotic twins) or fertilization of one egg by two sperm. Usually detected by hematologists as anomalous blood groups,
or by endocrinologists as ambiguous sexual differentiation, chimerism is exceedingly rare. Findlay and Moores studied more than a million routine blood samples and found evidence of chimerism in only two. Pigmentary variation in chimerism typically presents in a checkerboard pattern; however, streaks along the lines of Blaschko and mixtures of light and dark skin that do not correspond to recognized patterns of mosaicism have also been described. In cases of XX/XY chimerism, the karyotype of dermal fibroblasts does not necessarily correspond to the overlying skin color, probably reflecting the different embryonic migration routes in the dermis and epidermis.
mouse Porcn blocks Wnt ligand secretion and reveals an ectodermal etiology of human focal dermal