Repigmentation of Human Skin Xenografted Onto Nude Mice

To the Editor.—  We read with interest the article of Gilhar et al¹ reporting the repigmentation of vitiligo and idiopathic guttate hypomelanosis (but not tyrosinase-negative albinism) in human skin 6 to 10 weeks after grafting onto nude mice. Using the same model we have observed, as Gilhar et al¹ did, that split-thickness normal human skin xenografts (NHSX) onto nude mice become macroscopically deeply pigmented 2 to 3 weeks after grafting.² Histologic study of these NHSX showed an abundance of melanin within the epidermis of NHSX and also within melanophages of the underlying dermis, whereas ultrastructural examination showed highly active denditric melanocytes, with melanosomes being found high up in the epidermis within granular-layer keratinocytes. We believe these alterations to be due to melanocyte stimulation rather than proliferation, since no obvious increase in their number was seen by electron microscopy. Furthermore, we have observed that, in the long term