The role of the tissue environment in the expression of spotting genes in the mouse

Genes causing white spotting in the mouse act in two major ways: some affect the melanoblasts, while others affect the tissue environment of the melanoblasts. The question is whether the normal tissue environment plays any role in the origin of spots in those mutants in which the melanoblasts are believed to be the site of gene activity. An earlier study, using the genotypes +/mi, Miwh/+, s/s, Wυ/+ and Wυ/Wυ (mi = microphthalmia; Miwh = white; s = piebald; Wr = viable dominant spotting), indicated that it probably does, the evidence largely consisting in the occurrence of extremely precise pigmentation patterns on a minute scale. It seemed that more direct evidence could be obtained by comparing the pigmentation of the iris with that of the choroid and the retina in the same eye in these and other genotypes. The outer and inner layers of the iris derive their pigment cells from the choroid and the retina respectively; therefore any clear and consistent differences between the behaviour of these cells in their original and their secondary place of activity would constitute evidence for the role of the tissue environment. Such differences were found. It was also found that in another genotype, Miwh/mi, the retinal pigment cells, although unpigmented, are clearly distinguishable. This casts serious doubt on the widespread assumption that melanoblasts which do not differentiate always die.