Effect of Single Gene Substitution on the Melanocyte System of the C57Bl Mouse : Quantitative and Qualitative Histology

Abstract

ON theoretical grounds, “racial” colour differences could be caused by a variety of factors (differences in the number of melanocytes, in melanin content, in melanosome morphology, in tyrosinase activity, and so on)¹. It is known^2–6 that colour differences in the guinea-pig and in man are not a consequence of any difference in the number of melanocytes. In these species, the inheritance of colour differences is more complicated than in the C57 mouse, where, for example, a single gene substitution at the D locus produces dramatic colour differences⁷. The C57Bl mouse (–DD–) is black, whereas the DBL mouse (–dd–) is dilute brown. It is not known, however, whether this gene substitution affects only the quality and quantity of pigment produced or is manifested by a reduction in the number of melanocytes. Furthermore, no thorough comparative study of the melanocyte system in the DBL strain, assessing melanocyte morphology in various body regions and thus determining possible effects of the tissue environment on the morphology of DBL melanocytes, has been carried out. Markert and Silvers reported⁸ that melanoblast differentiation in the “dilute” genotype is determined by factors intrinsic to melanoblasts, yet they later showed that, in certain cases (in transplants to the anterior chamber of the eye), the tissue environment also strongly influences the morphology of melanocytes⁹.