Chapter 7

The Pigment Patterns of Allophenic Mice and Their Significance

III. Inception of Gene Activity

In addition to demonstrating that all the melanocytes of mice are derived from 34 clonal initiator cells, Mintz's observations also demonstrate at approximately what time in development the specific gene loci responsible for melanoblast determination first become active. This follows from the fact that as the left and right sides of the animal can be of different colors, and hence must be derived from different clones, the clonal-initiator cells must arise before the neural crest cells can pass from one side to the other, i.e., before the neural folds on the two sides start to come into physical contact and fuse. This occurs on day 8 of embryonic life in the mouse, so that day 7 would presumably be the latest time at which clonal initiation of melanoblasts can occur ( Mintz, 1970). On the other hand, inasmuch as there must be a sufficient number of cells present in the embryo to produce not only the pigmentary system but other systems as well, it is doubtful if the loci which are responsible for initiating melanoblast differentiation can become active earlier than the fifth day of development. Thus, it appears that it is sometime between day 5 and day 7 that melanoblast differentiation is initiated (Mintz, 1970, 1971a).