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Inasmuch as in mice melanoblasts must usually enter a hair follicle to produce any melanin at all, it is not surprising that the clonal history of the coat also contributes to the pigment patterns of allophenic animals. Indeed, the interaction between the independently derived melanoblast and hair follicle clones is yet another factor which increases the variability of allophenic phenotypes.
As in the case of the allophenic pigment pattern the archetypal and modified developmental hair patterns can be discerned only when the two contributing strains differ with respect to a suitable genetic marker, in this case one which affects either the morphology of the hair, such as fuzzy ( fz/fz) and normal ( Fz/Fz), or by a melanocyte marker which acts via the hair follicle, such as agouti ( A/A) and nonagouti ( a/a). When such a marker is present the pattern produced, in its maximal or standard form, is similar to the melanoblast pattern in that it too is characterized by a number of transverse bands with frequent mid-dorsal and mid-ventral asymmetries on the left and right side. However these bands are noticeably narrower and much more numerous than the melanoblast bands. The left-right asymmetry again indicates that the bands on each side of the mid-dorsal line are independently derived and therefore must have originated before the fusion of the neural fold. Although it is difficult to accurately determine the number of bands (clonal-initiator cells) there are approximately 170, 85 on each side. The numbers of these bands on each side of the trunk correspond to the somite number (approximately 30 per side) and, assuming a similar situation for the tail, it is judged to be about 35 clones on each side. Eighteen pairs of bands occur in the head region ( Mintz, 1974). 2 As in the case of the melanoblast clones, these hair follicle clones also can vary in their proportions, positions, proliferative capacities, etc. and by so doing provide a continuous and almost unlimited number of phenotypes. 3
The interaction between melanoblast clones and hair follicle clones can be observed in allophenic mice produced from embryos known to bear both melanocyte and follicular cell markers, such as brown nonagouti ( b/b;a/a) and black agouti ( B/B;A/A), and in such animals it is found that each melanoblast clone physically overlaps about six hair follicle clones ( Mintz, 1969b). Consequently, in the standard pattern of a/a;b/b <--> A/A;B/B allophenics a b/b melanoblast clone includes about three brown pigmented hair clones ( aa) and about the same number which display the brown agouti ( A/A) pattern. These subdivisions of the melanoblast clone are known as "subclones" because they can be shown by graft tests to be reversible modifications ( Mintz and Silvers, 1970). Figure 7-4 illustrates the tremendous range of phenotypic variability which is realized in single allophenic animals as a result of the interaction between the independently originating melanoblast clonal and hair follicle clonal patters.
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