Mammalian chimaeras / Anne McLaren.

  • McLaren, Anne.
Date:
[1976]
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Mammalian chimaeras / Anne McLaren. Source: Wellcome Collection.

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    62 Pigment patterns abnormalities, presumably due to partial destruction of an initially composite hair follicle. When chimaeras are made between strains differing at two alleles, both act¬ ing through the hair follicles, the relative distributions of the two phenotypes can be examined. Mintz & Kindred (cited by Mintz, 1970/)) combined strains differing both at the agouti locus, and at the fuzzy locus that affects hair structure. The component strains were fuzzy non-agouti and normal agouti, but the chimaera also showed all intermediate types of hair structure and agouti banding, in various combinations. The appearance of intermediate types establishes that a hair follicle originates from more than one cell. The variety of combinations of hair structure and banding reported raises the suspicion that fuzzy and agouti may operate on two different cell populations within the hair follicle: no information was given on the relative incidence of the various combinations, nor on the incidence of 'component' (fuzzy non-agouti and normal agouti) versus 'recombinant' (fuzzy agouti and normal non-agouti) hairs. In a similar experiment a strain homozygous for waved-2 and non-agouti was combined with a straight-haired agouti strain (McLaren & Bowman, 1969). Agouti pigmentation is controlled by the dermal component of the hair follicle (Mayer & Fishbane, 1972); waved-2 is thought to involve a lack of keratinization, due presumably to a defect in the 'keratogenous zone' of the hair follicle, situated above the dermal papilla. Although the pattern of distribution of waved hairs at a gross level proved impossible to ascertain, individual zig-zag hairs could be scored both for the presence of an agouti band and for the waved phenotype (McLaren, 1971). If both characters were determined by the same component of the hair follicle, a clonal theory would predict that the non-agouti hairs in the chimaeras should be waved, while the agouti hairs should be normal, as in the component strains. If the characters were determined by different clonal populations assorting randomly with respect to one another, e.g. mesoderm and ectoderm, the distributions of waviness and agoutiness should be unrelated. If one character were determined clonally, and the other systemically, their distributions should again be unrelated. Examination of individual hairs (McLaren, unpublished observations) showed no tendency for the proportion of waved hairs to differ as between agouti and non-agouti hairs, within a single patch of hairs. On the other hand if different patches within the same animal were sampled, making allowance for the different proportions of the two phenotypes in the animal as a whole, a significant inverse relationship (P<0.05) between the proportion of waved and the pro¬ portion of non-agouti hairs was seen (Fig. 22b). Why should this be? One possibility is that both characters are determined systemically, responding to some environmental variable. For example, low concentrations of sulphydryl groups are known to discourage the expression