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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    5 Pigment patterns The value of chimaeras for the study of development, and in particular for the analysis of cell lineages, depends on the availability of suitable markers. One of the most satisfactory intrinsic markers available for this purpose is pigment (usually melanin). As pointed out in Chapter 2, two locations in which pigment remains within the melanocyte that produces it are the inner ear and the pigmented retina of the eye. Here pigment can act as a cell marker, enabling the distribution of the two chimaera components to be analysed at a cellular level. This is not so in hair or in the choroid layer of the eye, where pigment is secreted by melanocytes into extracellular spaces; but even here, pigment patterns in chimaeras may provide information on developmental processes and on gene action. Similar information can be derived from the analysis of females heterozygous for X-linked genes affecting pigment, owing to the phenomenon of X-inactivation. A comparison between chimaeras and X-inactivation mosaics will be made in Chapter 9. Hair For diagnosing chimaerism, coat colour offers a more obvious and easily visible system than any other character. The first adult mouse chimaeras to be made and identified were between an agouti strain and a non-agouti (black) strain. They were mottled, showing both agouti and black coloration arranged in a striking pattern of markings' (Mintz, \965b). In the mammalian coat, pigment is secreted into hairs by melanocytes, descendants of the melanoblasts that migrate down from the neural crest during embryonic development, to colonize the hair follicles. Thus a single hair follicle in a chimaeric individual may contain not only two genetically differing populations of epidermal cells, but also two independently derived populations of melanocytes. Coat colour can be affected by genes (e.g. brown, albino, dilute) acting through the melanocytes to determine the type or quantity of pigment synthesized, or the form in which it is laid down; it can also be affected by genes acting through the epidermal cells of the hair follicle. Transplantation experiments (Silvers & Russell, 1955) suggested that non- agouti acts through the hair follicle; this has been fully confirmed by chimaera studies. Cattanach, Wolfe & Lyon (1972) have suggested, on the basis