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.

    35/168 (page 25)
    Differentiation of 1С M and trophectoderm 25 by autoradiography after tritiated-thymidine labelling of one partner, or through the characteristic granularity of the cytoplasm of one strain, or by taking one embryo of the homozygous lethal t^^lt^^ genotype (Mintz, 1964a). The results were not conclusive, but no evidence of any 'ordered deployment' of the two types of cells was seen. Later work, in which isotopically labelled and unlabelled 8-cell embryos were aggregated and the resulting blastocysts serially sectioned and sub¬ jected to autoradiography, confirmed that little or no cell mingling takes place up to the blastocyst stage (Garner & McLaren, 1974). The proposal that location within the embryo, either internal or external, at a particular stage of development, might be the factor determining whether a particular blastomere differentiated as inner cell mass or trophectoderm, was put forward by Tarkowski & Wroblewska (1967) as a result of their experi¬ ments on the isolation of blastomeres at the 4- and 8-cell stage. They suggested that any cell on the outside after three to four cleavage divisions (normally, the 8-16-cell stage) differentiated as trophectoderm, while those on the inside formed the inner cell mass (Fig. 116). If cell number were experimentally reduced at the critical period, all cells would be on the outside, so that a 'trophoblastic vesicle' would be formed, lacking an inner cell mass completely. This hypothesis was strongly supported by the elegant recombination experiments of Hillman et al. (1972). To test the effect of 'outside' location, 4- and 8-cell mouse embryos were labelled with tritiated thymidine, then disaggregated and each blastomere stuck onto the outside of a different un¬ labelled zona-free 4- to 16-cell embryo. Autoradiography at the blastocyst stage revealed that 90 % of the labelled cells were in the outer trophectoderm layer; in no embryo were the labelled cells found only in the inner cell mass, and in the case of three donor embryos all the labelled derivatives were in the outer layer, although had they been left in situ some of their descendants would of course have formed the inner cell mass. When a similar experiment was performed with donor and host differing by an electrophoretically de¬ tectable variant of the enzyme GPI, descendants of the donor blastomeres were always present in the trophoblast and/or yolk sac on the 10th day of gestation, and only in four out of twelve cases did they form part of the embryo proper. Conversely, when labelled 8-cell stages were entirely sur¬ rounded with fourteen unlabelled embryos (Fig. 12), the labelled cells were found only in the inner cell mass in four out of seven attempts, and in both inner cell mass and trophectoderm in the remaining three. After transfer, such giant chimaeric blastocysts proved capable of developing at least until the 13th day of gestation. The conclusion that the location of a blastomere during cleavage could determine the direction of its differentiation was underlined by the study of Wilson, Bolton & Cuttler (1972). Droplets of silicone fluid injected as