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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    3 Early development This chapter is concerned with experimental embryology, with the pre-im- plantation and early post-implantation stages of development of the mam¬ malian (in effect mouse) embryo, and with the light that studies of chimaeric embryos have been able to throw on the underlying causal basis of the developmental events taking place during this period. Table 2 gives the timing of some of the landmarks of mouse development that will be discussed. The very fact that embryo aggregation is possible in the mouse, and that it results in a single adult individual of normal size and shape rather than any form of double monster, bears witness to the astonishing regulative powers of mammalian embryos. This aspect was stressed in the early papers on aggre¬ gation chimaeras (Tarkowski, 1961, 1965; Mintz, 1962¿, 1964а, b, 1965a); it has been fully borne out by later work. Tarkowski (1961) thought that the developmental lability of the blastomeres of the mouse embryo was lost during the mitotic cycle following the 8-cell stage, but subsequent experiments showed that successful aggregation could be obtained between 16-cell and even 32-cell morulae (Mintz, 1964a). Stern & Wilson (1972) report some aggregation between early blastocysts, but these were probably still at the 16-cell stage, as they were only 12 hours older than the 8-cell embryos used in the same study. Reaggregation of later stages following complete dissocia¬ tion in versene has been reported (Stern, 1972); possibly the versene treat¬ ment alters the properties of the cell surface. Heterochronic aggregations and reaggregations, between embryos or blastomeres of différent ages, are also possible (Mintz, 1962¿>; Mulnard, 1971; Stern & Wilson, 1972; Stern, 1972), but the proportion developing successfully to the blastocyst stage is lower than when the two component embryos are the same age as one another. The participation of heterochronic partners in post-implantational development has not been examined. The developmental potential of individual blastomeres at the 4-cell stage has been investigated by Kelly (1975) in the mouse. After dissociation, each of the four blastomeres was surrounded by 'carrier' cells from another embryo, distinguishable both by coat colour genes and by an enzyme marker. When all four composite embryos successfully continued their development after transfer to a uterine foster-mother, each blastomere was found to have contributed to the foetus, to the yolk sac, and to the placenta; in several cases