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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    Subsequent fate of trophectoderm and ICM 27 into giant cells. When inner cell mass and polar trophectoderm are cut off, the remaining trophectoderm rounds up into a trophoblastic vesicle, which is capable of inducing a decidual reaction but shows no further cell division (Gardner & Johnson, 1972). If, however, a substitute inner cell mass is injected into such a trophoblastic vesicle, the reconstituted blastocyst develops normally, with proliferation of trophoblast to form a normal ectoplacental cone (Gardner, 1971). The proliferating ectoplacental cone tissue could, of course, be derived directly from the inner cell mass, rather than from trophoblast under the influence of the inner cell mass. This possibility was tested by recolonizing trophoblastic vesicles of one GPI type with inner cell masses of an electro- phoretically distinct GPI type (Gardner, Papaioannou & Barton, 1973). When examined at the early somite stage the ectoplacental cone tissue turned out to be almost exclusively of the trophoblast donor type. The authors conclude that the inner cell mass makes little or no cellular contribu¬ tion to the ectoplacental cone, but controls trophoblast proliferation, per¬ haps by inhibiting giant cell transformation in those trophoblast cells adjacent to it. On the other hand the embryo and extra-embryonic membranes, be¬ lieved to be derived entirely from the inner cell mass, turned out also to include a substantial proportion of the trophoblast donor enzyme. Subse¬ quent studies of mouse-rat composites, with a rat inner cell mass injected into a mouse blastocyst, suggest that the trophectoderm-derived component is in fact the extra-embryonic ectoderm (Gardner, 1975Ò). The development of composite mouse-rat embryos has been followed up to the 30-somite stage (Gardner & Johnson, 1973, 1975; Johnson & Gardner, 1975; Gardner, 1975û). From fifty-six chimaeric blastocysts transferred, twenty-eight living embryos were recovered. Chromosomal analysis showed chimaerism in the first four embryos tested; a further seventeen were there¬ fore sectioned and treated with fluorescent antisera, with anti-mouse and anti-rat applied to adjacent serial sections. Five showed convincing evidence that rat cells as well as mouse were present in the endoderm, and three of these had both rat and mouse cells in the mesoderm and ectoderm also. Positive staining for rat antigens in one section was associated with deficiency of staining for mouse antigens in corresponding areas of the adjacent section, and vice versa (Fig. 13). Although the rat cells were transferred as a clump, they were detected subsequently as several discrete patches, indicating that cell migration had occurred. Some of the patches were large, so some coherent clonal growth must have occurred, and the rat patches in one germ layer showed no obvious spatial relationship to those in the adjacent cell layer. Rat cells from the injected inner cell mass contributed not only to the embryo, but also to the amnion, distal endoderm and yolk sac. Chimaeric embryos pose a problem in growth regulation, since 'double' blastocysts formed by aggregating two 8-cell stages contain twice the normal 3 M MC