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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    4 What is a cliimaera ? Experimental chimaeras in animals Experiments on lower vertebrates, in which embryos or parts of embryos are aggregated together, have mainly used later embryos, forming 'secondary chimaeras' in the sense defined earlier. The early work by Spemann and his colleagues on determination of the neural plate (see Spemann, 1938) involved grafts at the gastrula stage between Trituras embryos differing in degree of pigmentation, while in later induction experiments composite embryos were formed at the gastrula or early neurula stage, with ectoderm and mesoderm from one individual and endoderm from another (e.g. Okada, 1955). Studies on the origin of primordial germ cells in Amphibia have involved grafts at the neurula stage, either of the actual area containing primordial germ cells in Xenopus (Blackler & Fischberg, 1961; Blackler, 1962) or of the entire endoderm or mesoderm in urodeles (Nieuwkoop, 1946). The trans¬ plantations were made between different subspecies or species differing in egg size and colour, and the chimaeric individuals were reared to the adult stage so that the developmental origin of their eggs could be determined. Attempts to produce primary chimaeras in invertebrates and lower verte¬ brates have rarely been successful. Early experiments on sea urchins and Amphibia have been described by Morgan (1927). George (1958) treated the egg capsules of snails with sodium tauroglycocholate in order to weaken the vitelline membrane and thus facilitate aggregation of the eggs, and obtained two conjoined twin monsters, showing head-to-head union, out of 200 treated capsules (Fig. 2). Presumably the determinate mode of development characteristic of most invertebrate species does not permit the full regulation of two embryos into a single normal individual, as can occur in mammals. On the other hand, some insect gynandromorphs, with two cell lines of dif¬ ferent sex, are thought to be chimaeras formed as a result of double fertiliza¬ tion (for review, see Stern, 1968). Recently, chimaeric Drosophila have been produced by transplanting nuclei into specific regions of the fertilized egg (Okada, Kleinman & Schneiderman, 1974). In birds, secondary chimaeras have been produced by joining the blood circulations of two fertilized eggs, a procedure known as 'parabiosis'. After hatching, the chicks can both be shown to have not only two populations of blood cells, but also two populations of germ cells, since in birds the pri¬ mordial germ cells circulate in the embryonic blood. Primary chimaeras have been produced by Marzullo (1970) by injecting chick blastodisc cells from pig¬ mented Barred Rock or Rhode Island Red eggs into unincubated host eggs of the unpigmented White Leghorn breed. Of 239 eggs injected, nineteen survived to the stage of feather formation, but only three showed evidence of pigmentation, indicating chimaerism (Fig. 3). In each case the pigment was of donor type. The systematic study of primary chimaeras in mammals began in 1961,