Mammalian chimaeras / Anne McLaren.

  • Anne McLaren
Date:
[1976]
Catalogue details
    13/168 (page 3)
    Some definitions 3 occasionally show blood chimaerism and freemartinism, because placental vascular anastomoses seldom occur, but the rare twin pairs studied in horses show no signs of freemartinism even when vascular anastomoses and blood chimaerism are present. Marmosets produce twins in almost every pregnancy, and blood chimaerism is the rule, but they show no abnormalities of the reproductive system. Primary chimaeras can be formed by aggregating or combining cells from different embryos at a very early stage of development, or can arise spon¬ taneously at fertilization if more than one spermatozoon fertilizes a single egg, or an egg and its polar body. 'Fertilization chimaeras' will be considered further in Chapter 11, in the context of spontaneous chimaerism in Man and other animals. The first category of primary chimaerism, in which cells from different embryos are brought together experimentally to form a single indivi¬ dual, constitutes the subject matter of most of the rest of this book. A different term, 'mosaic', is used to describe a composite individual derived from a single fertilized egg, that is from the union of one egg with one spermatozoon. Benirschke (1970) has argued that there is no difference in principle between chimaeras and mosaics, and certainly it is often difficult in spontaneously composite individuals to ascertain how the condition arose. In practice, however, the distinction can be useful. In a mosaic individual, the different cell populations must arise during the course of development, by some process such as somatic mutation, somatic recombination or non-disjunction, spontaneous or induced. For example, a mosaic with an XX (or XY) and an XO cell line would be formed if an XX (or XY) zygote lost a sex chromosome at an early cleavage division, giving rise to an XO cell line as well as the original XX (or XY) line. On the other hand the occasional individual reported to have both an XX and XY cell line is unlikely to be a mosaic, since such a situation cannot arise by any simple non-disjunctional event: it could however be a chimaera, formed by aggregation of two separate zygotes, or by double fertilization, with two separate spermatozoa fertilizing an egg and its polar body. To take another example, female mammals (eutherians anyway) are mosaic for any X-linked locus for which they happen to be heterozygous, since half of their cells will have one X chromosome inactivated and half the other. For the same X- linked locus, a chimaera could also in principle be constructed, by aggregating embryos from two strains homozygous for different alleles at that locus. Both the mosaic and the chimaera would have two cell populations, with alter¬ native alleles active; the mosaicism would have been initiated whenever X chromosome inactivation occurs, probably soon after implantation, while the chimaerism would have begun earlier, as embryo aggregation is usually carried out at the 8-cell stage. The similarities and differences between mosaics and experimental chi¬ maeras in mammals will be discussed further in Chapter 9.