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.

    40/168 (page 30)
    30 Early development addition the intermediate band that indicated the presence of hybrid enzyme. The authors concluded that the multinucleate myotubes of skeletal muscle must arise in vivo by fusion of separate myoblasts, as they were already known to do in vitro. The absence of hybrid enzyme in other chimaeric tissues implies that, in heterozygotes, both alleles must be active in the same skeletal muscle cell, since assembly of hybrid molecules evidently cannot take place by diffusion of subunits across cell boundaries. Its presence in multinucleate skeletal muscle myotubes implies that each subunit is separately coded for by the genetic material, with assembly taking place in the cytoplasm. To investigate the genetic control of another enzyme, NADP-dependent malate dehydrogenase (MDH), Baker & Mintz (1969) again made chimaeras between strains differing for electrophoretically distinguishable variants of the enzyme, and examined homogenates of skeletal muscle and other tissues (liver, kidney, lung, salivary gland, mammary gland, cardiac muscle). Hetero¬ zygotes for the two MDH alleles show a five-banded pattern on electro¬ phoresis, suggesting a tetrameric structure for the molecule, with two subunits A and B, giving three 'hybrid' bands, ABBB, AABB and AAAB. Skeletal muscle from the chimaeras showed a similar pattern to that seen in hetero¬ zygotes, but other tissues again showed only the two pure bands. This result not only confirmed the tetrameric structure, but indicated that the MDH molecule is built up, not from two dimers, but from monomeric subunits assembled independently in the cytoplasm. Cell hybridization, followed by the restoration of diploidy, has been shown to take place among somatic cells in vitro, but enzymic and chromosomal studies (Mintz, 1970ö) have so far yielded no evidence for any such process in normal mammalian development. The largest cells in the body are probably trophoblast giant cells, the nuclei of which contain in the mouse up to a thousand times the haploid DNA content. Using a very sensitive electrophoretic assay system for glucose phosphate isomerase (GPI), it proved possible to determine the GPI-1 type of individual ectoplacental cones (trophoblast) as early as the 8th day of preg¬ nancy, and to detect a minor component constituting 1 % or more of the total GPI-1 activity of the sample (Chapman et al, 1972). When 8-cell stages from strains differing at the GPI-\ locus were aggregated, two enzyme bands, indicating chimaerism, were found in embryos, embryonic membranes and trophoblast at the 11th day of pregnancy, but no trace of a hybrid band was detected. Trophoblast outgrown from chimaeric blastocysts in vitro also failed to show any hybrid enzyme. It was concluded that trophoblast cells do not fuse with one another either in vivo or in vitro, and that the increase in DNA probably occurs by endoreduplication. A similar result was obtained by Gearhart & Mintz (1972Ò) on trophoblast taken from chimaeric con- ceptuses on the 10th and 19th days of pregnancy.