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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    Celi fusion during development 29 number of cells (Buehr & McLaren, 1974,) yet by late gestation foetal and placental weight is indistinguishable from that of control 'single' embryos (Bowman & McLaren, 1970). Tarkowski (1963) reported that 'double' embryos 'fall within the normal range of variation in size' from 6^ days p.c. onwards; Mintz (1971Ò) states: 'normal embryo size is restored during im¬ plantation', perhaps by 'origin of the embryo itself from a small fixed number of cells.. .in the inner cell mass, regardless of the total cell number in the blastocyst'. In the normal mouse embryo differentiation of endoderm is followed by downgrowth of the egg cylinder into the blastocyst cavity, while within the egg cylinder segregation of embryonic and extra-embryonic ectoderm occurs. At about 5^ days p.c. the proamniotic cavity is first seen. Volume estimates from serial sections (Buehr & McLaren, 1974) indicate that throughout this period the egg cylinders of 'double' embryos are at least twice as large as those of'single' embryos. At later stages, however, no size difference between 'double' and 'single' egg cylinders could be detected, suggesting that, in accordance with the observations of Tarkowski (1963), growth regula¬ tion takes place 5^6 days p.c. Since regulation appears to involve the entire egg cylinder, it is unlikely to be due to the embryo originating from a small fixed number of cells. Cell fusion during development Some cell types in the body contain substantially more than the 2c-4c amount of DNA characteristic of normal diploid cells. The additional DNA may all be in one large nucleus, or may be distributed among several nuclei. Two developmental paths could lead to such a situation: fusion between cells, or continued replication of the DNA within a cell ('endoreduplication'), with or without nuclear division, but without cell division. The occurrence of fusion can be detected if the cells concerned differ with respect to electrophoretic variants of a dimeric enzyme, as synthesis of both forms of the enzyme in a single cell (for example, in heterozygotes) yields an intermediate 'hybrid' band on gel electrophoresis. One way of achieving a genetically mixed popula¬ tion of cells for this purpose is by making chimaeras. This principle was first applied by Mintz & Baker (1967) to investigate the developmental history of the multinucleate myotubes found in skeletal muscle, using the dimeric enzyme, NADP-dependent isocitrate dehydrogenase (IDH). Aggregations were made between embryos of strains C3Hf {IdA^lId-V) and DBA/2 {Id-\'lId-\'), and C57BL/6 {IdA'^jldA'') and CBA {Id-\^'IId-\''). Various tissues of the adult chimaeras were homogenized and analysed by starch gel electrophoresis: two bands, corresponding to the two enzyme variants Id-X and Id-\*', characterized homogenates of chimaeric liver, kidney, lung, spleen and cardiac muscle, but skeletal muscle yielded in 3-2