Influence of embryo size on lactate dehydrogenase isozyme expression in giant mouse chimaeras.

Chimaeras consisting of 2, 4, 8 and up to 16 mouse embryos were formed at day 3 and cultured in vitro up to day 8. The influence of size and cell number on the activation of the embryonic genome for LDH-5 was studied in non-attaching giant chimaeric blastocysts and compared to control embryos implanting in vivo or growing out in vitro. Day 5 chimaeric blastocysts consisting of 4 to 8 embryos had a similar size as early in vivo egg cylinders, but still expressed exclusively LDH-1 as normal preimplantation embryos do. Only during further ageing of non-attaching chimaeric blastocysts, a few samples were positive for LDH-5. There is no convincing evidence that the experimental increase of the cell number triggers a premature activation of the embryonic LDH genes. These results are discussed in comparison to other possible mechanisms regulating stage-specific activation of the embryonic genome.