Size regulation in the mouse embryo

The development of mouse embryos formed by the aggregation of four 8-cell-stage eggs was analysed in comparison with control single embryos. The analysis revealed that: (1) Quadruple aggregates undergo size regulation over several days, starting before implantation and being completed by 6·5 days post coitum. (2) The attainment of recognizable postimplantation morphological stages is independent of size. (3) Regulation is not brought about by disproportionate alterations in the size of the internal cavities. (4) Regulation in both inner cell mass (ICM) and trophectoderm derivatives is completed between 5·5 and 6·5 days post coitum. (5) Despite the abnormal proportions of ICM and trophectoderm in quadruple blastocysts, the proportions of the tissues derived from them are already normal by 5·5 days. The possibility that down regulation in size of aggregate embryos occurs as a consequence of limited nutrient supply is discussed.