Control of events during early cleavage of the mouse embryo: an analysis of the '2-cell block'.

Embryos from certain strains of mice do not develop into blastocysts when cultured in vitro from the 1- or 2-cell stages but arrest development as 2-cell embryos--a phenomenon referred to as the '2-cell block'. Reciprocal crosses between eggs and sperm of a 'blocking' (CFLP) and 'non-blocking' (F1) strain show that in this combination the genotype of the egg alone determines whether the embryo 'blocks' at the 2-cell stage (or continues retarded development to the 4- to 6-cell stage in a minority of cases). A comparison of molecular and cellular development in normal and 'blocked 2-cell' embryos was therefore undertaken to investigate the influence of these maternal components on early mouse development. The results show that the majority of 'blocked 2-cells' arrest development at a stage equivalent to the late 2-cell stage in terms of cellular and nuclear division, DNA synthesis, activation of the embryonic genome, qualitative and quantitative changes in amino acid uptake, polypeptide synthesis and morphological maturation of organelles. These observations are compatible with the notion that maternally inherited developmental information plays an important role in controlling early cleavage of the mouse embryo.