Development of single mouse blastomeres enlarged to zygote size in conditions of nucleo-cytoplasmic synchrony

Abstract

The following blastomeres were enlarged to the size of the zygote by one, two or three rounds of blastomere enucleation and electrofusion: (1) from the 2-cell stage (referred to as 2/1 embryos), (2) from the 4-cell stage (referred to as 4/1 embryos), (3) from the 8-cell stage (referred to as 8/1 embryos). Such single enlarged blastomeres developed into blastocysts in vivo in 55.5% (2/1), 28% (4/1) and 6.6% (8/1) of cases. Their mean cell numbers were 45.3, 24.5 and 13.0 in 2/1, 4/1 and 8/1 embryos, respectively. When a blastomere nucleus from another mouse strain (heterologous nucleus) was substituted for a blastomere's own (homologous) one, then fewer blastocysts were formed from 2/1 embryos (34.6%), but not from 4/1 and 8/1 embryos. Five young (10.4%) were born from 2/1 embryos with a homologous nucleus, and nine (8.3%) from 2/1 embryos with heterologous nuclei. Four young (7.1%) were born from 4/1 embryos with heterologous nuclei. No young were obtained from 8/1 embryos. Incorrect cavitation resulting in trophoblastic vesicles and false blastocyst formation was common in 4/1 embryos (18.7% of those with homologous nuclei and 41.3% with heterologous nuclei) and in 8/1 embryos (53.3% and 43.7%, respectively). The results show that neither enlargement to zygote size nor nucleo-cytoplasmic synchrony improve postimplantation development of 4- and 8-cell stage blastomeres when compared with less enlarged non-synchronous ones; therefore, it appears that an insufficient number of inner cell mass cells in blastocysts and not too small a size of isolated blastomeres precludes their postimplantation development.