Similar effects of osmolarity, glucose, and phosphate on cleavage past the 2-cell stage in mouse embryos from outbred and F-1 hybrid females

One-cell-stage embryos derived from most random-bred and inbred female mice exhibit an in vitro developmental block at the two-cell stage in classical embryo culture media. However, embryos derived from many F, hybrids develop easily past the two-cell stage under the same conditions. This has given rise to the commonly accepted idea that there exist blocking and non-blocking types of female mice, with only the former being prone to a two-cell block. Recently, culture media have been improved to the point that even embryos prone to the two-cell block will develop past the block in vitro, making it possible to study its etiology. Here, we show that either increased osmolarity or increased glucose/phosphate levels induced the expected two-cell block in random-bred CF1 embryos and the two-cell block at increased osmolarities could be rescued by the organic osmolyte glycine. Surprisingly, one-cell embryos from B6D2F(1) (BDF1) F-1 hybrid females, considered to be nonblocking, also became blocked at the two-cell stage when osmolarity or glucose/phosphate levels were increased. They were also similarly rescued by glycine from the osmolarity-induced block. The most evident difference was that the purportedly nonblocking embryos became blocked at a higher threshold of osmolarity or glucose/phosphate level than those considered prone to this developmental block. Thus, both blocking and nonblocking embryos actually exhibit a similar two-cell block to development.