Effect of phosphate on the second cleavage division of the rat embryo

Development of the rat embryo is arrested at the 2-cell stage in vitro in the presence of inorganic phosphate (Pi). Rat embryos were affected by exposure to 1.19 mM KH2PO4 in modified hamster embryo culture medium-1 at the late 2-cell stage only. When exposure durations were 6 h, embryos whose exposure timings were prior to cleavage had a reduced rate of development to the blastocyst stage (2-8%) when compared with embryos with no exposure to Pi (97%, P < 0.05). When exposure durations were 18 h, all embryos were arrested at the 2- to 4-cell stage. These timings would correspond to the G2 to M phase of the second cell cycle. Maturation-promoting factor (MPF), which is regulated by a phosphorylation cascade, controls cell division, and its kinase activity is necessary in order for the cell to enter the M phase. However, the histone H1 kinase activity levels and the patterns of the state of phosphorylation of cdc2 were the same in blocked and non-blocked embryos. Because MPF was active in blocked embryos, the developmental block in rat 2-cell embryos caused by phosphate was not due to MPF activity or its phosphorylation cascade.