A reversible block at the G sub(1)/S border during cell cycle progression of mouse embryos

Late 2-cell stage mouse embryos were cultured in M-199 plus 100 mu g/ml Na pyruvate 25 mu g/ml gentamycin and 0.3% BSA with or without mimosine (200 mu M, 150 mu M, 100 mu M and 50 mu M) for a short (4-5 h) or long (18-20 h) culture period; after drug removal subsequent embryo development was evaluated. Late 2-cell stage mouse embryos treated with mimosine were blocked at the 4-cell stage. Autoradiographic studies show that mimosine inhibits cell cycle progression in mouse embryos at the G sub(1)/S boundary. The onset of DNA replication occurs within 15 min of releasing the embryos from mimosine block. Embryos pretreated with mimosine at 200 mu M and 150 mu M for 4-5 h progress after 3-4 days in culture to hatched blastocyst (71% and 79%, respectively) compared with control (90%). However a longer pretreatment (18-20 h) with mimosine at 200 mu M was significantly detrimental to the subsequent developmental progression to hatched blastocyst (2% vs 81%, p less than or equal to 0.05); the proportion of degenerated embryos was significantly increased with mimosine at 200 mu M and 150 mu M compared with control (57% and 28% vs 4%, p less than or equal to 0.05) after 3-4 days in culture. Preliminary studies with mimosine treatment at 100 mu M and 50 mu M for 18-20 h show that 70% and 37% of the embryos were blocked at 4-cell stage, respectively. These results indicate that mimosine inhibits cell cycle progress in mouse embryos at the G sub(1)/S border and thus induces a reversible arrest in a dose- and time-dependent manner.