Seasonal variation in cell cycle during early development of the mouse embryo

Studies of the cell cycle of mouse embryos before implantation were conducted using Giemsa and DAPI stains. The time of embryo recovery did not affect the success rate of cultures during the winter, but embryos cultured during the summer showed the 'two-cell block' phenomenon at the early two-cell stage, 30-37 h after the injection of human chorionic gonadotrophin. There was no significant difference in the number of embryos collected per mouse between summer and winter, but cleavage from the two-cell to the four-cell stage occurred later in the summer than in the winter. Cell cycle of mouse embryos may therefore show seasonal variation.