Mechanisms of cell number regulation in the peri-implantation mouse blastocyst.

Low viability of manipulated or in vitro cultured is caused primarily by the reduced cell number in the implanting blastocysts. In order to investigate the effect of implantation delay on embryo viability and cell number, mouse blastocysts were transferred into oviducts of day 0 pseudopregnant females. This type of transfer improved embryo survival rates, indicating that embryos retarded by in vitro culture restored their viability during 3 days of delayed implantation. Our results showed that even in the cases when the initial cell count was as low as 28.2 +/- 0.7 cells per blastocyst (vs 60.5 +/- 1.4 cells in the control blastocysts, developed in vivo), implantation delay increased this number to 107.2 +/- 3.5 cells (control blastocysts had at this stage on average 111.0 +/- 3.7 cells). Half-blastocysts, developed from the single blastomeres of the 2-cell embryos or from experimentally produced tetraploids, had around 50 cells after 3 days of implantation delay. This indicates that the start of blastocyst dormancy is triggered during the eighth cell cycle and independent of the absolute cell number or the number cytokineses. Implantation-delayed blastocysts, developed from the half-embryos with the doubled volume of cytoplasm, had on average 70.5 +/- 2.4 cells, suggesting that embryo fall into quiescence is also dependent upon the attainment of a definite nucleo-cytoplasmic ratio. We conclude that blastocyst readiness for implantation is determined by two factors: number of cell cycles and nucleo-cytoplasmic ratio.