Instructions for use Title STUDIES ON DEVELOPMENTAL POTENTIALS OF BISECTED MOUSE EMBRYOS IN VITRO AND IN VIVO

To determine the critical cell numbers needed to form blastocyst stage embroys, bisected mouse embryos of 8-cell stage and compacted morula (approximately 16 cells) containing different numbers of blastomeres were cultured in vitro. The percentage of bisected compacted morulae with 7-9 blastomeres that developed to eu-blastocysts was 98.8%, while only 64.2% of the bisected 8-cell embryos with 4 blastomeres developed to eu-blastocysts. The percentage of eu-blastocysts decreased, while that of pseudo-blastocysts and trophectodermal vesicles increased as the number of blastomeres decreased in the bisected embryos of the two stages. Inner cell masses were observed in all the eu-blastocysts and in 87.3% of the pseudo-blastocysts that developed from bisected embryos. The cell number of inner cell mass in the pseudo-blastocysts was significantly less than that of eu-blastocysts (P< O. 01). The ratios of inner cell mass to total cell number of eu-blastocysts that developed from 4 blastomeres in 8-cell embryos and from 7-9 blastomeres in compacted morulae were similar to those of control embryos, respectively. However, the ratios decreased in proportion to the decrease in the number of blastomeres in the two groups. Five sets of monozygotic twins and 4 singletons of live fetuses were obtained after transfer of the 20 twin sets of eu-blastocysts that developed from the bisected compacted morulae with 7-9 blastomeres. However, no implantation occurred when the pseudo-blastocysts that developed from bisected compacted morulae with 6 or fewer blastomeres were transferred. Weight of fetuses that developed from bisected embryos on Day 18 of pregnancy were significantly lower than those of the control.