Morula and blastocyst stage embryos recovered from B6D2F_1 mice were bisected with a metal microblade in M2 medium with or without sucrose and/or cytochalasin B supplementation. Cell lysis was determined by staining the embryos with Hoechst 33258 and propidium iodide. Lysed cells take up both stains but non-lysed cells only the Hoechst 33258 stain, resulting in pink fluorescence for lysed cells and blue fluorescence for non-lysed cells under UV excitation. During bisection of morulae, the

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... ce of cytochalasin B decreased the proportion of lysed cells in both the absence (P=0.0001) and presence of sucrose (P=0.001). During bisection of blastocysts the average proportion of lysed cells was slightly lower in the presence of cytochalasin than that in the control medium, but the effect was not statistically significant (P=0.34). No effect of sucrose was observed in either demi-morulae or demi-blastocysts. These results are essentially similar to those obtained in simultaneous experiments with cattle embryos, suggesting that the simpler mouse model might be useful for developing less traumatic bisection protocols for cattle embryos.