Cell allocation in twin half mouse embryos bisected at the 8-cell stage: implications for preimplantation diagnosis.

Previous work in the mouse and human has shown that removal of cells from 2- and 4-cell embryos disproportionately reduces the inner cell mass (ICM), from which the fetus is derived, suggesting that preimplantation biopsy at these stages would be inadvisable. Here we have examined allocation in twin half mouse embryos produced by bisection at the 8-cell stage. Half embryos and intact zona-free control embryos were differentially labeled at the time of cavitation using polynucleotide-specific fluorochromes, and the numbers of cells in the ICM and trophectoderm (TE) counted. Adding the numbers of cells in the twin halves together, the numbers of ICM cells are not significantly different from those in intact controls. In contrast, numbers of TE cells were significantly higher (P < 0.01), producing a small increase in the total cell number of half embryos. There is no concomitant decrease in ICM cell number. We propose that as the surface area of a half embryo is greater than half an intact control, TE cells in half embryos divide faster in order to reach an optimal packing density similar to that in intact control embryos. Furthermore, we suggest that preimplantation biopsy only be performed at the 8-cell stage or later.