Cell allocation in half- and quadruple-sized preimplantation mouse embryos.

The size of preimplantation mouse embryos was experimentally manipulated in order to examine the consequences for the allocation of cells to the two primary tissues, trophectoderm and inner cell mass (ICM). Half embryos were produced by the mechanical lysis of one cell at the two-cell stage and quadruple embryos by the aggregation of four whole eight-cell embryos. Such procedures are shown not only to alter the absolute number of cells that are assigned to the trophectoderm and ICM, but also to disturb significantly the proportions of these two tissues in the blastocyst. The proportion of trophectoderm is directly related to the surface area of the morula, as is predicted by a purely epigenetic scheme for cell allocation.