During pre-implantation mammalian development, a blastocyst forms that contains three distinct cell types: trophectoderm (which gives rise to the placenta) and the primitive endoderm and epiblast (which together form the inner cell mass from which the fetus develops). But does blastocyst patterning originate in the egg (the prepatterning model) or do differences between blastomeres only appear after the 8-cell stage of embryonic development (the regulative model)? On , Dietrich and Hiiragi now provide evidence that patterning in pre-implantation mouse embryos involves stochastic (random) processes. To identify the critical events in blastocyst patterning, the researchers analyzed the expression of Oct4, Cdx2 and Nanog (three transcription factors that help to specify the blastocyst lineages) at precisely defined times during pre-implantation development. Their results lead them to propose a regulative model for blastocyst patterning in which the first lineages in the early mouse embryo emerge after the 8-cell stage as a result of random processes and/or of earlier asymmetric cell divisions.
