Alfonso Martinez Arias, PhD, is a developmental biologist whose work over the past fifteen years has focused on using Embryonic Stem Cells (ESCs) to investigate cell fate decisions during mammalian development. Recently, his team has developed a novel experimental system that is transforming the study of early mammalian development. By aggregating defined numbers of mouse ESCs under controlled culture conditions, they have generated structures that mimic the organization of the vertebrate embryo over time. These structures, termed "gastruloids," recapitulate the process of gastrulation.