Although fate maps are powerful tools available for most model organisms, there are currently no fate maps for human development, largely because of the ethical constraints on culturing human embryos. Now, Iain Martyn, Eric Siggia and Ali Brivanlou construct a rudimentary fate map of the human primitive streak using gastruloids: self-organised structures derived from human embryonic stem cells with embryo-like organisation. By using different levels of BMP, WNT and NODAL, the authors derive different subpopulations of endoderm and mesoderm that, when compared with data from mice, correspond to cells found at different positions along the anterior-posterior axis of the primitive streak. For example, BMP4 induces distinct radially organised populations that express markers associated with extra-embryonic mesoderm, lateral mesoderm and posterior endoderm, whereas WNT induces anterior endoderm. In addition, they use a fluorescent protein photo-conversion technique to trace the migration of the different populations. Finally, they examine the 3D structure of the gastruloids and, by combining these data, piece together a fate map of the human primitive steak. Although a full picture can only be obtained from in vivo experiments, this research provides the first step towards understanding the behaviour of different cell lineages during human gastrulation.
