A modular organization of mammalian gastrulation and the Spemann-Mangold organizer

Studies in amphibian embryos revealed the existence of groups of cells, organizers, that play a central role in laying down the body plan. One, the Spemann-Mangold Organizer (SMO) is associated with the induction of the nervous system and the development of the head, whereas a second one has been linked with the development of the trunk and the tail. Homologues of these organizers have been described in other vertebrates. In the mouse, the SMO organizer has been associated with a region of the mid- early gastrula and the tail-trunk Organizer with the node. Altogether these observations suggest a modular organization of the vertebrate body plan into three domains. One, most anterior, competent to form the brain, a middle one, associated with neural induction and the head, and one dedicated to axial extension. Work with gastruloids, pluripotent stem cell models of mammalian development, reveal that these modules are independent developmental units. Here we discuss the relationship of the gastruloids findings to the activity of organizers in embryos and the implications of this modular organization for the evolution of the vertebrate body plan.