Notochord and axial progenitor generation by timely BMP and NODAL inhibition during vertebrate trunk formation

Abstract

The formation of the vertebrate body involves the coordinated and progressive production of trunk tissues from progenitors located in the posterior of the embryo. In vitro models based on pluripotent stem cells (PSCs) replicate aspects of this process, but they lack some tissue components normally present in the trunk. Most strikingly, the notochord, a hallmark of chordates and the source of midline signals that pattern surrounding tissues, is absent from current models of human trunk formation. To investigate how trunk tissue is formed, we performed single-cell transcriptomic analysis of chick embryos. This delineated molecularly discrete progenitor populations, which we spatially locate in the embryo, compare across species, and relate to signalling activity. Guided by this map, we determined how differentiating human PSCs develop a stereotypical spatial organization of tissue types. We found that LATS1/2 repression of YAP activity, in conjunction with FGF-mediated MAPK activation, induced the transcription factor Bra/TBXT and facilitated WNT signaling. In addition, inhibiting a WNT-induced NODAL and BMP signaling cascade at the appropriate time regulated the proportions of different tissue types produced, including notochordal cells. We used this information to create an integrated 3D model of human gastrulation undergoing morphogenetic movements to produce elongated structures with a notochord and spatially patterned neural tissue formation. Together the data provide insight into the mechanisms responsible for the formation of the tissues that comprise the vertebrate trunk and pave the way for future studies of patterning in a tissue-like environment.