Molecular control of macroscopic forces drives formation of the vertebrate hindgut

The embryonic gut tube is a cylindrical structure from which the respiratory and gastrointestinal tracts develop¹. Although the early emergence of the endoderm as an epithelial sheet^2,3 and later morphogenesis of the definitive digestive and respiratory organs^4-6 have been investigated, the intervening process of gut tube formation remains relatively understudied^7,8. Here we investigate the molecular control of macroscopic forces underlying early morphogenesis of the gut tube in the chick embryo. The gut tube has been described as forming from two endodermal invaginations-the anterior intestinal portal (AIP) towards the rostral end of the embryo and the caudal intestinal portal (CIP) at the caudal end-that migrate towards one another, internalizing the endoderm until they meet at the yolk stalk (umbilicus in mammals)^1,6. Migration of the AIP to form foregut has been descriptively characterized^8,9, but the hindgut is likely to form by a distinct mechanism that has not been fully explained¹⁰. We find that the hindgut is formed by collective cell movements through a stationary CIP, rather than by movement of the CIP itself. Further, combining in vivo imaging, biophysics and mathematical modelling with molecular and embryological approaches, we identify a contractile force gradient that drives cell movements in the hindgut-forming endoderm, enabling tissue-scale posterior extension of the forming hindgut tube. The force gradient, in turn, is established in response to a morphogenic gradient of fibroblast growth factor signalling. As a result, we propose that an important positive feedback arises, whereby contracting cells draw passive cells from low to high fibroblast growth factor levels, recruiting them to contract and pull more cells into the elongating hindgut. In addition to providing insight into the early gut development, these findings illustrate how large-scale tissue level forces can be traced to developmental signals during vertebrate morphogenesis.