LAUSR

Significance Previous studies have suggested that the amniote node (Hensen’s node) contains a small population of self-renewing resident cells whose progeny progressively lay down axial tissues, including notochord and somites. This can only be demonstrated definitively at the level of single cells. Here we ask whether the node is an environment that can confer this behavior on cells that enter it. We challenge single cells in vivo and mRNA-profile these cells to demonstrate that the node can indeed do this, and thus show that the node acts as an instructive niche. In warm-blooded vertebrate embryos (mammals and birds), the axial tissues of the body form from a growth zone at the tail end, Hensen’s node, which generates neural, mesodermal, and endodermal structures along the midline. While most cells only pass through this region, the node has been suggested to contain a small population of resident stem cells. However, it is unknown whether the rest of the node constitutes an instructive niche that specifies this self-renewal behavior. Here, we use heterotopic transplantation of groups and single cells and show that cells not destined to enter the node can become resident and self-renew. Long-term resident cells are restricted to the posterior part of the node and single-cell RNA-sequencing reveals that the majority of these resident cells preferentially express G2/M phase cell-cycle–related genes. These results provide strong evidence that the node functions as a niche to maintain self-renewal of axial progenitors.