The embryonic node behaves as an instructive stem cell niche for axial elongation

Abstract

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.

Quantitative Experimental Embryology: A Modern Classical Approach

“…This approach has the additional advantage of being compatible with live imaging approaches, unlike post-fixation staining (e.g., quail-chick chimeras). A particularly impressive use of the GFP transgenic chicken embryos for fate mapping is described by Solovieva et al [ 2 ]. The Hensen’s node fate map [ 39 ] was revisited using single cell grafts to define the location of self-renewing stem cells, resulting in their precise localisation to the posterior portion of the node [ 2 ].…”

A niche for axial stem cells - A cellular perspective in amniotes

“…Another well characterized role of the node is its cellular contribution to axial (notochord, floorplate) and paraxial (somite) tissues and definitive gut endoderm ( Rosenquist, 1966 , 1983 ; Schoenwolf et al., 1992 ; Selleck and Stern, 1991 ; Spratt, 1955 ). More recently it has also been shown to act as a niche for resident axial stem cells in chick ( Solovieva et al., 2022 ), a novel role which we will discuss in more detail.…”

“…While many cells leave the node to give rise to axial and paraxial structures, some cells remain resident within the node, regressing node and tailbud; we will refer to these as ‘resident cells’ or cells with ‘resident behaviour’. From the collective point of view of the cell population that makes up these structures, resident behaviour in the node, regressing node and tailbud has been demonstrated through homotopic grafts ( Cambray and Wilson, 2007 ; Catala et al., 1995 , 1996 ; McGrew et al., 2008 ), electroporation ( Mathis et al., 2001 ) and DiI labelling ( Knezevic et al., 1998 ; Selleck and Stern, 1991 ; Solovieva et al., 2022 ; Wilson and Beddington, 1996 ). Tracking cells and their descendants from sub-regions of the regressing node and tailbud has revealed that longer term resident cells reside only in the posterior regions of the regressing node ( Cambray and Wilson, 2007 ; Solovieva et al., 2022 ; Wymeersch et al., 2019 ), and in the chordoneural hinge (CNH) and dorsal part of the posterior tailbud ( Cambray and Wilson, 2002 ; McGrew et al., 2008 ) ( Fig.…”

“…Caudal part of chick (A–B) and mouse (C–D) embryos shown during regressing node stages (A, C) and tailbud stages (B, D). All areas in shades of red show regions containing cells with resident behaviour, as shown through homotopic grafts ( Cambray and Wilson, 2007 ; Catala et al., 1995 , 1996 ; McGrew et al., 2008 ), electroporation ( Mathis et al., 2001 ) or DiI labelling ( Knezevic et al., 1998 ; Selleck and Stern, 1991 ; Solovieva et al., 2022 ; Wilson and Beddington, 1996 ). The location of resident cells in the mouse tailbud was inferred from heterochronic grafts ( Cambray and Wilson, 2002 ; McGrew et al., 2008 ).…”

“…Dashed red highlights regions containing long-term self-renewing stem cells (at least at the population level), as suggested by serial transplants of cell populations ( Cambray and Wilson, 2002 ; McGrew et al., 2008 ). Solid red marks regions containing an instructive stem cell niche, as suggested by heterotopic grafts of single and/or groups of cells ( Cambray and Wilson, 2007 ; Solovieva et al., 2022 ), and cell-cycle analysis ( Bellomo et al., 1996 ; Solovieva et al., 2022 ; Wymeersch et al., 2019 ). Black dashed lines outline the node.…”