Do All Somites along the Anterior–Posterior Axis Come from NMPs?

Vertebrate somites are present along most of the body axis, including the trunk and tail. NMPs exist in the post-gastrulation tailbud and give rise to the posterior trunk and tail of the body. NMPs have been best studied in this post-gastrulation context, but there is experimental evidence to suggest that they are present during gastru- lation and give rise to anterior trunk somites. During zebrafish gastrulation, the prospective anterior spinal cord and trunk somites reside within the dorsal-lateral margin of the embryo (Kimelman and Martin, 2012). Lineage analysis in zebrafish shows that cells in the margin of the gastrula are generally mono-fated, giving rise to either spinal cord or mesoderm (Attardi et al., 2018). However, these mono-fated cells could maintain neuromesodermal plasticity, but due to their signaling environment, predominantly give rise to only one cell type. The role of Wnt signaling in post-gastrulation neuromesodermal fate decision suggests that this is the case. But the role of Wnt signaling during the neuromesoderm fate decision is stage-specific. When cells containing an inducible cell-autonomous Wnt inhibitor are transplanted into the margin of host embryos, where the future spinal cord and somitic cells reside, inhibition of Wnt signaling at the end of gastrulation causes NMPs to contribute to tail spinal cord and inhibits contribution to tail somites, whereas transplanted cells that had already made the decision to become paraxial mesoderm when Wnt signaling was inhibited joined the trunk somites normally (Martin and Kimelman, 2012). On the other hand, if Wnt signaling is inhibited in cells transplanted into the margin of wild-type host embryos at earlier stages during gastrulation, transplanted cells contribute to trunk spinal cord and fail to contribute to trunk somites (Martin and Kimelman, 2012). The continuous role of Wnt signaling in the neuromesoderm fate decision before and after gastrulation suggests that NMPs exist during gastrulation and are allocated to spinal cord or mesodermal fate depending upon their local Wnt signaling environment, although further work is required to definitively demonstrate the gastrula stage existence of NMPs.

In mouse embryos, there is single-cell clonal descendant contribution to anterior trunk mesoderm and spinal cord (Tzouanacou et al., 2009). During gastrulation, these cells originate in the epiblast, and at the completion of gastrulation, NMPs are deposited in the tailbud of the embryo (Cambray and Wilson, 2007; Rodrigo Albors et al., 2018; Wymeersch et al., 2016). These results suggest that there exists NMPs during mouse gastrulation that contribute to anterior somites, in addition to post-gastrula NMPs that give rise to the posterior somites. Thus, in both mouse and zebrafish it appears that somitic cells along the entire anterior-posterior axis originate from NMPs that exist during and after gastrulation.

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