Dynamics of primitive streak regression controls the fate of neuromesodermal progenitors in the chicken embryo

Abstract

In classical descriptions of vertebrate development, the segregation of the three embryonic germ layers completes by the end of gastrulation. Body formation then proceeds in a head to tail fashion by progressive deposition of lineage-committed progenitors during regression of the primitive streak (PS) and tail bud (TB). The identification by retrospective clonal analysis of a population of neuromesodermal progenitors (NMPs) contributing to both musculoskeletal precursors (paraxial mesoderm) and spinal cord during axis formation challenged these notions. However, classical fate mapping studies of the PS region in amniotes have so far failed to provide direct evidence for such bipotential cells at the single-cell level. Here, using lineage tracing and single-cell RNA sequencing in the chicken embryo, we identify a resident cell population of the anterior PS epiblast, which contributes to neural and mesodermal lineages in trunk and tail. These cells initially behave as monopotent progenitors as classically described and only acquire a bipotential fate later, in more posterior regions. We show that NMPs exhibit a conserved transcriptomic signature during axis elongation but lose their epithelial characteristicsin the TB. Posterior to anterior gradients of convergence speed and ingression along the PS lead to asymmetric exhaustion of PS mesodermal precursor territories. Through limited ingression and increased proliferation, NMPs are maintained and amplified as a cell population which constitute the main progenitors in the TB. Together, our studies provide a novel understanding of the PS and TB contribution through the NMPs to the formation of the body of amniote embryos.

Cell-to-cell heterogeneity in Sox2 and Bra expression guides progenitor motility and destiny

“…Biasing progenitors with mesenchymal properties toward a neural state is therefore much more likely to give a difference in motility than a change toward another mesodermal state. In line with this explanation is the fact that during the course of axis elongation posterior progenitors undergo an epithelial-mesenchymal transition before reaching their full potential to give rise to progeny in the NT and the PSM ( Guillot et al, 2021 ; Goto et al, 2017 ; Dias et al, 2020 ). Therefore, it is likely that even though Sox2/Bra heterogeneity is present since stage HH5, regulation of progenitor destiny by cellular motility is mostly active after stage HH8 when the progenitors have become mesenchymal with cellular properties that are closer to PSM cells.…”

“…We electroporated quail embryos at stage HH5 with a vector encoding for nuclear GFP and performed time-lapse imaging experiments from stage HH8 to stage HH12. At these stages (from stage HH8 onward), posterior progenitors are no longer located in the dorsal epithelium but rather within a dense and internal mesenchymal structure that prefigures the embryonic tailbud ( Schoenwolf and Delongo, 1980 ; Guillot et al, 2021 ). In order to compare migration properties between tissues, we focused on the PZ , the PSM and on the posterior NT ( Figure 3A , Figure 3—video 1 ).…”

“…Heterogeneity in the progeny of PZ cells was further confirmed by retrospective clonal analysis studies performed in the mouse embryo which revealed the existence of single progenitors, giving rise either to neural or mesodermal cells, but also of bi-potent progenitors, named neuro-mesodermal progenitors (NMPs), that generate both neural and mesodermal cells ( Tzouanacou et al, 2009 ). The existence of bi-potent progenitors has since been shown at earlier stages of zebrafish development ( Attardi et al, 2018 ) and in bird embryos ( Solovieva et al, 2020 ; Wood et al, 2019 ; Guillot et al, 2021 ) (for reviews Wymeersch et al, 2021 ; Sambasivan and Steventon, 2020 ). Thus, to sustain the formation of tissues that compose the vertebrate body axis, the heterogeneous population of posterior progenitors must maintain an appropriate balance between the two choices of staying in place and self-renew or exit the progenitor region to contribute to the formation of the NT and the PSM.…”

From Bipotent Neuromesodermal Progenitors to Neural-Mesodermal Interactions during Embryonic Development

“…Whether NMPs constitute a population of bipotent cells or alternatively, a mixture of precursors separately committed to each lineage remained unclear. Recent lineage tracing in the chick embryo using a barcoded retroviral library and the Brainbow method for in vivo clonal analysis, revealed a resident cell population, mapped to the anterior primitive streak epiblast, that contains single cells contributing to both neural and mesodermal lineages in trunk and tail, and further characterized their dynamics and molecular profile [61]. NMPs can either self-renew to maintain the bipotent state, or differentiate either as paraxial mesoderm or neuroectoderm.…”

“…Clonal approaches in vitro and in vivo recently showed the bifated nature of NMPs [61] and additional studies begun addressing the molecular network responsible for fate segregation (see Section 2). Much is still to be uncovered regarding the relationship between cell fate acquisition and morphogenesis, by addressing dynamic processes such as EMT and cell migration.…”