Axis Specification in Zebrafish Is Robust to Cell Mixing and Reveals a Regulation of Pattern Formation by Morphogenesis

Abstract

Summary A fundamental question in developmental biology is how the early embryo establishes the spatial coordinate system that is later important for the organization of the embryonic body plan. Although we know a lot about the signaling and gene-regulatory networks required for this process, much less is understood about how these can operate to pattern tissues in the context of the extensive cell movements that drive gastrulation. In zebrafish, germ layer specification depends on the inheritance of maternal mRNAs [ 1 , 2 , 3 ], cortical rotation to generate a dorsal pole of β-catenin activity [ 4 , 5 , 6 , 7 , 8 ], and the release of Nodal signals from the yolk syncytial layer (YSL) [ 9 , 10 , 11 , 12 ]. To determine whether germ layer specification is robust to altered cell-to-cell positioning, we separated embryonic cells from the yolk and allowed them to develop as spherical aggregates. These aggregates break symmetry autonomously to form elongated structures with an anterior-posterior pattern. Both forced reaggregation and endogenous cell mixing reveals how robust early axis specification is to spatial disruption of maternal pre-patterning. During these movements, a pole of Nodal signaling emerges that is required for explant elongation via the planar cell polarity (PCP) pathway. Blocking of PCP-dependent elongation disrupts the shaping of opposing poles of BMP and Wnt/TCF activity and the anterior-posterior patterning of neural tissue. These results lead us to suggest that embryo elongation plays a causal role in timing the exposure of cells to changes in BMP and Wnt signal activity during zebrafish gastrulation. Video Abstract

Tissue tectonics and the multi-scale regulation of developmental timing

“…cells must be able to determine the state of embryo elongation and tissue morphogenesis to coordinate these processes with cell specification and patterning. A recent study has approached this question using explants of zebrafish embryonic cells that were cultured away from the yolk and yolk syncytial layer [74]. Such aggregates go on to break morphological symmetry and generate multiple germ layers in an organized manner [74][75][76].…”

Pescoids and chimeras to probe early evo-devo in the fishAstyanax mexicanus

“…A. mexicanus gastruloids were produced following a recent description in zebrafish (Fulton et al, 2020; Figure 1A). Briefly, at the 512-1K cell stage the yolk was carefully removed from embryos using eyebrows knives (Supplemental video 1).…”

“…In zebrafish pescoids (yolk-less blastoderm explants) the main aspects of early development observed in intact embryos are recapitulated. Symmetry breaking, axis elongation and neural specification occur despite the absence of extraembryonic signaling (Fulton et al, 2020;Schauer et al, 2020). In fish, these events are controlled maternally (Marlow, 2020;Solnica-Krezel, 2020) and A. mexicanus with its two morphotypes has become an excellent model to challenge the role of maternal determinants in embryogenesis (Ma et al, 2020(Ma et al, , 2018Torres-Paz et al, 2019).…”

“…Here, we describe the implementation in A. mexicanus of methods used in well-established fish models to probe mechanisms of cell/tissue specification during early embryogenesis. First, we have adapted a recent method of embryonic explant culture developed in zebrafish and known as "pescoids" to grow the blastoderm after removal of the extraembryonic yolk cell (Fulton et al, 2020;Schauer et al, 2020). Under these conditions of altered embryonic geometry and physical constraints, the explants are able to recapitulate the main processes observed in intact embryos such as symmetry breaking, germ layer specification and elongation.…”

A temporally resolved transcriptome for developing “Keller” explants of theXenopus laevisdorsal marginal zone

“…More recently, there has been renewed interest in the study of early embryo explants from zebrafish (Fulton et al, 2020;Schauer et al, 2020;Williams and Solnica-Krezel, 2020;Xu et al, 2014), which has prompted comparisons to mammalian gastruloids, in which pluripotent stem cells are directed toward early embryonic fates (Simunovic and Brivanlou, 2017). Indeed, explants of the fish blastoderm have even been referred to as "pescoids" and the hugely tractable explants of pluripotent blastula ectoderm from Xenopus (so-called "animal caps") as "Xenoids" (Moris et al, 2020).…”