Resilience in zebrafish embryoids

It is unresolved how morphogenetic cell movements and Nodal signalling are involved in the formation of the embryonic body during gastrulation. We address this question by using the zebrafish embryoid model system, which consists of cultured blastula cells lacking yolk and signalling derived from the yolk syncytial layer (YSL). Zebrafish embryoids form a cylindrical-like structure under serum-free growth culture conditions. We find that embryoids elongate without directed convergence and maintain their proliferation rate and cell mixing behaviour. Surprisingly, they undergo budding activity that has not been reported in zebrafish embryos yet. Here, the constitutively active Nodal receptor type I promotes internalisation-like gastrulation movements and induces the expression of shield genetic markers in embryoids. This work demonstrates that the absence of the yolk and signalling derived from the YSL does not affect the cell behaviour, while the constitutively active Nodal receptor type I impacts budding, internalisation movement and the emergence of the second shield. Summary statementWe demonstrate that the cell behaviour during gastrulation is robust to the deficiency of yolk topology and signalling originating from the yolk syncytial layer.