Extra-embryonic tissue spreading directs early embryo morphogenesis in killifish

Abstract

The spreading of mesenchymal-like cell layers is critical for embryo morphogenesis and tissue repair, yet we know little of this process in vivo. Here we take advantage of unique developmental features of the non-conventional annual killifish embryo to study the principles underlying tissue spreading in a simple cellular environment, devoid of patterning signals and major morphogenetic cell movements. Using in vivo experimentation and physical modelling we reveal that the extra-embryonic epithelial enveloping cell layer, thought mainly to provide protection to the embryo, directs cell migration and the spreading of embryonic tissue during early development. This function relies on the ability of embryonic cells to couple their autonomous random motility to non-autonomous signals arising from the expansion of the extra-embryonic epithelium, mediated by cell membrane adhesion and tension. Thus, we present a mechanism of extra-embryonic control of embryo morphogenesis that couples the mechanical properties of adjacent tissues in the early killifish embryo.

Cell cycle dynamics during diapause entry and exit in an annual killifish revealed by FUCCI technology

“…Random movements of EL cells in the dispersed phase were originally reported by Lesseps et al, in the '70s [45] by means of bright-field microscopy and were later confirmed by us [6]. instructed by mechanical interactions between EVL and EL cells [8].…”

“…It is known that diapause II is characterized by reduced protein synthesis, cell cycle arrest and remodelling of mitochondrial physiology and it is controlled by insulin-like growth factor 1 signalling [29][30][31][32][33][34][35].Recently, RNAseq studies have shown that , gene-expression patterns during diapause resemble those observed during aging [36] and vitamin D signalling controls to the choice between direct development and diapause II [37]. While early embryonic development following fertilisation have been in part studied [7][8][9], the characterization of the physiological and molecular events occurring during diapause I has been started to be investigated only recently [35,38,39]. Here, to investigate entry and exit form diapause I and II, we created a stable transgenic line in N. furzeri that expresses fluorescent reporters for cell cycle phases [40].…”

“…When epiboly is ongoing, both green and red cells are present in dFUCCI embryos (A). As long as epiboly proceeds (B, C) the number of green EL cells gradually and slowly decreases over time while the number of red EVL cells increases until the reaching of a plateau (graph hours[7][8][9][10][11][12]. The cells in the field of view were easily tracked and counted transforming the dots in particles with IMARIS(D,E,F).…”

E-cadherin expression pattern during zebrafish embryonic epidermis development

“…EVL arises at mid-blastula stage (2.5 hours post fertilization; hpf) covering the whole embryo 5 . During epiboly EVL maintains tight joins to the yolk syncytial layer (YSL), and becomes the migration substrate for the underlying deep cells spreading during gastrulation 6 . At the tail bud stage (8–10 hpf), the epiblast forms the outer germ layer, the ectoderm, characterized as a pseudo-epithelial germ layer.…”

“…E-cadh is required for blastomeres adhesion during the cleavage stage and later during gastrulation and epiboly 19– 21 . Indeed, as epiboly proceeds, EVL directs cell migration and the spreading of cells of the deep cells layer (DCL) in a process that requires dynamic cell contacts remodeling mediated by E-cadh 6 . Once the bi-layered epidermis is established, E-cadh role becomes more refined by keeping its integrity while actively remodeling cell-cell contacts within each layer.…”