Cellular and molecular mechanisms of convergence and extension in zebrafish

Abstract

Gastrulation is the period of development when the three germ layers, mesoderm, endoderm and ectoderm, are not only formed, but also shaped into a rudimentary body plan. An elongated anteroposterior (AP) axis is a key feature of all vertebrate body plans, and it forms during gastrulation through the highly conserved morphogenetic mechanism of convergence & extension (C&E). As the name suggests, this process requires that cells within each germ layer converge toward the dorsal midline to narrow the tissue in the mediolateral (ML) dimension and concomitantly extend it in the AP dimension. In a number of vertebrate species, C&E is driven primarily by mediolateral intercalation behavior (MIB), during which cells elongate, align, and extend protrusions in the ML direction and interdigitate between their neighbors. MIB is only one of many complex cellular mechanisms that contributes to C&E in zebrafish embryos, however, where a combination of individual cell migration, collective migration, random walk, radial intercalation, epiboly movements, and MIB all act together to shape the nascent germ layers. Each of these diverse cell movements is driven by a distinct suite of dynamic cellular properties/activities, such as actin-rich protrusions, myosin contractility, and blebbing. Here, we discuss the spatiotemporal patterns of cellular behaviors underlying C&E gastrulation movements within each germ layer of zebrafish embryos. These behaviors must be coordinated with the embryonic axes, and we highlight the roles of Planar Cell Polarity (PCP) in orienting and BMP signaling in patterning C&E cell behaviors with respect to the AP and dorsoventral axes. Finally, we address the role of GPCR signaling, extracellular matrix, and mechanical signals in coordination of C&E movements between adjacent germ layers.

Keywords: Axis extension; Cell intercalation; Cell migration; GPCR; Gastrulation; Germ layers; Morphogenesis; Nodal; PCP; Planar cell polarity.

Fig. 1.

Cell behaviors underlying convergence & extension (C&E) of zebrafish mesoderm. Gastrulation movements prior to (A) and after the onset of C&E in lateral (B) and dorsal (C) view. The purple stripe in C represents the axial mesoderm. (D) Cell behaviors underlying C&E movements of mesoderm in each of the five regions indicated in (B) and (C): vegetal migration in region I, dorsal migration in regions II and III, cell intercalations in regions IV and V, and anterior migration in region V. Dorsal is to the right in (A), (B), and (D); anterior is up in all panels.

Fig. 2.

Signaling pathways regulating C&E cell behaviors. (A) Within regions IV and V, non-canonical Wnt/PCP signaling regulates ML cell elongation and ML-biased cell protrusions via a number of downstream modules including JNK, Ca²⁺, and RhoA. (B) Within region V, anteriorly-directed cell migration is regulated by sphingosine-1-phosphate (S1P), Wnt/PCP, and PDGF signaling that ultimately promote directed protrusions via localized Rac1 and RhoA activity. (C) Within region II, cells converge slowly by alternating phases of “tumble”—blebbing induced by loss of membrane-cortical attachment, and “run”—directed protrusion-driven migration requiring Apelin receptor signaling. Black hashes represent the actin cytoskeleton, green hashes represent non-muscle myosin.

Fig. 3.

Cell behaviors underlying convergence & extension of zebrafish endoderm. Endoderm cells move by random walk prior to the onset of C&E (A), then switch to dorsal migration at mid-gastrulation (B). (C) Endoderm migration is regulated by Nodal and Apelin/Toddler signaling and is tethered to the underlying mesoderm by increased Integrin-based adhesion downstream of Cxcl12/Cxcr4 chemokine signaling.

Fig. 4.

Cell behaviors underlying convergence & extension of zebrafish neuroectoderm. (A) The zebrafish neuroectoderm transforms from a neural plate into the neural keel after completion of gastrulation. Convergence of neuroectoderm cells toward the dorsal midline first drives extension (B), then extension stops while convergence continues, instead driving internalization of cells at the midline to form the neural keel (C). Once the neural keel is established, mirror-image divisions across the midline maintain the width of the neural keel/neural rod (D).