Models of convergent extension during morphogenesis

Abstract

Convergent extension (CE) is a fundamental and conserved collective cell movement that forms elongated tissues during embryonic development. Thus far, studies have demonstrated two different mechanistic models of collective cell movements during CE. The first, termed the crawling mode, was discovered in the process of notochord formation in Xenopus laevis embryos, and has been the established model of CE for decades. The second model, known as the contraction mode, was originally reported in studies of germband extension in Drosophila melanogaster embryos and was recently demonstrated to be a conserved mechanism of CE among tissues and stages of development across species. This review summarizes the two modes of CE by focusing on the differences in cytoskeletal behaviors and relative expression of cell adhesion molecules. The upstream molecules regulating these machineries are also discussed. There are abundant studies of notochord formation in X. laevis embryos, as this was one of the pioneering model systems in this field. Therefore, the present review discusses these findings as an approach to the fundamental biological question of collective cell regulation. WIREs Dev Biol 2018, 7:e293. doi: 10.1002/wdev.293 This article is categorized under: Early Embryonic Development > Gastrulation and Neurulation Comparative Development and Evolution > Model Systems.

Figure 1

Convergent extension (CE) during the formation of Xenopus laevis notochord. (a) General cell movements exhibited during CE. The cells move bidirectionally along the future short axis of the elongating tissue (horizontal axis in this scheme, green arrows) and intercalate between each other. The continuous intercalation allows the tissue to elongate along the perpendicular axis (blue arrows). (b, b′) Notochord formation during gastrulation in the X. laevis embryo. The region that develops into the notochord is marked with a pink color. The notochord elongates along the anteroposterior axis of the embryo by cells intercalating along the mediolateral axis. (c–c") Immunostaining of embryos injected with membrane‐GFP mRNA. The notochord dramatically narrows during neurulation. Arrowheads indicate notochord–somite boundary, and the yellow arrows indicate the width of the notochord. A, anterior; P, posterior; M, medial; L, lateral; St, embryonic stage.

Figure 2

Two modes of convergent extension. (a) Driving force and cell movements in the crawling mode. The cells elongate along the mediolateral axis (short axis of the elongating tissue) and crawl and tug the neighboring cells by actin‐rich protrusions located at both tips of the elongated cells (red parts) that lead to the intercalating movements. (b, b′) Driving force and cell movements in the contraction mode. Activation of actomyosin along the cell–cell junction constricts the junction (red line) and pulls the neighboring cells (dark and light gray colored cells) to the center in this scheme. The actomyosin is activated at the cell–cell junction aligning along the short axis of the elongating cell. After the neighboring cells meet together, these two cells construct a new cell–cell junction (junction remodeling, green short line), and actomyosin starts constricting the other cell–cell junction aligned to the short axis of the tissue. Repeating these events allows the cell to establish intercalating movements. (B′) is a scheme of rosette formation.

Figure 3

Driving forces of the two modes of convergent extension. (a) Actin‐rich protrusions allow the cell to crawl along the neighboring cells. Actin is polymerized at the protrusions and pushes the membrane forward between the neighboring cells. The actin cables provide the resistant force for cell elongation, in order for the cells to tug the adjacent cells by the crawling motion. In this model, the cells move actively. (a′) A model of actin filament polymerization and branching. Actin‐binding proteins such as Arp2/3, Formin, or Cofilin function to branch, polymerize or sever the actin filament. (b) Actomyosin activation constricts the cell–cell junction. The neighboring cell (gray colored) is pulled by the shrinking cell–cell junction. In this model, the cells move passively. (b′) A model of actomyosin contraction. Phosphorylation of myosin light chain triggers the actin filaments to slide toward the myosin complex.

Figure 4

Various tissues showing convergent extension. Classification of CE models based on the crawling and contraction modes. Membrane protrusions are observed during CE in the mouse neural plate, Xenopus laevis notochord, and Ciona intestinalis notochord (i.e., crawling mode: blue circle). The formation of X. laevis notochord also exhibits the contraction mode, and it is thought that the mouse neural plate and C. intestinalis notochord also display the contraction mode. Cell–cell junction contractions are observed during CE in Drosophila melanogaster germband extension, chick neural tube elongation, and X. laevis kidney tubule elongation (i.e., contraction mode: red circle). Although there are other developmental processes showing CE, their modes are unclear (green box).

Figure 5

Cell adhesion properties for the two modes of convergent extension. (a) Cell adhesion properties suggested for the crawling mode. Loose adhesion is required for the cell (colored gray) to crawl between neighboring cells (colored white). (b) Cell adhesion properties suggested for the contraction mode. Dynamic adhesion (green line) is required for the cells (colored white) to attach together and shrink the shared cell–cell junction, while not impeding contraction. Tighter adhesion (red line) between the cells exhibiting cell–cell junction shrinkage (white) and the cells consequently being pulled (colored gray) is required for the cell (gray) to be pulled by the contraction occurring between cells (white).

Figure 6

Planar cell polarity (PCP) pathway regulates actin dynamics via small guanosine triphosphatases (GTPases). Downstream pathways of the PCP pathway for cytoskeletal regulations. Activation of (a) actin polymerizing factor Daam1 by Rho, and (b) actin branching factor Arp3 by Rac. (c) Phosphorylation of myosin light chain (Myl) by Rho and Rock (Rho‐Rock‐Myl cascade). (d) Phosphorylation and activation of LIM‐kinase (LIMK) controlling F‐actin severing cofilin function by Rho and Rock. (e) A feedback loop of cofilin on PCP proteins. (f) A feedback loop of contractile forces of actomyosin on PCP proteins.

Figure 7

Interplay between the regulators of cytoskeleton components and cell adhesion molecules. Interplay between the planar cell polarity (PCP) pathway, Shroom, and fibroblast growth factors (FGF) for the regulation of downstream cytoskeleton components and cell adhesion molecules. The nature of each relationship is classified by localization (blue line), expression (red line), or physical interaction (yellow line). Solid and dotted lines are indicative of the relationship being demonstrated by studies investigating convergent extension (CE) and non‐CE, respectively.

Figure 8

Tissue explant isolation from Xenopus laevis embryos for live imaging. (a) Procedure of isolating Keller explants. The explant is cut out at embryonic stage 10.5. Incisions are made on both sides of the blastopore lip, and the dorsal region is opened after cutting the ectoderm. The dorsal region is discerned by cutting along the blastopore lip. (b) Trimming the Keller explant and imaging the notochord. The endoderm is removed to expose the mesoderm (notochord) before mounting on a fibronectin‐coated dish. The mesoderm is placed face down for the purpose of live imaging through inverted confocal microscopy. (c–d′) Images of the cell membrane and F‐actin captured by live imaging of Keller explants. (c) and (c′) are images of the cells at the surface of the explant, where the cells adhere to the fibronectin. (d) and (d)′ are images of the same cells as in (c) and (c′), but focused 5 µm deep from the plane of (c) and (c′). The membrane protrusions (arrowheads) and actin cables are more obvious in (c) and (c′) images, whereas the borders of the cells are more obvious in (d) and (d’) images.