Epithelial machines that shape the embryo

Abstract

Embryonic form and the shape of many organs are the product of forces acting within and on epithelial sheets. Analysis of these processes requires both consideration of the mechanical operation of these multicellular machines and an understanding of how epithelial sheets are integrated with surrounding tissues. From the diverse array of epithelial morphogenetic movements seen during embryogenesis we review examples of epithelial sheet bending, Drosophila ventral furrow formation and ascidian gastrulation, and direct measurements of epithelial mechanics from Xenopus laevis. We present these examples as works-in-progress and highlight opportunities for future studies into both the direct consequence of force production and embryonic tissue mechanics and potential roles of signaling from biomechanical processes.

Figure 1. Anatomy and mechanics of epithelial sheet morphogenesis

A) Three axes of the epithelial sheets and the localization of distinct molecular complexes along the radial axis (Axis I) as well as along the two planar axes (Axis II and III). B) In-the-plane force-generating mechanisms can alter the geometry of an epithelial sheet. Asymmetries in force generation along the radial axis or constraining boundary conditions can cause out-of-the-plane changes in geometry, such as bending. C) Boundary conditions preventing or limiting the movement of an epithelial sheet can dramatically alter the final shape of the sheet after the application of a force; for instance, a fixed boundary can lead to sheet bending. D) Asymmetrical distribution of forces, either externally applied or internally generated, can also drive bending.

Figure 2. Cellular behaviors, molecular mechanism and a case of epithelial folding

A) A hexagonal `unit-cell' in an epithelial sheet consists of discrete mechanical elements. In addition to the contents of the cell, the circum-apical junctions, apical cortex, lateral cortex and basal cortex (not shown) have specific geometries and material properties. These structural elements all contribute to the stiffness of the unit-cell. B) Either in response to internal or external stresses, cells may adopt a variety of shapes. Internally driven forces may produce counter-intuitive changes in cell shapes as stresses distributed throughout the sheet adjust to boundary conditions. C) Initial anatomy of ascidian embryo before gastrulation. Cells in the ectoderm, mesoderm and endoderm are arrayed in a single layer. Ectoderm and endoderm cells are in direct contact at their basal surfaces. Sherrard and colleagues suggest cells undergo a two-step program of apical constriction followed by basolateral contraction. For this program to move the endoderm into the embryo requires boundary conditions provided by a contractile ectodermal sheet. These differential programs of contractility can be seen in the relative abundance of F-actin and active non-muscle myosin II, and are thought to drive multiple phases of out-of-the-plane folding during gastrulation in the ascidian (C reproduced with permission from [25]).

Figure 3. Measuring physical mechanics of epithelial morphogenesis

A) Marginal zone explants from gastrulating Xenopus laevis embryos can be microsurgically isolated and placed onto force-reported polyacrylamide gels conjugated with fibronectin (PAG-FN). Cells in explants expressing membrane-targeted GFP (mem-GFP) bind fibronectin and displace red fluorescent beads embedded within the PAG-FN. B) Color-encoding shows regions of high traction force (arrows) on the mem-GFP image and can also be seen in a frequency histogram. C) Both the viscoelastic properties and the force-generating potential of an embryonic epithelial sheet can be recorded in a combination stimulatormicroaspirator. A patch of the embryo can be drawn into a microchannel by reduced pressure in the channel. D) The full shape of the aspirated cells expressing mem-GFP and their 3D conformation within the channel may be recorded with a laser scanning confocal microscope. E) A kymograph of a transverse view of the microaspirated tissue reports deformation after a simple drop in pressure (ΔP - no stimulus) and after a brief electrical pulse is used to stimulate contraction (ΔP + stimulus). (A and B modified from [36]; C and E modified from [33]; D courtesy of M. von Dassow).