Mechanics of blastopore closure during amphibian gastrulation

Abstract

Blastopore closure in the amphibian embryo involves large scale tissue reorganization driven by physical forces. These forces are tuned to generate sustained blastopore closure throughout the course of gastrulation. We describe the mechanics of blastopore closure at multiple scales and in different regions around the blastopore by characterizing large scale tissue deformations, cell level shape change and subcellular F-actin organization and by measuring tissue force production and structural stiffness of the blastopore during gastrulation. We find that the embryo generates a ramping magnitude of force until it reaches a peak force on the order of 0.5 μ Newtons. During this time course, the embryo also stiffens 1.5 fold. Strain rate mapping of the dorsal, ventral and lateral epithelial cells proximal to the blastopore reveals changing patterns of strain rate throughout closure. Cells dorsal to the blastopore, which are fated to become neural plate ectoderm, are polarized and have straight boundaries. In contrast, cells lateral and ventral to the blastopore are less polarized and have tortuous cell boundaries. The F-actin network is organized differently in each region with the highest percentage of alignment occurring in the lateral region. Interestingly F-actin was consistently oriented toward the blastopore lip in dorsal and lateral cells, but oriented parallel to the lip in ventral regions. Cell shape and F-actin alignment analyses reveal different local mechanical environments in regions around the blastopore, which was reflected by the strain rate maps.

Decoupling the roles of cell shape and mechanical stress in orienting and cueing epithelia mitosis

“…Whilst informative, these techniques are invasive, perturbing the stress field through the measurement, and usually require constitutive modelling for the measurement to be interpreted (Stooke-Vaughan et al, 2017; Sugimura et al, 2016). However, mathematical modelling combined with high quality fluorescence imaging now provides the possibility of non-invasively inferring mechanical stress in tissues (Brodland et al, 2014; Chiou et al, 2012; Feroze et al, 2015; Ishihara and Sugimura, 2012; Nestor-Bergmann et al, 2017; Xu et al, 2015).…”

A toolbox to explore the mechanics of living embryonic tissues

“…Some of the global forces driving gastrulation [163] and blastopore closure [164] in Xenopus have been recently measured in vivo using calibrated cantilevers. Besides forces, the mechanical properties of embryonic tissue explants of several amphibian species ( Xenopus [165] and newt [166]) have been measured ex vivo using cantilevers under controlled applied forces.…”

Xenopus as a model for studies in mechanical stress and cell division

“…Force production over long time‐scales can be accurately measured using a calibrated cantilever: if embryonic tissue pushes or pulls against the cantilever, the force produced by the morphogenetic movement can be determined by the distance the cantilever moves. Calibrated cantilevers have been inserted into Xenopus embryos to directly measure the forces driving blastopore closure (Feroze et al, ). Cantilevers have also been used to measure forces produced by extending Xenopus tissue explants (Moore, ).…”

“…In Xenopus, strain mapping has been used for a number of different purposes. By assessing the speed and movement of cells during blastopore closure, the location, direction, and relative magnitude of morphogenetic forces driving gastrulation have been mapped (Feroze, Shawky, von Dassow, & Davidson, 2015). Similarly, comparing cell level strain rates between epithelial and neural precursor ectoderm suggests a greater level of tension in neural precursor ectoderm (Yamashita, Tsuboi, Ishinabe, Kitaguchi, & Michiue, 2016).…”