Studying gastrulation by invagination: The bending of a cell sheet by mechanical cell properties using 3D deformable cell based simulations
Fig 5
Cell Stiffness and constriction factor.
This figure shows cross sections through blastulas that have constricted the apices of the endodermal plate. The endodermal plate (salmon) is oriented to the left. Below each blastula is the total number of cells in the blastula (32-1024) and number of endodermal cells between parentheses (). (A) Uniform cell stiffness constriction factor 0.1. Simulating with the following parameters: Cell stiffness 0-100% k=0.5. Constriction factor 0.1 and endodermal adhesion region 20-65%. All blastulas invaginated, but the global shape was bowl-like with a large opening. In the 1024 blastula the endodermal and ectodermal layer did not establish contact. (B) Uniform stiffness with constriction factor 0.05. Simulating with the following parameters: cell stiffness 0-100% k = 0.5, constriction factor 0.05, endodermal adhesion region 20–65%, The endodermal cells elongated and expanded more basally due to the smaller edge constriction factor. This placed more strain on the smaller embryos, therefore the number of endodermal cells were reduced to allow better invagination into the blastulas. Reduction of endodermal cells strongly influenced the global shape. The final gastrula was rounder than seen in Fig 5A but now no germ layer alignment occurred. All blastulas invaginated, but the larger the embryo size, the more bowl shaped the embryo became with a larger opening. (C) Non-uniform cell stiffness. Simulating with the following parameters; constriction factor 0.05, endodermal adhesion region 20–65%. The cells are divided into three stiffness zones (apical region: 0-30% of the spherical cell, lateral region: 30–70% and basal region: 70–100%). The cell stiffness is stiffest in the apical region (k = 1) in the lateral region the stiffness is k = 0.5 and the basal region is the softest (k = 0.1). All the blastulas invaginated and the endoderm and ectoderm aligned completely in the smaller gastrulas. The larger gastrulas (512 en 1024 cells) approached alignment. The global shape became bowl-like, with a large opening. (D) Increased cell stiffness over time. The endodermal cell stiffness is increased apically and decreased basally over time. Simulating with the following parameters: Constriction factor 0.1, endodermal adhesion region 20-65%. The cell stiffness at the start of the experiment is k = 0.5. For the endoderm cells the stiffness of the apical top (0–40%) is slowly increased to k = 1.5 and the basal side (40–100%) decreased to k = 0.1. For the ectoderm cells the apical stiffness is increased to k = 1.4 and for the basal side it is decreased to k = 0.35. The cells showed a more bottle like appearance close to the ectoderm (blastoporal lip cells). The layers did not connect. The smaller gastrulas remained rounder, but the largest gastrula (256 cells) had a strong bowl-shape and large opening. (E) Number of endoderm. Image E shows the 256 celled blastula from row C, but now the number of endodermal cells is reduced from 58 to 36. This results in more blastocoel space remaining between the endodermal plate and the ectodermal layer. The smaller number of endodermal cells resulted in one more ectodermal ring that could move into the blastoporal opening.