Fig. 8.

a Temporal evolution of a crenel during contraction along the D-V line. The crenel tends to round off, with a pear like shape having the narrow end oriented ventrally (Video 6). b In vivo time-lapse video-microscopy of ear formation in the chicken embryo. The initial crenel like domain is deformed and takes a pear like shape with the narrow end oriented ventrally (Video 7). One observes a progression of morphogenesis from the median axis, down. The morphogenesis correlates also with a progression in cell stacking in the presumptive ear ridge. c Slowing down of the morphogenesis in the numerical simulation as the contracting segment slows down. Following the initial deformation, the dynamics becomes close to an exponential slowing down (to the right the log-linear plot), linked to the fact that the smaller the segment, the smaller the force. d Effect of a gradient of tension in the ear edge (Video 8). During embryo development, there exists a media-lateral bias : the ear starts to form close to the neural tube, and patterning progresses from the neural tube downwards. Surface movement, buckling and cell alignment progresses in the same direction (see Fig. 8b). We assume the existence of a tensile gradient oriented from the median axis and outwardly away laterally with a maximum at the median axis. We introduce such a tension gradient as a surface shear oriented tangentially in the direction of the neural tube. This gives rounder ears. The pattern corresponds also to the deformation of the eye territory observed in Fig. 7b. e Effect of tension asymmetry (Video 9). During embryo development, there exists an A-P asymmetry of tension forces due to the heart pull along the ventral side. We assume an asymmetry between the tension along the DV line, between anterior and posterior, and also an asymmetry in the crenel tension, and follow the evolution of the ear form, the boundary conditions being fixed. We find a biased ear pattern reminiscent of ear form in vertebrates. f Comparison of an embryonic ear at day 3 of development (chicken embryo) and of a simulated ear. The arrow points to the vestige of the crenel which is advected towards the top in the simulation, and in the in vivo time-lapse movie. g If we assume a very soft, or even absent tissue in the trapeze, we observe the formation of a slit, instead of the formation of an ear (Video 10). This may explain the lateral collapse of the ear at the moment of inward buckling in 8B bottom right