Résumé
The ascidian embryo exhibits a highly reproducible pattern where homologous cells can be identified across different embryos, allowing consistent cell naming. This developmental stereotypy of the embryo depends strongly on the orientation of cell divisions. To better understand the embryonic organization, identifying the cues that control division orientation by attempting to predict it is of great interest. Before the 64-cell stage, when the embryo still has a spherical shape, the longest axis of a cell's apical surface during interphase (around 20 minutes before cell division) predicts its division orientation (Hertwig's rule). After the 112-cell stage, testing this rule becomes more difficult due to the local tissue deformations occurring between a cell's interphase and its division. Here, we propose a method to systematically test the Hertwig rule in 3D+t ascidian embryos between the 64-and 300-cell stages. Our results suggest that some neural plate cells division orientation may follow nongeometric cues, and some cell divisions exhibiting two distinct orientations may arise from apical surface geometry. This quantitative analysis contributes to our understanding of the factors controlling cell division orientation.