Fig. 1. Cells in the Xenopus blastula divide according to their shape. (A) Xenopus blastula cells have different shapes; arrows indicate cells with small apical surfaces and a long axis perpendicular to the surface. (B,C) Examples of single cells (B) and a pair of cells (C) dissected out (arrowhead indicates flat cell with a large apical surface; arrow indicates apicobasally elongated cell with a small apical surface). (D,E) Confocal image stacks of embryos stained for -tubulin (red in D, white in E) and DNA (yellow in D). A top view section and the corresponding side projection of the stack are shown. In D, the spindle is oriented parallel to the surface; in E it is perpendicular to the surface. The cell in E has a small apical surface and a spindle aligned with the long axis that is perpendicular to the surface. Such cells will undergo a fate-asymmetric division. Spindle alignment with the long axis of the cell is also observed in cells with parallel spindles (D). (F-H) Spindle orientation was visualised in vivo using tau-GFP-injected embryos and alignment was analysed at late anaphase between the 8th and the 10th division. Spindles aligned within 25° of the long axis were scored as aligned with the long axis (LA) (F,H). Alignment within 25° of the short axis was scored as aligned with the short axis (SA) (G,H). n=216 divisions in 11 embryos. Scale bars: 50 µm. (I,J) 3D reconstructions of sister cells that were identified by the remnant of the midbody (white arrow). (I) A pair of flat cells that are the product of a parallel division and (J) a flat cell and an apicobasally elongated cell, the product of an oblique division. The previous cleavage plane can be deduced from the position of the midbody remnant. In all these cells, the axis of the spindle (red line) is aligned close to the long axis of the cell. Movies 1 and 2 in the supplementary material correspond to these images. (K,L) Computational analysis of spindle alignment with the long axis in 3D reconstructions shows that the median deviation is 12° at metaphase and most spindles are within 20° of the long axis. The distribution of angles was non-random, as determined by Watson's U²_n test (P<0.005). (M) Correlation of the elongation factor with spindle alignment to the long axis. (A value of 1 corresponds to a sphere.) Elongated cells show better alignment with the long axis.