Fig. 4. MT behaviour during mitosis entry. Upper panel (A-H): Mitosis entry and spindle rotation. (A-C) 22 hours AF. (A) Projection of all sections. Nuclear MTs are nucleated at centrosomes and are connected to the cortex of the rhizoid collar and to the rhizoid tip. (B) Projection of 6 upper cortical sections. Cortical MTs are nucleated at the rhizoid collar and are oriented parallel to the growth axis. (C) Nuclear area (projection of 6 sections). One of the centrosomes is connected via two long MT bundles to peripheral MTs. (D) 23 hours AF, the centrosomal axis has rotated slightly. The rhizoid has elongated and the rhizoid MT array is nucleated from a cortical site at the rhizoid collar. (E) 26 hours AF: Prophase spindle and cortical MT structures at the cortex of the collar (projection of all sections). All other cortical MTs have disappeared. Outline of the zygote is shown white. (F,G,H) 30 hours AF, respectively, all sections, seven upper cortical sections and nuclear area (projection of 12 sections). The metaphase spindle has rotated to become almost aligned with the growth axis. Cortical MT structures are present at the rhizoid collar and the rhizoid apex (arrows). (I-L) Projection of all sections during mitosis exit: (I) telophase spindle at 22 hours AF. Spindle asters are separated by a thick MT bundle. (J) At 23 hours AF nuclear envelopes are reformed and MTs appear at the rhizoid apex. In the inset, the focus on one deep section shows faint MT connections between the new rhizoid nucleus and the cortex. (K) At 24 hours AF MTs extend from both nuclei toward the future division plane at the centre of the embryo while cortical MTs reappear in the rhizoid. (M) Different sections of the zygote shown in (K) sampled every 6 µm. In the upper section cortical MTs are detected in both the thallus and the rhizoid. Numerous fine MTs connect both nuclei and interdigitate at the centre of the embryo. (L) 26 hours: cytokinesis is achieved. In the rectangle contrast has been enhanced to show MTs organised in the spindle interzone at the division plane. MTs extend horizontally to the edge of the embryo and perpendicularly other nuclear MTs reach the same plane. Bar, 15 µm.