Pre- and postmitotic reorientation of microtubule arrays in youngSphagnum leaflets: Transitional stages and initiation sites

Summary The microtubules (MTs) of developingSphagnum leaflets rearrange from the interphase array into the preprophase band without obvious participation of definite initiation sites. At late prophase, additional MTs appear along the nuclear envelope, with the same orientation as in the peripherally situated preprophase band. Spindle formation begins along the nuclear envelope; spindle MTs run perpendicular to preprophase band MTs and converge in several focus points with indistinct polar bodies. After cytokinesis, most spindle and phragmoplast MTs disappear. Interphase MTs reappear at first along the central part of the new cell wall, in a region which was occupied before by the initial phragmoplast; their orientation is perpendicular to the phragmoplast MTs. Also here, distinct MT organizing centers could not be observed. Then the MT spread out over the cell periphery. The observations suggest that diffuse MT organizing zones rather than definite MT organizing centers play a role in the rearrangement of the different MT arrays during the cell cycle.