POLAR ORGANIZERS MARK DIVISION AXIS PRIOR TO PREPROPHASE BAND FORMATION IN MITOSIS OF THE HEPATIC REBOULIA-HEMISPHAERICA (BRYOPHYTA)

Changes in the pattern of microtubules during the cell cycle of the hepatic Reboulia hemisphaerica (Bryophyta) were studied by indirect immunofluorescence using conventional and confocal laser scanning microscopy (CLSM). The first indication that a cell is preparing for division is fusiform shaping of the nucleus accompanied by the appearance of well-defined polar organizers (POs) at the future spindle poles. Microtubules emanating from the POs ensheath the nucleus and eventually develop into the half-spindles of mitosis. Some of the microtubules from each PO pass tangential to the nucleus and interact in the region of the future mitotic equator. A preprophase band (PPB) forms in this region later in prophase and coexists with the prophase spindle. Thus, the plane of division appears to be determined by interaction of opposing arrays of microtubules emanating from POs. Prometaphase is marked by disappearance of the POs, loss of astral microtubules, and conversion of the fusiform spindle of prophase to a truncated, barrel-shaped spindle more typical of higher plants. Restoration of cortical microtubules in daughter cell occurs on the cell side distal to the new cell plate, but nucleation of microtubules is associated with the nuclear envelope and not with organized POs. At the next division POs appear at opposite poles of preprophase nuclei with no evidence of division and migration that is characteristic of cells with centriolar centrosomes. These data lend additional support for the view that mitosis in hepatics is transitional between green algae and higher plants. © 1990 Springer-Verlag.