ACTIN MICROFILAMENTS ARE ASSOCIATED WITH THE MIGRATING NUCLEUS AND THE CELL CORTEX IN THE GREEN-ALGA MICRASTERIAS - STUDIES ON LIVING CELLS

Rhodamine-phalloidin or FITC-phalloidin has been injected in small amo unts into living, developing cells of Micrasterias denticulata and the stained microfilaments visualized by confocal laser scanning microsco py. The results reveal that two different actin filament systems are p resent in a growing cell: a cortical actin network that covers the inn er surface of the cell and is extended far into the tips of the lobes in both the growing and the nongrowing semicell; it is also associated with the surface of the chloroplast. The second actin system ensheath es the nucleus at the isthmus-facing side during nuclear migration. It s arrangement corresponds to that of the microtubule system that has b een described in earlier electron microscopic investigations. The spat ial correspondence between the distribution of actin filaments and mic rotubules suggests a cooperation between both cytoskeleton elements in generating the motive force for nuclear migration. The function of th e cortical actin network is not yet clear. It may be involved in proce sses like transport and fusion of secretory vesicles and may also func tion in shaping and anchoring the chloroplast.