Transformation of trans-Golgi Network During the Cell Cycle in a Green Alga, Botryococcus braunii

trans

-Golgi network (TGN), and the changes in its structure and behavior throughout the cell cycle of a unicellular green alga, Botryococcus braunii, were examined with deep-etching replicas and in cryo-fixed/freeze-substituted specimens. In interphase cells, the TGN consisted of a hemispherically shaped cisterna (TGN-cisterna) with regularly distributed pores on the surface and a tubular network (TGN-tubules) with clathrin-coated vesicles. The TGNs changed their structure drastically throughout the cell cycle. The TGN-cisterna disappeared from the beginning of nuclear division to the completion of the cell wall, in contrast that TGN-tubules with the clathrin-coated vesicles were always observed. The TGN-tubules produced at least five other kinds of vesicles depending on the stage of the cell cycle: 200-nm vesicles with fibrillar substances and multivesicular bodies in interphase, 180–240 nm vesicles during cell division, and 400–450 nm vesicles containing fibrils and small masses of electron-dense substances, and 200-nm vesicles containing electron-dense spherical substances just after cell division. During cell wall formation, TGN-tubules were small and had only a few clathrin-coated vesicles. After cell wall formation, TGN-tubules grew and a TGN-cisterna with pores appeared again.