GOLGI-APPARATUS ACTIVITY AND MEMBRANE FLOW DURING SCALE BIOGENESIS IN THE GREEN FLAGELLATE SCHERFFELIA-DUBIA (PRASINOPHYCEAE) .2. CELL-WALL SECRETION AND ASSEMBLY

Secretion of the cell wall (theca) in the scaly green flagellate Scherffelia dubia (Prasinophyceae) has been examined by electron microscopy during cytokinesis. The bi-laminate wall forms by the extracellular amalgamation of two layers of scales produced in the Golgi apparatus (GA). Each mature GA cisterna contains ca. 12,000 scales of two distinct varieties arranged in two layers on the cisternal membrane. GA cisternae undergo turnover and one scale containing cisterna matures from the trans-face of each dictyosome every 3-4 minutes. Cisterane then fuse with the plasma membrane at the anterior end of the cell releasing the scales onto the cell surface. The two layers of wall scales integrate on the cell surface in a time-dependent self-assembly process. The first scales deposited commencce assembly at the cell posterior and the wall develops anteriorly by edge growth. The daughter cell wall is composed of ca. 1.2 million scales deposited in about 3 hours. Calculations of net membrane flow strongly indicate extensive endocytosis during wall deposition.