Scale formation in algae.

Scale biogenesis in algae represents a unique model system to study the transport of secretory macromolecules through the Golgi apparatus (GA) and their exocytosis. The larger scales can be visualized in the light microscope, and thus the kinetics of scale assembly, transport, and secretion can be studied in vivo. In addition, scales are osmiophilic and readily visible in conventional transmission electron microscopy; thus, details of scale assembly and sorting can be studied without invoking immunolabeling techniques. The following are distinctive features of scale biogenesis in algae: 1) transport of scales through the GA-stack occurs by cisternal progression; 2) scale secretion may be very rapid (in some cases a single GA-cisterna leaves the stack every 15-20 s); 3) sorting of different scale types does not occur in the GA, but in a post-GA-compartment. Recent progress in the analysis of scale formation in the green flagellates Tetraselmis and Scherffelia is reviewed.