Studies on scale morphogenesis in the Golgi apparatus of Pyramimonas tetrarhynchus (Prasinophyceae)

Ultrastructural investigations of scale formation in the cisternae of the Golgi apparatus have been carried out on the prasinophycean flagellate Pyramimonas tetrarhynchus, whose cell surfaces are covered with 6 different scale types, 3 on the flagella and 3 on the cell body. Our results suggest that at least 4 and probably all 6 scale types can be formed together within the same cisterna and that there is some degree of intracisternal differentiation, since the formation of 2 scale types (the small underlayer scales on cell body and flagella) is restricted to the cisternal peripheries, whereas the remaining scale types are formed in the more central portions. Detailed studies of morphogenesis of the larger body scales reveal the earliest identifiable stages as 8-armed figures, with 8 thin arms in the intermediate body scales (IBS), and 4 thick and 4 thin arms in the outer body scales (OBS). From these incipients structures that bear little resemblance to the finished products, the complex, 3-dimensional mature body scales are elaborated in each of the cell's 4 distyosomes, and maintain throughout their different developmental sequences a close relationship to the inner surfaces of the cisternal membranes, as well as a particular orientation within the dictyosomes. Preliminary calculations of total numbers of scales that cover cell and flagellar surfaces are included. The small, underlayer scales, which on the flagella are shown to be arranged in 24 rows, number about 350000; larger scales of more complex construction number about 20 000.