Architecture of the Golgi apparatus of a scale-forming alga: biogenesis and transport of scales

Mechanisms of transport of secretory products across the Goigi apparatus (G A) as well as of scale formation in prymnesiophytes have remained controver sial. We have used a quantitative morphological approach to study formation and transport of scales across the GA in haploid cells of Pleurochrysis sp . The GA of these cells differs from the GA of higher plants in at least si x morphological characteristics. Our results show that scales form in the t rans-Golgi network (TGN) and transit the TGN in heretofore unrecognized pro secretory vesicles. Prosecretory vesicles differentiate into secretory vesi cles prior to exocytosis of scales to the cell surface. Because prosecretor y vesicles are only fragments of TGN cisternae, the classical model of cist ernal progression is not a valid mechanism of transport in this alga. TGN t ransport vesicles are also involved in scale formation; however. the role o f tubular connections between cisternae of a single stack-TGN unit is not c lear. The relationship of two morphological types of cisternal dilations to a membrane-associated bottlebrush-shaped macromolecule of novel morphology suggests a new hypothesis for the biogenesis of scales.