Structure of dictyosomes in Scenedesmus acuminatus observed by the freeze-substitution method.

The ultrastructure of dictyosomes in Scenedesmus acuminatus was studied by the freeze-substitution method. The cisternal membranes of dictyosomes clearly exhibited three-layered structures. Electron-dense particles were visible on or in the proximal cisternal membranes. The upper and lower membranes of the distal cisternae were in close contact with each other at the cisternal centre. The electron density of transfer vesicles, secretory vesicles and cisternal margins was very high. These findings differ considerably from those obtained by conventional chemical fixation, and suggest that freeze-substitution is superior to chemical fixation for preserving structures, without distortion and preventing dissolution of constituent substances.