Ultrastructure of the concentric membrane system in asci of Arthroderma vanbreuseghemii.

An ultrastructural study was performed on the ascosporogenesis in Arthroderma vanbreuseghemii. Young gymnothecia of the fungus were treated with four different schedules: (1) glutaraldehyde and osmium tetroxide fixation, (2) potassium permanganate fixation, (3) tannic acid, with or without glutaraldehyde, and osmium tetroxide fixation, (4) periodic acid-alkaline bismuth (PABi) staining after glutaraldehyde and osmium tetroxide fixation. The glutaraldehyde- and osmium tetroxide-fixed asci had a concentric membrane system (CMS), concomitantly with the double membrane system (DMS) that eventually delimited ascospores. The CMS consisted of multiple membranes of a myelinoid appearance, and continued to the ascus plasma membrane and also to the DMS. In contrast, potassium permanganate fixation and tannic acid and osmium tetroxide fixation without glutaraldehyde failed to reveal such a structure as the CMS within the asci. PABi-positive materials were found on the marginal glycogen granules, the CMS, the DMS, and the ascus plasma membrane. These observations suggest that the CMS is lipid-poor and not composed of true membranes. It is not the direct source of the DMS, but merely participates in the production of cell wall materials during ascosporogenesis with the marginal glycogen granules as a substrate.