ABSTRACT
The second type of ER aggregates is even more unique; these aggregates penetrate the entire cytoplasm of the developing microspores of Liriodendron chinensis (Gabarayeva, 1990, 1991a). They consist of stacks of long tubular cistemae arranged parallel to one another. The number of cistemae in aggregates is normally odd, ranging from 3 to 13. The central cistern invariably contains an electron-dense, osmiophilic substance; in polycistemal aggregates, electron-dense cistemae usually alternate with electron-transparent ones, which is why they have been termed zebra aggregates (Gabarayeva, 1991a). These large aggregates form undulations in the cytoplasm, branch or form rings. Association of the ends of osmiophilic cistemae with lipid globules invariably present nearby in the cytoplasm is clearly discernible. The outer electron-transparent cistemae of the aggregates carry ribosomes, with the ends of the peripherally located aggregates lying immediately adjacent to the plasmalemma. Given the stages of Liriodendron chinensis microspore development at which zebra aggregates occur, the contact of the aggregate cistemae with the plasmalemma and the particularly intense accumulation of sporopollenin in the exine occurring at the time, it would appear reasonable that these structures should be involved in synthesis of sporopollenin precursors. Assumption of the lipoprotein nature of the substances synthesized, as a step in the biosynthetic pathway of this biopolymer, has been substantiated by an electron cytochemical enzymatic reaction with pronase (Gabarayeva, 1990). Structures somewhat similar to those described above (twin ER cistemae with osmiophilic material in-between and ribosomes on the outer surfaces) were detected by Fleurat-Lessard (1986) in parenchymal cells of Mimosa pudica. He believes them to be involved in synthesis of proteins, presumably contractile ones. ER aggregate cistemae have been found in cancer cells (Schaff et al., 1972; cited in Fleurat-Lessard, 1986). Structures like these have also been reported in encysting zoospores of the fungus Pythium aphanidermatum (Grove and Bracker, 1978); however, their function is not discussed by the authors. The occurrence of this type of ER aggregate in organisms as dissimilar as higher plants, fungi and animal cancer cells is noteworthy. Such ER aggregates are likely to be of general cytological significance; they arise where and when there is a need for intense synthesis of particular compound proteids, the synthesis requiring spatial separation of the initial components (proteins and prosthetic groups).
