The biogenesis, assembly, and degradation of ejectisomes of Pyramimonas grossii were investigated by conventional transmission electron microscopy. Premature ejectisomes were mainly found beneath the cell envelope, often in close proximity to the nucleus, and as vesicles with diameters of 100 to 400 nm. Ejectisomes in the early stages of development contained only a few (2-4) turns of the ejectisome tapes. In the course of the ejectisome development, the number of turns and the widths of the coiled tapes increased. It is likely that vesicles, which were up to 650 nm in diameter, with granule- and plate-like structures inside, delivered additional preassembled ejectisome polypeptides to these premature stages. Both types of vesicles, those containing early stages of ejectisomes and those delivering additional ejectisome material, are believed to originate directly from the endoplasmic reticulum. Mature ejectisomes were mainly registered at the apical periphery of the cells. Up to 11 ejectisomes were found within a single cell. Ejectisomes that were most likely being in the process of degradation were registered within the cytoplasm and within vesicles, often together with material which resembled body scales. Mature ejectisomes which were still furled or which were arrested in the process of discharge were also found outside the cells in the medium.
Keywords: Ejectisome; Endoplasmic reticulum; Golgi; Organelle biogenesis; Prasinophyte, protein assembly; Pyramimonas grossii, transmission electron microscopy.