Trichites of Strombidium (Ciliophora, Oligotrichida) are extrusomes

The trichites of Strombidium and related genera have been considered either as a cytoskeletal armature or as extrusomes. To demonstrate their true nat ure, a study was undertaken on two marine Strombidium species by ultrastruc tural and cytochemical analysis as well as in vivo experiments. Trichites, extending from the cortex into the cell, are rod-shaped, membrane-bounded, and have a complex structure. The following elements of the trichites, are distinguishable: an electron-transparent lumen, a laminated layer, and a co mpact layer. In trichites of one species, thin "rings" surround the lumen. Numerous short, curved tubules with a polysaccharide wall are present in th e cytoplasm surrounding the trichites. At the conical end, each trichite is enveloped by a ''cap'' of electron-dense proteinaceous material. In some c ases, the cortical alveoli appear interrupted, forming a "hole" for trichit e ejection. Ejection of rod-shaped structures, up to 5 times longer than re sting trichites, was obtained by in vivo treatments with dextran and aminoe thyldextran. Negative staining indicated that these structures were transfo rmed trichites. As no other possible extrusive structures were observed in the cytoplasm of Strombidium, trichites were considered extrusomes.