Observations on the ultrastructure of extrusomes in the hypotrichous ciliate Architricha indica (Protist, Ciliophora)

Extrusomes of different categories in ciliates have been suggested to have different structures and functions. To investigate the characteristics of extrusomes in the hypotrichous ciliate, we examined Architricha indica using scanning and transmission electron microscopy and electron microscopic enzymo-cytochemistry. The results showed that the mature extrusomes were ellipsoidal vesicles with structureless cavities on their anterior parts and arranged in rows beneath the pellicle; the extruded structures of extrusomes were cyathiform and can remain outside of the cell surface after extrusion. These findings differ distinctly from that of previously reported extrusomes in other ciliates and thus suggest a new type of extrusomes. These organelles originated from endoplasmic reticulum in the deeper part of the cytoplasm, underwent a serious of development, gradually migrated to the cortical cytoplasm and finally positioned in their functional areas. In contrast with lower forms of ciliates, density, arrangement, and localization of extrusomes in A. indica showed obvious differences between the ventral and dorsal surface of cells. This may be in some way related to the differentiation of the dorsoventral cortex, and functional ciliature happened to the hypotrichous ciliates. In addition, the extrusomes may perform protective or defensive function, which associates with the activity of mitochondria and contributes to the pellicle retrieval or renewal.