Resorption of microtubules and nucleus during the encystation process in Echinosphaerium nucleofilum

The cyst formation process was investigated in a large heliozoan Echinosphaerium nucleofilum, with particular reference to the resorption of axonemal microtubules and nuclei into the cytoplasm. As the encystation proceeded, the axonemal microtubules were found to disassemble from their distal ends by being surrounded or invaded with many tubular vesicles, followed by detaching at their proximal ends from the nuclear envelope and by disassembling completely at the final stage. On the other hand, several nuclei were found to be enclosed by large vacuoles before they were destroyed or digested and finally absorbed into the cytoplasm. At this stage, one or two large multi-layered bodies were formed inside the nucleus. The meanings of the resorption of microtubules and nuclei were considered and discussed.