SEM observation of non-fixed and water freeze-dried Spirostomum ambiguum

We aimed to freeze-dry the ciliate Spirostomum ambiguum, obtained from water, without fixation and observe it using scanning electron microscopy (SEM). Living cells were placed on a specimen stub and frozen upon contact with a Cu block kept at either -80 °C or -100 °C. Samples were then freeze-dried and observed by SEM. In most cases, no damage to the specimen due to ice crystal formation was observed. Because of the instantaneous freezing, the metachronal wave of cilia on the body surface of the ciliate was well preserved. Approximately 70-80% of cells were preserved in the contracted state due to inevitable exposure to mechanical vibration immediately before freezing. The remaining samples were preserved in a fully-extended state. Morphometric measurements of the cell surface showed that in the extended state, ciliary rows were almost parallel to the long axis of the cell, whereas in the contracted state, they were twisted in a left-handed helix at an angle of 45-65°. The distance between adjacent cilia along a ciliary row was 1.88 ± 0.43 μm in the extended state and 1.32 ± 0.41 μm in the contracted state (mean ± SD). However, the spacing between adjacent ciliary rows remained unchanged.