Ultrastructure and calcium-dependent contraction of the myonemal network in a heterotrich ciliate, Blepharisma japonicum

Summary This laboratory has previously demonstrated that shortening of the cell body of a heterotrich ciliate, Blepharisma japonicum, could be induced as a step-down photophobic response. Here, we examined the structure and contractility of the myonemes in detergent-extracted cell models and in isolated cortical fragments. Ultrastructural observation showed that the myoneme was connected to the basal ends of the posterior kinetosomes and constructed a systematic network as a whole. Shortening of the cell model was induced by > 10−4 M Ca2+, while the rounded cell model did not re-elongate even when it was washed in a calcium-free solution either with or without addition of ATP. Fluffy fibrils, which were tentatively identified as aggregated bundles of the myonemes, were isolated with the kinetosomal complex and showed calcium-dependent and ATP-independent contraction. The minimum concentration of Ca2+ required for inducing contraction was at the level of 10−6 M. These results suggest that the cell body shortening in Blepharisma is caused by the Ca2+-dependent contraction of the myonemal network.