Ultrastructure of paired coniform 9 + 0 sensory cilia: a new type in the organ of Bellonci of the marine amphipod Gammarus setosus

Summary The structure of modified 9 + 0 cilia in the organ of Bellonci was studied in Gammarus setosus from late embryonic development to adult after routine fixation, fixation with lanthanum treatment, and prefixation with ethylene diamine tetraacetic acid and sodium dodecyl sulphate. The cilia are distinct from known sensory cilia in that they occur in pairs and lack centrioles. The basal bodies are at right angles to each other. The basal body cylinders consist of dense microtubule doublets and have 3 regions: the basal cartwheel, the middle pinwheel and the distal transitional. The pinwheel, which has 9 fins of dense material attached to the doublets, is differentiated into a spiral attachment of the ciliary roots whose periodicity is 70 nm. The scanning electron microscope shows the roots as beaded, tapering ribbons. The coniform outer segments give rise to tubules, each with 1 or 2 single or double microtubules in its core. The tubules are in contact with extracellular chains of calcium granules inside the organ. A bend in the axoneme brings the paired outer segments together. Lamellar bodies develop from the ciliary tubules in embryos and juveniles, but not in adults, except after exposure to lanthanum.