Ultrastructure and membrane topography of special ciliary organelles in the ciliate Eufolliculina uhligi (Protozoa)

Summary In Eufolliculina uhligi and other folliculinid ciliates, a territory has been identified that differs ultrastructurally from other areas of the cell, and that is especially sensitive to mechanical stimuli. This territory is located around the anterior oral apparatus of the loricate trophont and posterior to the membranellar spiral of the swarmer. Each cilium in this territory is closely apposed to a small membrane-covered pin that is supported by transverse microtubules of the cilium. In front of the pin, the base of the cilium bulges out; the ciliary membrane is interconnected with the axoneme by filamentous material. Freeze-fractured cilia show a large rectangular particle array at the site of the basal swelling. Only scattered particles have been observed in the pin membrane. It is suggested that the cilium and the pin act as a unit, which has therefore been named the ciliumpin-complex. Comparison with ciliary organelles of unicellular and multicellular organisms indicates that, because of their polar organization, the complexes are involved in the transduction of oriented, presumably mechanical, stimuli.