Ultrastructure of the proximal region of somatic cilia in Paramecium tetraurelia.

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Abstract
The morphology of the transition zone between the terminal plate of the basal body and the 9 + 2 region of the somatic (non-oral) cilium was examined in P. tetraurelia. Freeze-fracture and thin-section techniques disclosed both membrane specializations and various internal structural linkages. Freeze-fracture material revealed sets of particles interrupting the unit membrane. The more distal of these formed plaquelike arrays while the proximal set of particles formed the ciliary necklace. The plaque regions corresponded to anionic sites on the outer membrane surface as revealed by binding of polycationic ferritin. Both the plaque particles and the necklace particles appeared to be in contact with outer doublet microtubules via a complex of connecting structures. In the interior of the transition zone an axosomal plate supported an axosome surrounded by a ring of lightly packed material. Only 1 of the 2 central tubules of the axoneme reached and penetrated the axosome. Below the axosomal plate 4 rings, each about 20 nm wide, connected adjacent outer doublets. An intermediate plate lay proximal to these rings, and a terminal plate marked the proximal boundary of this zone. Nine transitional fibers extended from the region of the terminal plate to the plasmalemma. These observations were used to construct a 3-dimensional model of the transition region of wild-type Paramecium somatic cilia. This model will probably be useful in future studies concerning possible function of transition-zone specializations, since Paramecium may be examined in both normal and reversed ciliary beating modes, and since mutants incapable of reverse beating are available.