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Fig. 1. The axoneme, transitional region and basal body. (A-J) Series of transverse sections (section numbers 1, 10, 13-20) showing the axoneme (A,B), transitional region with the distal stellate structure (C,D), transitional plate (E) and proximal stellate structure (F), and distal part of the bb (G-J). Within the axoneme, doublet 1 (A,B, arrowhead) can be identified by a bridge extending from its A-tubule to the B-tubule of doublet 2. This doublet (doublet/triplet 1 is marked with an arrowhead in C-J) is continuous with the middle of the three triplets to which the distal connecting fiber (dcf; J) is attached (J, arrowhead). In the transitional region, short, T-shaped fibers (the doublet outer projections; C,D,F,K, small arrows) are attached to the A/B-tubule junction on the outside of the doublets. In the area of the proximal stellate structure, the doublet outer projections are sometimes interconnected, forming a ring (F, small arrows). In the region of the transitional plate, the lumen of the transitional region is filled with electron-dense material that extends between the microtubular doublets into the doublet outer projections, which, at this level, form saw-tooth-like structures projecting from the microtubular cylinder (E). The transitional fibers (G-I,K, large arrows) are attached to the distal half of the bb. Within the microtubular cylinder – exactly at the transition between the microtubular doublets of the transitional region and the microtubular triplets of the bb – filaments are attached in a rotationally asymmetric pattern to the microtubular doublets/triplets (G) (Fig. 2D,E). A filament outlining the shape of an acorn is attached to the A-tubules of doublets 7, 8, 9, 1 and 2 (G, doublets 7, 9 and 2 are numbered). The lumenal side of the acorn is crossed by a V-shaped filament system; two filaments extend from triplets 4 and 5, and, in the center of the bb lumen, connect to another filament that runs perpendicular to the two filaments, extending down from the proximal end of the stellate structure in the transitional region (F,G,K, small arrowheads). The acorn and V-shaped filament system are already present at the distal end of the pbbs (Fig. 2A). (K) Median longitudinal section through the axoneme and bb showing the transitional region (vertical line labeled tr) and doublet outer projections (small arrows), the transitional fibers (large arrows), and the cartwheel (vertical line labeled cw). Sectional planes corresponding to transverse sections B-J are marked by dotted lines. Two rows of projections can be seen attached to the triplets and extending into the lumen of the bb (between two pairs of dashes). A fine filament (small arrowhead) interconnects the proximal end of the proximal stellate structure with the system of filaments present in the distal lumen of the bb (compare with F and G, showing this filament in transverse section, small arrowheads). Scale bar, 250 nm.
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