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Fig. 2. The probasal body. (A-C) Consecutive transverse sections through a pbb; the two bbs are sectioned at an oblique angle. The microtubular roots (A-C, d for d-roots, s for s-roots) and the distal connecting fiber (B, dcf) are visible. At the distal end of the pbb (A, open arrow), the acorn and V-shaped filament system, which are also present at the distal end of the bb, can be seen (compare to bbs shown D and E; see also Fig. 1G). More proximally (B, pbb triplets are numbered), the microtubular triplets and the hub and spoke complex of the cartwheel are visible. The B- and C-tubules are filled with electron-dense material. An electron-dense patch is overlying the C-tubule of triplet 9 and filaments of the lateral striated fiber link at least triplets 7, 8 and 9 to the corresponding d-root (B, small arrowhead; see also Fig. 6E,F). Triplets 1 and 2 lie close to the d-root and corresponding striated microtubule-associated fiber (SMAF), and are connected to the SMAF by fine filaments (B, double arrowhead; see also Fig. 6H). At the most proximal end of the pbb, only the hub and spoke complex of the cartwheel, protruding from the microtubular cylinder, is visible (C, compare with longitudinal sectioned pbb in F, open arrows). The lateral striated fiber is labeled (C, large arrowhead). (D,E) Transverse sections through the most distal end of bbs showing the acorn and V-shaped filament system (triplet numbers are indicated, compare with Fig. 1G). The transitional fibers are marked (large arrows). In E, the two filaments that extend from triplets 4 and 5, and fuse in the center of the bb, continue and make contact with doublet 9 (small arrowhead). (F) Longitudinal section through a bb and pbb (open arrow). The cartwheel of the pbb consists of six tiers, of which only three are bound by the short microtubular triplets. Also, in longitudinal section, the filamentous system present at the distal end of the pbb clearly resembles that present within the lumen at the distal end of the bb. This becomes clear from a comparison of the pbb shown in F with the bb shown in G. The acorn is seen as two asymmetrically positioned electron-dense dots in the bb (Fig. 2G, small arrowheads) and pbb (Fig. 2F, small arrowheads). The filaments extending from triplets 4 and 5 at the level of the acorn (small arrow at the left side of the pbb in F and bb in G) bend downwards with an angle of about 35° (ref erring to the transverse axis of the pbb/bb) after reaching the lumenal side of the acorn and continue to the other side of the pbb/bb. In the pbb, these filaments make contact with triplets and the cartwheel (F, small arrow at the right side of the pbb). In the bb, these filaments make contact with triplets at the level of the dcf attachment with the bb (G, small arrow at the right side of the bb). (H) Section through a basal apparatus; one bb is sectioned longitudinally, the other one transversely. The proximal connecting fibers interconnecting the bbs are visible (pcf). The transversely sectioned pbb (open arrow) is attached by the lateral striated fiber (see also C, large arrowheads) to the corresponding d-root (d). The lateral striated fiber also interconnects the pbb with parts of the sf2 and probably the pcf. The limits of the sf2 to the pcf are outlined by white squares. Scale bar, 250 nm.
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