SFs contact the pcMTs and the cell cortex. (A) i: Schematic of T. thermophila cortical array (cross-sectional interface) illustrating BBs (red) and their associated appendage structures, including SFs (green), pcMTs (cyan) and transverse microtubules (tMT; black). (ii) Schematic depicting the SF–cell cortex longitudinal interface of a single T. thermophila cortical unit. Boxes mark the interfaces of interest. (B) EM tomographic images of the SF–pcMT interface between a pair of BBs at steady-state (25°C). Electron-densities (red arrowheads) link SFs to pcMT bundles (EM tomographic slice: i and iv; model: ii and iii). (C) i: Fluorescence images illustrating the SF position relative to the cell cortex (epiplasm; EpiC-mCherry [EPI]) at steady-state (25°C) and elevated ciliary force-induced state (37°C). (ii and iii) Averaged fluorescence images show that SFs extend toward (y axis) and along (x axis) the epiplasm at both steady-state and elevated force-induced state. BB, magenta; SF, green; EPI, red. Dashed line indicates the peak fluorescence intensities of EpiCp-mCherry. (iv) Quantification of SF position relative to the epiplasm. Black dashed line marks the positions where the SF distal ends start to extend along the epiplasm. Steady-state, n = 104 SFs (53 cells); elevated force-induced state, n = 94 SFs (53 cells). Mann-Whitney test. Mean ± SD. (D) EM tomographic images of the longitudinal SF–epiplasm interface. Linkages were not detected between SFs and the epiplasm (white arrowheads; EM tomographic slice: ii and iv; model: i and iii). (E) EM tomographic images of the cross sections of the SF–epiplasm interface at the ciliary pocket at steady-state (25°C) and elevated ciliary force-induced state (38°C). Electron-dense linkages (red arrowheads) were observed between SFs and the epiplasm only at the elevated force-induced state (EM tomographic slice: i and iv; model: ii and iii). B, D, E: BB, red; SF, green; pcMTs, cyan; epiplasm, white; linkages, magenta; axoneme (AXO), dark blue. Scale bars, 200 nm.