Force on individual mammalian k-fibers suppresses depolymerization at both ends, leading to net k-fiber lengthening, related to Fig. 3 . Additional example time lapse of photomark (PA-GFP-tubulin, white) movement during microneedle (Alexa 555, yellow) manipulation of a k-fiber (SiR-tubulin, magenta). The distance between the photomark and the pole remains constant (orange line) while the distance between the photomark and plus end increases (red line). Scale bar, 4 µm.