Figure 4.

Luminal particles in different cell types revealed by CEMOVIS and cryo-ET. (A) Cryo-EM projection image of a vitrified section (CEMOVIS) of neuronal tissue, showing an area within a process containing microtubules with luminal material, seen in both microtubule top views (black arrows) and side views (black arrowhead). Some microtubules have little detectable luminal material (empty arrow). The insets show density profiles across the microtubules indicated with a white arrow (left), an empty white arrow (middle), and white arrowhead (right). The arrows in the plots indicate the position of the microtubule protofilaments; asterisks denote the central density (if detected), generated by the luminal material. (B) CEMOVIS of neuronal tissue, showing a region within the cell body, as indicated by the proximity of the nucleus (N), where microtubules containing luminal material are also observed (white arrow). Asterisk, nuclear envelope. The plot shows the density profile through the microtubule indicated with the arrow. (C) Microtubules from rat HTC cells. The plot shows a density profile through the transversely sectioned microtubule (indicated with an empty white arrow). The density of material inside the microtubule lumen is similar to that of the surrounding cytoplasm. The images in A–C were CTF corrected. (D) Tomographic slice, showing microtubules in an astroglial cell in dissociated hippocampal culture. (E) Tomographic slice, showing a region in a process of a P19 cell grown on an EM grid (left). The two microtubules indicated with arrowheads were surface rendered (right). A 3D view of this tomogram is shown in Video 2. (F and G) Tomographic slices of a microtubule from a PtK2 cell (F) or HeLa cell (G) cultured on EM grids, lacking regular dense particles in the lumen. (H) A tomographic slice of a microtubule nucleated in vitro, devoid of any luminal material. Video 2 is available at http://www.jcb.org/cgi/content/full/jcb.200606074/DC1. Bars, 50 nm.