VWF tubule morphogenesis and structure. (A-B) In vitro assembly of D1D2 and (D′D3)₂ fragments at pH 6.2 in Ca²⁺ into tubules (A) and helical 3-dimensional reconstruction (B) showing external view (right) and a cross-section through the hollow tubule (left) colored from red to blue based on distance from the helical axis.⁶  (C) Single tubule in a clathrin-coated immature WPB in the juxta-Golgi.¹¹⁰  (D-E) Schematic of tubule assembly.⁶  Helical assembly is shown as progressing from bottom to top. Each successive pro-VWF dimer is numbered and shown alternately with solid or striped diagonal fill. Interdimer disulfide crosslinks form at the twofold symmetry axis between D′D3 domains (red SS). VWF helices are 1-start, that is, contain a single VWF molecule. Because of twofold symmetry, the 2 ends of the helix are identical.⁶  (F) Helical reconstruction from cryoelectron tomography of tubules in endothelial cell WPB (see panels H-I).¹⁹  Panels B and F are aligned vertically to show similar structure of tubules formed in vitro and in vivo. (G) Another example of tubule biogenesis in the juxta-Golgi.¹¹⁰  (H-I) Cryoelectron tomograms of a mature WPB in an endothelial cell (H) and a reconstruction (I) showing individual tubules (colored).¹⁹  (J) A WPB with increased spacing between tubules and a stalk (arrowhead).¹⁹  The WPB may have been captured during secretion and the stalk may be a secretion pore. (K-L) VWF tubules in porcine platelet α-granules, in EM sections that run parallel (K) or normal (L) to the tubule axis.²¹  In contrast to WPB, α-granules contain other components that segregate away from the paracrystalline VWF tubules (T, marked with arrows). (M) A cultured human umbilical vein cell immunofluorescently stained with anti-VWF to visualize WPB. Reprinted from Tom Carter, National Institute of Medical Research, United Kingdom, with permission.