Electron Tomography of Fusiform Vesicles Figure 1. Comparison of chemically and HPF fixed umbrella cells of the urothelium. mFVs are the most prevailing compartments in chemically (A, C) and in HPF fixed umbrella cells (B, D). Regarding the ultrastructure, mFVs are more dilated in chemically fixed (A, C) than in HPF fixed samples (B, D). Regarding immunolabelling with anti-AUM antibody, pattern and density of labelling is comparable on chemically (C) and HPF fixed samples (D). The organization of mFVs into stacks is better preserved in HPF fixed samples (red over-colour in B and D). Bars: 500 nm. doi:10.1371/journal.pone.0032935.g001 surrounded by small round vesicles (Fig. 5A). The latter were either uroplakin-negative or uroplakin-positive. This observation pointed to the dynamics of membrane traffic between FVs and other cellular compartments during possible maturation process of FVs, which was recently described [18]. In the intermediate cells, typical mFVs were not found. Instead, uroplakin positive vesicles were smaller (up to 400 nm in length), and not as flat as mFVs of superficial umbrella cells (Fig. 5B). They contained smaller urothelial plaques compared to plaques of mFVs. These iFVs were concentrated beneath the plasma Figure 2. Structure of a mFV. (A) mFVs are flattened vesicles with two opposing plaques of thickened asymmetric unit membrane (AUM, blue) and slightly dilated, omega shaped hinge region of un-thickened membrane (yellow). (B) AUM and hinge regions can be seen also in the apical plasma membrane. (C) By freeze-fracturing, uroplakin particles are seen concentrated in the centre of the mFV (blue), while thin hinge region contains particle-free membranes (yellow). (D–G) Three-dimensional model of mFV shows that it has the shape of a flattened disk. In D, a slice from a tomogram is shown. In E–G, a 3D model of a mFV in different projections is shown (green). Bars: 100 nm in A, C; 50 nm in B. doi:10.1371/journal.pone.0032935.g002 PLoS ONE | www.plosone.org 3 March 2012 | Volume 7 | Issue 3 | e32935