Three-Dimensional Ultrastructural Characterization of Fibroblastic/Stromal Cell Processes in Mouse Ureteral Lamina Propria by FE-SEM Array Tomography.

3D-reconstruction analysis by array tomography using FE-SEM was applied to fibroblasts/stromal cells localized close to the epithelial base in the lamina propria of the ureter of normal adult mice. The cell processes of 0.3 μm and smaller in size, which extended away from their somata and appeared as extremely thin threads under phase-contrast light microscopy, were revealed in ultrastructural 3D to largely consist of filmy sheet-like structures. However, their tip portions measuring 0.1 μm and smaller in size were actually thread-formed and directly contacted with the epithelial basal lamina. In the present study, such thin processes taking filmy sheet forms were tentatively termed "secondary processes," distinct from processes protruding from their somata, which were termed "primary processes." The actual end portions were termed "tertiary processes". The functional significance of these sheet-like cell processes is discussed. Possible reasons why such sheet-formed features of processes of fibroblastic/subepithelial stromal cells have been previously overlooked are also described.