ID structure and FEM representation. (A) Scanning electron microscopy (SEM) image of the ID from monkey ventricular myocyte. S and R indicate “steps” and “risers,” denoting the plicate and interplicate regions, respectively. Reproduced from Zhang et al. (1996) with permission. (B) TEM image of mouse ventricular myocyte ID, illustrating the plicate and the interplicate regions. (C) View of the 3-D FEM model, with plicate and interplicate regions and GJs highlighted. (D) Slice through the FEM mesh, illustrating a corresponding view to the 2-D TEM image. The sections in the gray boxes are enlarged in E. (E) Magnified view of TEM images and FEM mesh slices, showing structural details for the plicate folds (top) and the GJs (bottom).