Tetramer Identification and Classification . (A i) A single image plane extracted from the dual-tilt tomogram of a mouse (S2814A) left ventricular myocyte fixed in situ. Different orthogonal planes from within the volume of the tomogram are indicated by different colors; XY in red, YZ in green, and XZ in blue. The intersection point of all three planes lies within a single ryanodine receptor and the XZ plane (blue line) was positioned to parallel, as nearly as possible, the jSR (single arrowhead) and t-tubule (double arrowhead) membranes, but to be within the cleft and to bisect the ryanodine receptors. Scale bar is 100 nm. (A ii) An en-face view (XZ plane) of the indicated section (yellow arrows, A i) of this junction. (A iii) The position of the tetramers has been identified by red boxes 27 × 27 nm. (A iv) Tetramers were color-coded to indicate their orientation relative to their nearest-neighbor; checkerboard—aqua, side by side—orange, isolated—green. (B) Examples of the different tetramer–tetramer contacts.