Observations on Organization of Z-disk Components and on Rod-bodies of Z-disk Origin

Current concepts of Z-disk fine structure are based on studies by Knappeis and Carlsen (1), FranziniArmstrong and Porter (2), Reedy (3), and Kelly (4) . Knappeis and Carlsen found that I filaments terminated adjacent to the disk and, in cross-section, were positioned in corners of I squares with 220 f 35 A sides . Each I filament from one side of the disk faced the geometric center of the space between four I filaments from the opposite side and was joined to these four filaments by four Z filaments. Within the disk, Z filaments formed the sides of tetragons (Z squares) rotated 45 ° to, and measuring half the diagonal length of, I squares . Reedy later noted that Z filaments were not straight but were curved, imparting a woven pattern to the disk . This is the classic appearance of the transversely sectioned Z disk, forming what can be called a Z-1 pattern . Published pictures demonstrating this pattern have been of osmium tetroxide-fixed material from a variety of animal muscles. In normal skeletal muscle, we consistently encountered a different Z-disk image-a Z-2 pattern . Because our specimens were examined after aldehyde-osmium tetroxide fixation, it seemed possible that the difference resulted from different chemical properties of the fixatives . That the Z disk fixed in glutaraldehyde (G) plus osmium tetroxide (O) differs in morphology from that fixed in O alone has been reported recently also by Fardeau (5) and by Landon (6) . We are reporting results of a comparative study of Z-disk morphology in frog, rat, and human muscle after fixation in O, in aldehyde + O, and in aldehyde alone, and after low-ionic-strength extraction (LISE) prior to fixation. Parallel studies on a proliferative anomaly of the Z disk-rodlike structures arising from the organelle (7)-also are presented . Finally, possible ways in which divergent Z-disk images could arise are considered .