Dynamic organization of Physarum plasmodium.

Birefringent fibrils (BRFs) with a positive sign composed of bundles of F-actin were found throughout the Physarum plasmodium with the mode of existence differing regionally. In the zone behind the leading edge of an advancing plasmodium, where cytoplasmic sol and gel were still not well differentiated, more BRFs came to the foreground when the endoplasm flowed backward (emptying phase), and a substantial portion disappeared when the endoplasm flowed forward (filling phase), except for nodes, from which BRFs were reorganized in the early emptying phase of each cycle. BRFs found in the wall of the streaming channel in the posterior network and the branched vein section ran in parallel to or helically around the channel. They were much more stable and maintained strong birefringence irrespective of the direction of the cytoplasmic flow. When the fan-like expanse ceased moving forward, the BRFs no longer appeared and disappeared cyclically but persisted in the area which had previously been the front. We concluded that the site of the active contraction-relaxation rhythm in an advancing plasmodium with antero-posterior polarity is restricted to its frontal zone and that the rest of the plasmodium is in a state of "tonus" which continuously imparts a certain level of hydrostatic pressure to the interior. The meaning of the tonus and the mechanics of tensile force production in the plasmodium are discussed in terms of a working hypothesis arrived at from the phase relationship between isometric and isotonic contraction waves.