Syneresis in Ameboid Movement: Its Localization by Interference Microscopy and Its Significance

The periodic appearance of a hyaline cap at the front of advancing pseudopodia is an almost universal accompaniment of movement in ameboid cells. The fluid of the hyaline cap cannot be a simple filtrate of the ground substance pressed through the "plasmagel sheet" as Mast (11, 12) believed, because the dry mass of this fluid is less than half that of the ground substance of the rest of the cell (5). Neither can this fluid be derived from the environment, for the turnover rate of water is orders of magnitude too slow (10). The low dry mass of the hyaline cap fluid, therefore, indicates that it is produced by syneresis accompanying the contraction of a gelated region somewhere within the cell. The site of the contraction which serves as the motive force for ameboid movement has never been clearly established. The ectoplasmic contraction theory (or tail contraction theory) (7, 8, 11, 14) would lead one to predict that the origin of the hyaline cap fluid would be the contracting tail ectoplasm (7). This possibility seemed to receive strong support from the gradient of optical path difference in flattened amebae demonstrated by Allen and Roslansky (5). These data, in fact, suggested that contraction of the tail ectoplasm released a " t ide" of syncretic fluid which was pressed forward, its anterior margin visible as the hyaline cap. However, Allen and Roslansky pointed out several aspects of ameboid movement which were not in good agreement with the theory. Recently, one of us (2, 3) has proposed a new theory which is compatible with the various behavioral aspects of ameboid movement and with the limited amount of experimental data available. The new theory postulates a contraction at the front of each advancing pseudopod as the motive force for movement. This front contraction theory rests on the same two basic assumptions as the tail contraction theory: first, that the cytoplasm of ameboid cells can exist in at least two states, contracted and extended, and, second, that the driving contraction is propagated along the cytoplasm, as it is in muscle. According to the new theory, the contraction occurs in the endoplasm as the latter splits and becomes everted to form the ectoplasmic tube. The region just posterior to the hyaline cap where this is postulated to occur has been called the "fountain zone" (2, 3). The front contraction theory (or fountain zone contraction theory) demands that the endoplasm: (a) possess "structure capable of developing and transmitting tension," (b) be capable of conducting a wave of contraction, (c) actually shorten and thicken in the fountain zone in the process of forming new ectoplasmic tube, and (d) give rise to the hyaline cap fluid by syneresis localized in the fountain zone. The fi1~t three demands of the front contraction theory have been demonstrated to be fulfilled. Rheological experiments have proved that the endoplasm is not a sol, but rather contains pseudoplastic and /o r elastic components in its structure (I, 2, 6). Cytoplasm dissociated from Chaos chaos has been shown to stream outside the intact cell by means of a propagated contraction (4). The endoplasm does, in fact, shorten and thicken in passing through the fountain zone (2, 3). Unfortunately, most of the facts which these investigations have uncovered cannot be used as decisive evidence in choosing between the front and tail contraction theories, for these facts are compatible with both theories. In only two cases so far has it been possible to test the predictions of one theory in such a way as to yield evidence which excludes the other. In the first case, Allen, Cooledge, and Hall (4) have recently shown that the streaming of dissociated cytoplasm, unexplainable previously by the tail contraction theory, could be accounted for in all aspects by the front contraction theory. The second series of decisive experiments are those to be reported here. They concern the fourth prediction of the front contraction theory, namely, that the site of the syneresis giving rise to the hyaline cap must be the fountain zone. The prediction is testable by interference microscopy, for