| Abstract: | The viscoelasticity and contractility of amoeba cytoplasm has been studied in vivo and in vitro. A gradient of increasing viscoelasticity and contractility was identified in the endoplasm of intact cells from the uroid (tail) to the fountain zone (tip of advancing pseudopod). Anterior endoplasm, as well as all of the ectoplasm, contracted in response to the microinjection of a threshold calcium ion concentration (ca 7.0 × 10−7 M). In contrast, there were only delayed weak contractions in the uroid endoplasm upon the microinjection of a threshold calcium ion concentration. Contractions induced in the ectoplasm by microinjecting the contraction solution readily caused the endoplasm to stream. However, the endoplasm at the tips of the extending pseudopods were also contractile and transmitted applied tensions. Furthermore, the microinjection of subthreshold calcium ion concentrations caused the loss of distinct endoplasmic structure and the cessation of streaming in both the uroid and the anterior third of the cell. In addition, the relationship between contractility and cytoplasmic streaming was characterized in “relaxed” cytoplasm placed in a gradient of calcium ion concentration inside quartz capillaries. The results of these experiments demonstrated that the mechanochemical conversion of endoplasm to ectoplasm caused the cytoplasm to become more structured and contractile. Therefore, physiological contractions are possible during and after the conversion of endoplasm to ectoplasm. |
