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In this work we evaluate the cortical expansion model for amoeboid chemotaxis with regard to new information about molecular events in the cytoskeleton following chemotactic stimulation of Dictyostelium amoebae. A rapid upshift in the concentration of chemoattractant can be used to synchronize the motile behavior of a large population of cells. This synchrony presents an opportunity to study the biochemical basis of morphological changes such as pseudopod extension that are required for amoeboid chemotaxis. Changes in the composition and activity of the cytoskeleton following stimulation can be measured with precision and correlated with important morphological changes. Such studies demonstrate that activation of actin nucleation is one of the first and most crucial events in the actin cytoskeleton following stimulation. This activation is followed by incorporation of specific actin cross-linking proteins into the cytoskeleton, which are implicated in the extension of pseudopods and filopods. These results, as well as those from studies with mutants deficient in myosin, indicate that cortical expansion, driven by focal actin polymerization, cross-linking and gel osmotic swelling, is an important force for pseudopod extension. It is concluded that whereas three forces, frontal sliding, tail contraction, and cortical expansion may cooperate to produce amoeboid movement, the cortical expansion model offers the simplest explanation of how focal stimulation with a chemoattractant causes polarized pseudopod extension.

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The layer of cytoplasm underlying the plasmalemma of Xenopus eggs has contractile activity which is of vital importance in fertilization and early development, being involved in such processes as sperm engulfment, cortical granule exocytosis, development of the axes of embryonic symmetry and cleavage. In amphibian eggs this layer is also involved in wound healing and changes of cellular shape at gastrulation. Two kinds of contractile structures can be distinguished near the surface of Xenopus eggs. To characterize the mechanism and regulation of this contractile activity, we have experimentally induced cortical contractions in bisected living Xenopus eggs. We have shown previously that cortical contractions are induced by calcium ions in the bisected egg. Here we show that extraction of soluble cytoplasmic components prevents the calcium-induced contractions, but that addition of exogenous soluble myosin restores them. In oocytes, both soluble and insoluble components of the cortical cytoplasm are unable to support contraction. Thus, during meiotic maturation of oocytes into eggs, both of the components of the cortical cytoplasm must change so as to become competent for contraction.