Structural organization associated with pseudopod extension and contraction during cell locomotion in Difflugia

Abstract

In Difflugia corona, a free-living amoeboid cell, locomotion is hampered by a heavy shell or test made of sand grains and other debris. Locomotion involves pseudopod extension, attachment to the substratum, and forcible pseudopod retraction which pulls the shelled cell body forward. When observed through a polarizing microscope, the extending pseudopodia appear isotropic or very weakly birefringent. Upon attachment to the substratum a positively birefringent fibrillar array develops rapidly at the attachment point and extends from this region back to the cell body within the test. These birefringent fibrils extend through and parallel to the long axis of the pseudopod. As the pseudopod retracts, the birefringent fibrillar array disappears, and hyaline blebs, suggestive of syneresis, appear on the pseudopodial surface. The birefringent fibrils correspond in position and approximate diameter (1 μ) to refractile fibrils visible with the Nomarski differential interference microscope. Individual organisms were fixed for electron microscopy at a time when the pseudopodia were firmly attached to the substratum. Electron-microscopic examination of thin sections of pseudopodia revealed many 1-μ bundles of intimately associated, aligned, 55–75 Å micro-filaments. The orientation and size of the bundles indicate that they probably correspond to the birefringent, refractile fibrils observed in living cells. Microfilaments have also been observed both as randomly oriented and dispersed cytoplasmic components, and as aligned filaments in the ectoplasm adjacent to the plasmalemma. During pseudopod extension with sporadic streaming, birefringent ‘flashes’ have been observed at the front of the pseudopod. These flashes are believed to represent a photo-elastic phenomenon.