Polarized Structures of Cells in the Aggregating Cellular Slime Mold D. discoideum: An Electron Microscope Study

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Abstract
Ultrastructural features of the aggregation stream and the center of the cellular slime mold D. discoideum were examined with special reference to the polarized movements of cells toward the center. The contact between stream cells and the substratum differed in proportion to their distance from the aggregation center. Cells in the aggregation stream located at a distance from the center were in close contact with the substratum, but cells closer to the center tended to be separated from the substratum. Although cells in the stream were generally loosely packed with large intracellular spaces, some areas of the cell surface were in close contact with other cells. Two conspicuous differences were evident in the intracellular structures of the anterior and posterior regions of stream cells. Cytoplasmic fibrils of about 60 .ANG. in diameter were found more abundantly in the anterior region than in the posterior region, and a marked difference in the cell membrane structure was noted between the anterior and posterior ends. The biological significance of these differences was discussed in connection with the polarized movement and contact of cells.

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