Abstract

Rat leukemia cells IRC 741 in suspension culture form single cytoplasmic protrusions by which the cells preferentially adhere to one another. The induction and/or maintenance of these protrusions is sensitive to changes in intercellular contact, pH, temperature, and nutritional conditions. The protrusions are stable, rigid structures which take part in intercellular adhesion but not in adhesion to substrata. Movement on substrata occurs by means of ruffling membranes formed on the main cell body. This asymmetry in cellular form and function is associated with specialized cell surface regions. Ultrastructurally, the cell surface over the protrusions lacks microvilli, and is covered with a 3,000-4,000-A thick cell coat consisting of 200-500-A electron-dense particles in an amorphous matrix. In contrast, the surface over the main cell body has microvilli and a 200-A wide cell coat which lacks particles. The extracellular particles overlying the protrusions have electron-lucent cores, are protease- and pepsin-resistant, and do not stain with colloidal iron, while the matrix in which they are embedded is sensitive to proteolytic enzymes and contains acidic moieties. The negative surface charge density over the protrusions is higher than that over the main cell body, as shown by the orientation of the cells in an electric field. The unexpected observation that a region of higher charge density is one of increased intercellular adhesiveness might be explained, in part, by the rigidity of the protrusions and by the very small radius of curvature of the overlying extracellular particles. The protrusions permit the observation of discrete regions, differing in charge density, on the surface of living leukemia cells.

Highlights

  • Rat leukemia cells IRC 741 in suspension culture form single cytoplasmic protrusions by which the cells preferentially adhere to one another

  • This unusual asymmetry in shape and adhesiveness of the IRC cells was indicative of extensive specialized cell surface regions and suggested that their surface might be asymmetric in chemical composition, ultrastructure, or surface charges

  • The cytoplasmic protrusions formed by IRC 741 leukemia cells are responsible for the characteristic pear shape of the cells and represent sites of differential adhesiveness

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Summary

Introduction

Rat leukemia cells IRC 741 in suspension culture form single cytoplasmic protrusions by which the cells preferentially adhere to one another. We found that some of these difficulties could be overcome by using, as a model, suspension cultures of the leukemic cell line IRC 741 [10] This line seemed of particular interest because while most IRC cells in suspension assumed a round shape, a significant percentage of them had a stable cytoplasmic protrusion by which the cells preferentially adhered to one another. This unusual asymmetry in shape and adhesiveness of the IRC cells was indicative of extensive specialized cell surface regions and suggested that their surface might be asymmetric in chemical composition, ultrastructure, or surface charges. It was found that the cell surface overlying the protrusion appears to have a higher negative charge than does the remainder of the cell body

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