Cell prints on the surface of the slug of Dictyostelium discoideum: a Nessler-positive matrix substance.

A substance or mixture of substances that stains positively with Nessler's reagent is localized above the surface layer of cells of the migrating slug of Dictyostelium discoideum, possibly incorporated into the slime sheath and coating its surface as well. The substance is manifested as outlines ("cell prints") conforming to the profiles of the surface cells and as small globules over the external face of these surface cells. The entire circumference of the slug is covered by the Nessler-positive cell print substance, which is present from the slug tip to the rear of the slug; presumably all cells at the surface of the slug produce the substance. Cell prints are also present on the surface of the culminating slug as well as on the stalk sheath. No cell prints are seen on the slime sheath or in the slime trail, the collapsed sheath behind the slug. Either the Nessler-positive substance is not incorporated into the sheath or it is uniformly smeared into the sheath as the slug migrates through it. Analysis of size and orientation of cell prints reveals characteristic regional differences that correlate with the prestalk/prespore boundary but not with the location of "zones of adhesion." These patterns of cell prints are compared with some current models of slug cell patterns and movements. Nessler's reagent, a sensitive detector of free ammonia, is shown to react with the amino acids asparagine and glutamine as well as with peptides containing these amino acids. Nessler's reactivity to N-acetylglucosamine suggests that polysaccharides containing this amino sugar also react with Nessler's reagent. Several matrix proteins and polysaccharides are enriched in asparagine, and sheath polysaccharides contain N-acetylglucosamine. It is likely that Nessler's reagent is reacting with such matrix proteins and polysaccharides when cell prints are visualized. The possibility is raised of an interaction between such molecules and the ammonia concentration and flux at the slug surface.