Two contrasting mechanisms have been proposed for the establishment of the prestalk‐prespore pattern in the multicellular aggregate of the simple eukaryote Dictyostelium discoideum. One involves intermingled, non‐position‐dependent cell differentiation followed by sorting out which produces the pattern of prestalk cells in the anterior region and prespore cells posteriorly. The second mechanism involves patterning according to the position of cells within the aggregate, in which case intermingled cell types are not expected. Here we use a monoclonal antibody (MUD1), recognising a prespore cell surface antigen, to study the initial appearance of prespore cells in aggregates. Quantitative studies were made with a flow cytometer and frozen sections were used to localise the cells expressing the prespore antigen. This antigen first appeared at the onset of tip formation in the centre of aggregates in a position‐dependent fashion. The prespore antigen was not detected in the tip region or in streams of cells entering the aggregate. We re‐examined the evidence on which the non‐position‐dependent differentiation model is based. Our results support the positional model for pattern formation.
The effects of lymphokines on guinea pig peritoneal macrophages were measured via flow cytometry utilizing the three-parameter FLUVO-METRICELL flow cytometer. On the basis of cell volume three distinct macrophage populations could be distinguished. Three to 5 min after starting the incubation with lymphokines a hyperpolarization of all three macrophage populations took place which was followed by depolarization. After 60 min the transmembrane potential reached again its control values. The negative charge density of the cell membrane decreased shortly after beginning of the incubation to 70-80% of the initial value and then remained unchanged for the following 120 min. The phagocytic activity of the macrophages was diminished during the depolarisation phase but increased over control values after restoration of the transmembrane potential.
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