Quantitative analysis of α-DG aggregate parameters. The stimulus protocols for all panels are shown at the bottom: C control (no stimulus), A agrin, L laminin-1, AL agrin and laminin. A-D, numbers at the top indicate statistical groupings based on one factor ANOVA F tests, p < .05. A, density of α-DG (arbitrary units). Agrin clearly increased the expression of α-DG on the cell surface. The effects of LN1 cannot be assessed, because it competes for the binding of the immunological probe for α-DG. B, number of aggregates detected per cell. On this basis LN1, alone or in combination with agrin, significantly increased the number of aggregates; agrin by itself had no effect on aggregate number. C, aggregate size in micrometers. These results parallel those for aggregate number; laminin alone or in combination increased aggregate size. D, aggregate density (arbitrary units). Here we find a deviation from the pattern in A and B: only LN1 by itself causes density increase, while in the presence of agrin this increase is largely blocked. E, the mean Z corresponding to the average separation between aggregates. Significant deviations from random separation (p < .01) are marked with *. Unlike the AChR distributions shown in Figure 2D, α-DG aggregate distributions are found closer together than random under all experimental conditions. This cooperativity is enhanced by LN1 alone or in combination with agrin.