cAMP enhanced aggregation of cells from early chick embryos.

Whole chick embryos incubated for 24–36 hr were disaggregated with EDTA. The populations of single cells were incubated both in suspension and after being plated at various densities on agar blocks in a humid environment. In both cases aggregates formed. The aggregation was enhanced by cAMP and 3-isobutyl-1-methylxanthine (IBMX; a phosphodiesterase inhibitor). The density of aggregates which formed on the agar blocks decreased sharply at a critical cell density, suggesting that aggregation was mediated by a relayed signal. The critical density was decreased by IBMX and increased by phosphodiesterase (PDE), suggesting that aggregation was mediated by a cyclic nucleotide, most probably cAMP. Evidence was obtained for the presence of an extracellular PDE.