Formation and anatomy of the prestalk zone of Dictyostelium.

The pDd63 and pDd56 genes encode extracellular matrix proteins which, respectively, surround the migratory slug and mature stalk cells. Both genes are dependent for their expression upon, and rapidly induced by, DIF, the stalk cell inducer. Using these genes as cell-autonomous markers, we have defined three distinct kinds of 'prestalk' cells localized to different parts of the anterior region of the slug. At least one, and probably both, prestalk cell types initially differentiates at the base of the aggregate. The most abundant of the two prestalk cell types then migrates into the tip, the precursor of the prestalk zone which arises at the apex of the aggregate. Thus we believe that morphogenesis of the prestalk zone, the primary pattern-forming event in Dictyostelium development, involves a combination of positionally localized differentiation and directed cell migration. To account for the positionally localized differentiation of prestalk cells, we invoke the existence of gradients of the known antagonists of DIF-cAMP and NH3. We further suggest that differences in the motility of pstA and pstB cells might result from differences in their chemotactic responsiveness to cAMP signals propagated from the tip.