Cell differentiation without morphogenesis in Dictyostelium discoideum

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To understand pattern formation in an organism one needs to know what factors control the differentiation of each cell type and what elements are responsible for the spatial arrangement of these cell types. In the cellular slime mould Dictyostelium discoideum, the problem is relatively simple since the mature fruiting body consists of two basic cell types—stalk cells and spores. The stalk cells of the mature fruiting body are derived from the anterior cells of the multicellular masses formed by aggregation, whereas the spores are derived from the posterior cells1. Cells that are plated at a density too low for aggregation, or are allowed to aggregate under water do not normally differentiate into stalk or spore cells. In many of our experiments we have made use of the fact that cells plated on agar at any density fail to undergo development beyond the stage of aggregation if they are covered by a thin layer of Cellophane. In this paper we present observations on cell differentiation which derive from the original report of Bonner2 of stalk-cell induction by cyclic AMP in isolated cells. We have found that stalk-cell induction by cyclic AMP is markedly dependent on cell density, and present evidence for the involvement of a low molecular weight diffusible factor in this process. We also describe the isolation of a mutant which gives rise to spore cells under cellophane.