Early recognition of prespore differentiation in Dictyostelium discoideum and its significance for models on pattern formation.

An electron microscopic study revealed that during aggregation the cytoplasm of a number of cells increases in electron density. Increased electron density is shown to be the consequence of cell shrinkage, which causes a close packing of cytoplasmic components. Originally electron-dense cells are spread randomly over the aggregate. The anterior prestalk region of the slug is almost devoid of electron-dense cells. In the posterior prespore region, cells with varying degrees of electron density are intermixed with 15-20% electron-lucent cells. During culmination all cells of the prespore region become very electron dense. Besides introducing a new criterion to recognize prespore cells at an early stage of development, our data give further evidence that induction of prespore cell differentiation is not necessarily position dependent.