  The transition from single-cell to multicellular behavior is important in
early development but rarely studied. The starvation-induced aggregation of the
social amoeba Dictyostelium discoideum into a multicellular slug is known to
result from single-cell chemotaxis towards emitted pulses of cyclic adenosine
monophosphate (cAMP). However, how exactly do transient short-range chemical
gradients lead to coherent collective movement at a macroscopic scale? Here, we
use a multiscale model verified by quantitative microscopy to describe
wide-ranging behaviors from chemotaxis and excitability of individual cells to
aggregation of thousands of cells. To better understand the mechanism of
long-range cell-cell communication and hence aggregation, we analyze cell-cell
correlations, showing evidence for self-organization at the onset of
aggregation (as opposed to following a leader cell). Surprisingly, cell
collectives, despite their finite size, show features of criticality known from
phase transitions in physical systems. Application of external cAMP
perturbations in our simulations near the sensitive critical point allows
steering cells into early aggregation and towards certain locations but not
once an aggregation center has been chosen.
