How individual cells 'surf' chemical waves to form a collective

Scientists have devised a novel imaging method to observe single cells transitioning into multicellular organisms in greater detail. Collective cell movement is crucial for various bodily functions, such as immune responses and wound healing, as well as tumor progression. However, understanding how individual cells synchronize as a group has been elusive.

In a study published in Scientific Reports, researchers from Japan, Germany, and Bangladesh utilized an innovative technique to visualize the behavior of individual cells as they transition from solitary to coordinated movement. They focused on the soil-dwelling amoeba Dictyostelium discoideum, which releases a chemical signal, cyclic AMP (cAMP), when starved, prompting the cells to move in unison to form a multicellular organism, aiding their survival in harsh conditions.

The challenge lay in tracking the cells' movement while simultaneously measuring the cAMP wave's direction, which proved difficult as cells crowded together. To overcome this, the researchers employed fluorescent imaging to track cell movement and cAMP levels concurrently, capturing each frame. They then blurred these images to varying extents and applied particle image velocimetry—a method for tracing fluid flow—to each blurred version.

This approach allowed them to clearly distinguish individual cell paths from the smoother, larger-scale pattern of the cAMP wave. Their continuous tracking revealed that as a wave approaches, cells surge toward it almost head-on. However, once the wave peaks and starts to recede, the cells do not reverse direction; instead, they continue moving in the same direction, resting directionlessly in the wave's troughs until the next wave arrives, and the cycle repeats.

This behavior is likened to surfers paddling hard to catch a wave, riding it together, and then resting until the next one arrives. The findings provide the first systematic map of how amoebas' collective behavior emerges from individual actions. The team believes their imaging technique could be instrumental in understanding collective cell movement in other organisms, potentially identifying leaders and followers within a wave and informing future research in immune responses and cancer biology.