Steering yourself by the bootstraps: how cells create their own gradients for chemotaxis.

Chemotaxis, where cell movement is steered by chemical gradients, is a widespread and essential way of organising cell behaviour. But where do the instructions come from - who makes gradients, and how are they controlled? We discuss the emerging concept that chemotactic cells often create attractant gradients at the same time as responding to them. This self-guidance is more robust, works across greater distances, and is more informative about the local environment than passive responses. Several mechanisms can establish autonomous gradients. Best known are self-generated gradients, in which the cells degrade a widespread attractant, but cells also produce repellents and 'relay' by secreting fresh attractant after stimulation. Understanding how cells make and interpret their own chemoattractant gradients is fundamental to understanding the spatial patterns seen in all organisms.