Step by step: cells with multiple functions in cortical circuit assembly.

It is often thought that the construction of cortical circuits occurs as the result of an elegantly designed process that unfolds sequentially as an animal develops until adult functional networks emerge. In reality, cortical circuits are shaped by evolutionary mechanisms, changes in developmental programmes driven by neuronal activity or epigenetic mechanisms and the need to adapt to the external world, and must pass through several important phases and timely checkpoints as they form. Some cortical cell types serve multiple functions during this developmental journey and are then reused (or 'recycled') to perform different functions in the adult cortex. Understanding the different stages of the cortical construction process and taking into account the ways in which cellular functions change across time and space is therefore essential if we are to build a comprehensive framework of cortical wiring in both health and disease.