![Figure][1]
Coronal section of the neocortex in a juvenile mouse. Double immunostaining shows microglia (green) and inhibitory interneurons (red), whereas nuclear counterstaining is in blue.
IMAGE: PAOLA SQUARZONI
The human brain contains billions of well-connected neurons. Neural neighborhoods perform different tasks: Some coordinate movement, whereas others hum along planning dinner. The mature brain is a complex assembly of networks, structures, and tracts. Like cities and their neighborhoods, however, the brain does not arise fully formed. Rather, operational patterns and developmental constraints guide the proliferating neurons that build the typical adult human brain. Just as cities are governed by both hard and soft infrastructure—e.g.