By infusing cells with fluorescent dye, teams at Harvard Medical School and the Max Plank Institute for Medical Research pinpointed neurons in the mouse visual cortex and retina that responded to specific stimuli, like a beam of light moving in different directions. They then sliced the tissue and used electron microscopy, which can image some of the tiniest structures in cells, to map the connections between the neurons. Armed with knowledge of both how the cells connect and what they do, the teams gained insights into how the brain works.
This composite 3-D image from the Harvard team, which shows a half-millimeter-wide chunk of a mouse’s visual cortex, was created by stacking together millions of images snapped of more than 1,000 slices of brain tissue. Building a neural map is an arts-and-crafts project on a gargantuan scale; researchers go through every slice, identifying specific parts of each cell. The round, blue structures are the neuron bodies, the pink strands are blood vessels, and the long blue striations running up and down are the spindly arms known as dendrites and axons that project from nerve cells.