Image by Jeremy Petravicz, who explains:
This is a picture of a brain slice from one of the genetically modified mice I work with. They are a cell type called astrocytes, which are actually about 80% of your brain. The astrocytes in this mouse are expressing a protein that fluoresces green at certain wavelengths of light, called GFP. It originally comes from jelly fish, but since we’re scientist we like to stick the damn protein into everything.
The Y shaped thing near the middle is actually a blood vessel, which astrocytes extensively contact in the brain.