The mushroom body

What is a mushroom body? The mushroom body is a prominent and striking structure in the brain of several invertebrates, mainly arthropods. It is found in insects, scorpions, spiders, and even segmented worms. With its long stalk crowned with a cap of cell bodies, a GFP-labeled mushroom body certainly lives up to its name (Figure 1). The mushroom body is composed of small neurons known as Kenyon cells, named after Frederick Kenyon, who first applied the Golgi staining technique to the insect brain. The honey bee brain, for instance, contains roughly 175,000 neurons per mushroom body while the brain of the smaller fruit fly Drosophila melanogaster only possesses about 2,500. Kenyon cells thus make up 20% and 2%, respectively, of the total number of neurons in each insect's brain. Kenyon cell bodies sit atop the calyx, a tangled zone of synapses representing the site of sensory input. Projecting away from the calyx is the stalk comprised of Kenyon cell axons carrying information away to the output lobes.