3.2 Brain Soup: The Isotropic Fractionator

summary

Suzana Herculano-Houzel developed a breakthrough method to count neurons in the brain accurately and rapidly known as isotropic fractionator, or, as she likes to call it, “making brain soup.” The steps to make brain soup are:
- Step 1: Cut off the part of the brain of interest.
- Step 2: Chop that part of the brain into even smaller pieces.
- Step 3: Dissolve brain pieces in salty detergent.
- Step 4: Grind into homogenous suspension.
- Step 5: Add cell-type specific dye to make neuronal and other nuclei visible.
- Step 6: Count nuclei in a small known volume.
- Step 7: Extrapolate to total number.
By comparing the number of neurons across different species through this method, the following became evident:
- Larger cortices do not necessarily have more neurons than smaller ones.
- Neurons vary in size.
- Birds have more neurons than primates.