Honey, I shrunk the brain
When cryoprotectants are perfused through the blood vessels in the brain, they cannot cross the blood-brain barrier as fast as water can move in the opposite direction. And cryoprotectants generally have a much higher osmotic concentration than the typical blood plasma. For example, the cryoprotectant solution M22 has an osmotic concentration around 100 times higher. As a result, in a successful cryoprotectant perfusion (without fixatives), water rushes out of the tissue into the blood vessels, the tissue dehydrates, and you end up with a shrunken brain that is visibly pulled away from the skull. The brain weight goes down by 50% or more. This is currently considered a good sign of cryoprotectant perfusion quality. case report A-1002 is an example of a shrunken brain Far be it from me to say that a brain preservation method will not work because it seems weird. I myself have proposed that aldehyde fixation — something which is definitively lethal by contemporary medical criteria — may allow people to be revived with meaningful memories intact if humanity develops sufficiently advanced technology in the future. So I’m not going to use the absurdity heuristic here. Instead, the key question is what this severe dehydration does to the nanometer-scale structures in the brain, such as the connections between neurons, that are thought to be the key parts of the information that encodes long-term memories. Previous attempts at imaging this type of brain tissue were stymied because the severe dehydration made the tissue look unrecognizable. Synapses could be seen, but it wasn’t possible to clearly identify individual neurites or trace them to see whether the connectome is intact: https://www.brainpreservation.org/21cm-cryopreservation-eval-page/ A new paper from Greg Fahy et al at 21st Century Medicine provides the most detailed look yet at what happens to brain ultrastructure during vitrification. So naturally, I had a look at it. What do they think of non-vitrificat