A brain perfusion solution may clear blood vessels but damage membranes
A brain perfusion solution may clear blood vessels but damage membranes
On July 30, bioRxiv published a preprint describing five donor brains. CT showed contrast distribution after perfusion in four of them. In the fifth, a different solution caused similar damage. Electron microscopy revealed shrunken processes and damaged membranes.
After death, the brain can be perfused by pumping a fixative solution through its blood vessels to stop tissue decomposition. Researchers assess the procedure by looking at vessel clearance, brain color, and contrast enhanced CT. These features show where the solution has traveled.
In the preprint, a team from Apex Neuroscience and the Icahn School of Medicine at Mount Sinai studied three human brains and one canine brain. The researchers added 10% mannitol and 10% polyethylene glycol to 10% formalin. The authors attribute the tissue changes to osmotic dehydration: water leaves the tissue, reducing swelling around the blood vessels.
CT showed good contrast distribution in all four donors. Electron microscopy of the frontal cortex, white matter, and thalamus revealed enlarged spaces between cells, shrunken or fragmented processes, distorted membranes, and damaged myelin. In the fifth donor, a dog, a solution containing 20% mannitol without polyethylene glycol produced a similar pattern.
In these samples, favorable signs of perfusion coexisted with damage to the fine structure of the tissue. According to the authors' model, mannitol and the large polyethylene glycol molecules create a concentration difference across the blood brain barrier, the natural filter between the blood and the brain. Water leaves the tissue, while membranes and neuronal processes become deformed.
This adds detail to the problem of osmotic shrinkage during vitrification. In that case, shrinkage may have made neuronal processes impossible to distinguish even in electron micrographs. Here, electron microscopy already shows that the processes are fragmented. In both cases, successful passage through the blood vessels does not establish that a readable map of neural connections has been preserved.
For biostasis and connectomics, which seeks to reconstruct the map of neural connections, this distinction changes the order of evaluation. CT answers whether the solution reached a particular region of the brain. Electron microscopy answers a different question: whether the membranes and processes that carry neural signals remain distinguishable in that region. The authors propose testing both conditions when developing new solution formulations.
The Brain Preservation Foundation evaluates whole brain preservation by asking whether every neural connection can be traced in electron micrographs. Under this criterion, gross appearance and CT provide the first stage of evaluation. The next step is to examine the structure that the solution is intended to preserve.