You would think that once a human is dead, the body would be ready to do things; without blood and air circulation, the internal systems would be depleted quickly. But, due to a strange whim of biology, there are things like the undead – living cells, at least, inside a ready and dusty body.
Some cells within the human brain actually increase their activity after we die. These ‘zombie’ cells increase their gene expression and continue courageously trying to carry out their vital tasks, as if someone had forgotten to say that they are now redundant.
Neurologist Jeffrey Loeb of the University of Illinois and his colleagues watched as these cells stubbornly sprouted new tentacles and busied themselves with tasks for hours after death.
“Most studies assume that everything in the brain stops when the heart stops beating, but it’s not like that,” said Loeb. “Our findings will be necessary to interpret research on human brain tissues. We just haven’t quantified these changes so far.”
Much of the information we have about brain disorders, such as autism, Alzheimer’s and schizophrenia, comes from experiments performed on brain tissues after death; this approach is critical in the search for treatments, as animal models for brain studies often fail to translate us back.
Typically, this work is done on tissues of people who died more than 12 hours ago. By comparing the expression of the gene in fresh brain tissue (removed as part of the epilepsy surgery of 20 patients) with brain samples from deceased people, Loeb and the team found striking differences that were neither age nor disease specific.
They used data on gene expression, which they later corroborated by examining the histology of brain tissue, to understand changes in specific cell activity over time since death at room temperature.
While most of the gene’s activity remained stable for 24 hours, the team documented, the neuronal cells and their gene activity depleted quickly. Most notably, however, glial cells increased expression and gene processes.
The cells come to life after the death of the human brain. (Dr. Jeffrey Loeb / UIC)
Although surprising at first, it really makes a lot of sense, since glial cells, such as microglia and astrocytes that eat waste, are triggered when things go wrong. And dying is as “wrong” as living things can be.
“That glial cells increase after death is not very surprising, given that they are inflammatory and their job is to clean things up after brain injuries like oxygen deprivation or stroke,” said Loeb.
The team then demonstrated that the RNA expressed by the genes does not change within 24 hours of death, so any changes in its quantity must in fact be due to the continuation of biological processes.
“The complete gene expression of recently isolated human brain samples allows an unprecedented view of the genomic complexity of the human brain, because of the preservation of so many different transcripts that are no longer present in post-mortem tissues,” wrote the researchers in their article.
This has huge implications for previous and current studies that use brain tissue to understand diseases that involve immune responses – such as those ‘zombie’ glial cells that swell by uselessly devouring pieces of agonizing brains.
After 24 hours, however, these cells also succumbed and were no longer distinguishable from the degrading tissue that surrounded them.
“Researchers need to take these genetic and cellular changes into account and reduce the post-mortem interval as much as possible to reduce the magnitude of these changes,” explained Loeb.
“The good news about our findings is that we now know which genes and cell types are stable, which degrade and which increase over time so that the results of post-mortem brain studies can be better understood.”
Even in death, we biological entities are never totally static.
This research was published in Scientific Reports.