You would think that once a human dies, the body would stop doing things; without blood and air circulation, internal systems would quickly deplete. But due to a strange quirk of biology, there are such things as the undead: living cells, at least, within a made and dusted body.
Some cells of the human brain increase their activity after death. These ‘zombie’ cells increase their genetic expression and continue valiantly trying to perform their vital tasks, as if someone forgot to tell them 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 went about housework for hours after death.
“Most studies assume that everything in the brain stops when the heart stops beating, but it doesn’t,” Loeb said. “Our findings will be necessary to interpret research on human brain tissues. We just haven’t quantified these changes until now.”
Much of the information we have on brain disorders such as autism, Alzheimer’s, and schizophrenia comes from experiments performed on brain tissues after death; This approach is essential in the search for treatments, since animal models for brain studies are often not translated to us.
Typically, this work is done on tissue from people who died more than 12 hours ago. By comparing gene expression in fresh brain tissues (collected as part of epilepsy surgery from 20 patients) with the above-mentioned brain samples from deceased individuals, Loeb and his team found striking differences that were not age or age specific disease.
They used data on gene expression, which they then corroborated by examining the histology of brain tissue, to understand changes in the specific activity of cells over time since death, at room temperature.
While most of the gene activity was stable for the 24 hours the team documented, neural cells and their gene activity were quickly depleted. However, what is most notable is that glial cells increased gene expression and processes.
While surprising at first, this actually makes a lot of sense, given that glial cells, like debris-eating microglia and astrocytes, spring into action when things go wrong. And dying is as “bad” as living things can be.
“That glial cells enlarge after death is not too surprising given that they are inflammatory and their job is to clean things up after brain injuries like oxygen deprivation or stroke,” Loeb said.
The team then showed that the RNA expressed by genes does not change itself within 24 hours after death, so any change in its quantity must be due to the continuation of biological processes.
“Complete gene expression from freshly isolated human brain samples allows unprecedented insight into the genomic complexity of the human brain, due to the preservation of so many different transcripts that are no longer present in postmortem tissues,” the researchers wrote in their Article.
This has huge implications for past and present studies using brain tissue to understand diseases that involve immune responses, such as these ‘zombie’ glial cells that swell as they uselessly devour the surrounding fragments of dying brains.
However, after 24 hours, these cells also succumbed and could no longer be distinguished from the degraded tissue around them.
“Researchers should be aware of these genetic and cellular changes, and reduce the postmortem interval as much as possible to reduce the magnitude of these changes,” Loeb explained.
“The good news from our findings is that we now know which genes and cell types are stable, which are degraded, and which increase over time, so the results of post-mortem brain studies can be better understood.”
Even in death, biological entities are never quite static.
This research was published in Scientific reports.