While the onset of death is often defined as the time the heart stops beating, the body's cells do not all die immediately. Now, research from the University of Illinois, Chicago (UIC) suggests that important biological processes persist in the human brain for hours longer than previously thought.
'Most studies assume that everything in the brain stops when the heart stops beating, but this is not so,' said one of the researchers, Dr Jeffrey Loeb. 'Our findings will be needed to interpret research on human brain tissues.'
The UIC group took a novel approach of measuring gene expression in fresh brain tissue samples collected from 20 epilepsy patient brain surgeries.
'We decided to run a simulated death experiment by looking at the expression of all human genes, at time points from 0 to 24 hours, from a large block of recently collected brain tissues, which were allowed to sit at room temperature to replicate the postmortem interval,' said Dr Loeb.
The team sequenced over 18,000 genes from the fresh tissue samples and compared their expression with older postmortem brains. In older brains, the genes most degraded were associated with neuronal activity. Similarly, these genes were most rapidly degraded in fresh tissue, however, there was also a sharp increase in genes expressed in glial cells: the immune cells of the brain.
Upon imaging the fresh brain tissue at different time points, they found that whilst neurons decreased in number, glial cells grew in size – peaking at about 12 hours.
Dr Loeb explained 'That glial cells enlarge after death isn't too surprising given that... their job is to clean things up after brain injuries like oxygen deprivation or stroke.'
'The good news from our findings is that we now know which genes and cell types are stable, which degrade, and which increase over time so that results from post-mortem brain studies can be better understood' he said.
The research was published in the journal Scientific Reports.