We have been able to watch death travelling for the first time – and it moves in a wave at a pace of 30 micrometres per minute. At least, that is how fast death spreads across an egg cell.
In multicellular animals, cells often sacrifice themselves for the greater good. First, much of the machinery inside the cell self-destructs, and then the whole cell disintegrates. Some cells undergo this programmed cell death during development – it’s how our fingers separate in the womb, for instance – while other cells trigger it to prevent cancer or to stop viruses spreading.
It has been clear for a while that once programmed cell death has been initiated, the signal spreads rapidly within a cell, says James Ferrell of Stanford University in California. But no one had studied yet how it spreads.
Ferrell and his colleague Xianrui Cheng have now shown that rather than some chemical signal slowly diffusing through the cell, death spreads as a “trigger wave”, with the self-destruction of one part of the cell triggering the self-destruction of the next part. Other examples of trigger waves include nerve impulses and the spread of wildfires.
The pair discovered this by extracting the cytoplasm – the fluid inside a cell – from frog eggs, and placing it in a thin tube. The cytoplasm contained sub-cellular compartments full of a glowing green protein, which were visible as bright green dots along the tube.
They then placed one end of the tube in an extract from a cell that had already undergone programmed cell death, and to which a red dye had been added.
As the sub-cellular compartments self-destructed, the green dots disappeared, revealing the rate at which death spread along the tube. The wave of disappearing dots spread much faster than the red dye in the extract diffused along the tube.
Ferrell and Cheng then filmed this death wave spreading across an intact egg cell (above). As death moved across the cell, it caused the colour of the cell’s outer membrane to change colour.
In earlier work, Ferrell showed that the process of cell division also spreads through the cell in a trigger wave. “We think trigger waves are probably pretty common,” he says.
Journal reference: Science, DOI: 10.1126/science.aah4065