Neuroscientist Erik Herzog demonstrated tin 2005 that astrocytes, like neurons, have internal clocks. His test subjects made the cover of an issue of the Journal of Neuroscience that year.
To manipulate the astrocytes in the master clock independently of neurons, the scientists needed a way to target the astrocytes alone. The key turned out to a structural protein that helps to give astrocytes their branching structure, here linked to a protein that fluoresces green.
In this slice of the master clock, cells expressing an astrocyte-specific structural protein that had been stained red (top right panel) matched up well with cells that had been equipped to fluoresce green when they were expressing a clock gene (middle right panel), demonstrating that the scientists could watch astrocytes tick in the biological clock.