Pericytes, a little-understood type of cell on the brain's blood vessels, grow into the empty space when left neighboring pericytes die, such growth is a kind of brain plasticity that might be harnessed to fight age-related vascular disorders, such as Alzheimer's disease and stroke. The study findings were published in the  Cell Reports .

Researchers at the Medical University of South Carolina (MUSC), genetically modified adult miceso that their brain pericytes glowed brightly under a powerful two-photon microscope. Using this technique, Andy Y. Shih and Andrée-Anne Berthiaume, who performed the experiments, were able to take pictures over several weeks to see what happened to the brain when pericytes were lost.

In live mice, the pericytes were seen as oval cell bodies often located near junctions where two capillaries intersected, with long tentacle-like arms called processes extending outward along the capillaries. To study what happens when a pericyte is lost, Shih's team used a process called ablation . Intriguingly, when a pericyte was ablated, the capillary to which it had been clinging appeared to dilate.

As the researchers, they observed a curious pattern: over a period of days to weeks, the processes of neighboring pericytes grew to cover the capillaries where pericytes had been ablated. Those capillaries regained their normal tone, meaning that they were not longer dilated.

When their neighbors were lost, the surviving pericytes seemed to compensate for the job of keeping capillaries toned, a feature that is essential to maintain healthy blood flow in the brain. The images captured by Shih's group of pericytes extending their arms to cover exposed capillaries are the first of their kind. "Most methods to study pericytes give you a snapshot in time. To be able to do esta dynamically in a live beast's brain is an Important step forward," Said Shih.

Surprisingly, losing individual pericytes did not weaken the  blood-brain barrier  as expected. Shih speculates that other cell types that grow more rapidly towards injured sites in the brain, such as microglial cells, were acting quickly to protect the capillaries where pericytes were ablated. Unlike microglial cells , which must shrink parts of themselves to reach injured sites, pericytes grew larger to occupy the territory left by the neighbors.

Yet Shih suspects that there is a limit to how much pericytes can grow to compensate for their lost neighbors. Although these findings show that pericytes can take over the job of supporting capillary tone when it is a single neighbor is lost, it is still not clear what happens when larger numbers of pericytes die, as they do in Alzheimer's disease or stroke.

Further studies includes testing the health of blood vessels in the brain when larger numbers of pericytes are ablated. Shih and his group are also studying the plasticity of pericytes. Solving that puzzle could reveal ways to facilitate new pericyte growth, which in turn could combat the type of blood vessel dysfunction observed in Alzheimer's disease and stroke.