A lot of us grew up hearing that you can’t regrow brain cells -- usually after we bumped our head on something.

Scientists still debate whether that’s actually true, but one thing’s for sure: brains aren’t nearly as good at replacing cells as other parts of the body. And when the brain gets damaged by something like a stroke, that damage stays instead of getting fixed.

But a new study in the journal Nature Materials suggests a way of reversing that destruction. For the first time, by injecting a specially made gel into mice, scientists were able to successfully coax the mice’s brains into growing back cells and structures that were destroyed by a stroke. And with millions of people suffering from the long-term consequences of strokes, this could be a first step toward repairing their brains, too.

Strokes happen when blood is blocked from certain parts of the brain, cutting off oxygen and nutrients and eventually killing the starved cells. And if those cells do something important, like control your hands or safeguard your memories, that job stops being done — which is why people who have had strokes can have long-term memory issues or muscle control problems. While interventions like physical therapy can sometimes significantly improve people’s symptoms by teaching the brain to find ways around the stroke’s scar, there’s always a scar.

And for a little while, before that scar forms, there’s basically a hole where the cells once were. That’s because brains aren’t as good at replacing dead cells as other organs — many scientists say they can’t naturally do it at all — so new neurons don’t grow in to fill the gap. Adult human brains basically wall off the wounded area after cells die, keeping new blood vessels from bringing in nutrients and oxygen and preventing the hole from filling with the body’s structural goo that supports cell growth.

Scientists with UCLA and Duke University wondered if they could design a material that would essentially take that goo’s place and—with any luck—help the brain heal. What they came up with was a special kind of water-based gel which provides structure to the empty space left by the cells. They tried a few formulations, pitting the plain gel against ones with added goodies.

Those goodies included molecules that stimulate blood vessel growth and reduce inflammation, which helps bring down barriers between the gel and the rest of the brain and allow nutrients in to feed any new cells. They gave mice strokes, then injected a tiny amount of gel into the holes left behind five days later, with several mice receiving each formulation or nothing, as a control. Afterwards, they checked in on the mice for weeks, both by looking at their actual brains and by having them do certain kinds of physical tasks.

And over time, one thing became clear: one of their formulations was very effective. Whole new structures including neural networks formed in the place of the stroke-killed cells, and the mice mastered tasks their untreated counterparts couldn’t. But, there were some limitations to the study.

A big one is that the mice they used were young. They were given strokes when they were more like teens than adults, so the experiments happened as they grew into young adults — which is a stage in life when both mouse and human brains are thought to be better at regrowing brain tissue, and not the age when strokes usually occur. So more research will need to be conducted on adult mice to see if their brains are also able to benefit from the gel.

There’s also the always important caveat that mice aren’t people. Even adult rodent brains seem to be better at growing neurons than adult human brains, so what works for them might not work for us. And the scientists still aren’t sure exactly how the gel works, so they want to study it further to figure more of that out.

So there’s a lot to do before anything even approaching a human trial. But, the fact that any gel formulation worked at all, let alone so well, makes the study an exciting proof-of-concept. And it’s certainly a step in the right direction for millions of people who become disabled by strokes every year.

Unfortunately, it’s not all good news this week. Because a paper in last week’s issue of the journal Nature reports that someone, somewhere in the world, apparently decided that the ozone layer is overrated, and has been releasing alarming amounts of ozone-damaging CFCs into the atmosphere for the last few years. “CFC” is short for “chlorofluorocarbon” -- a huge class of molecules that, as the name implies, contain chlorine, fluorine, and carbon. They were invented in the 1920s.

And at first, they seemed perfectly inert, which made them ideal for applications where you don’t want chemical reactions: Firefighting, pressurizing hairspray, cooling refrigerators — you name it. But when CFCs reach the upper atmosphere, they’re broken down by ultraviolet light, creating super-reactive pieces. And those pieces eat away at our ozone layer, which is bad because that layer blocks a lot of harmful radiation before it reaches the ground and does terrible stuff like cause cancer in people or harm crops.

CFCs are also powerful greenhouse gases that trap thousands of times more heat than carbon dioxide. So they’re not something you want in your atmosphere. Which is why, in 1987, countries worldwide signed the Montreal Protocol, promising to phase out CFCs over time and switch to less-destructive molecules.

Since then, atmospheric CFC concentrations have gradually dropped while the ozone layer has gradually recovered. But a new paper published last week reports that since about 2012, scientists have detected far more CFCs in the atmosphere than there should be—about 13 million kilograms more. Concentrations are still dropping worldwide, but not as much as they should be, which is significantly slowing the progress of the last few decades.

And, maybe unsurprisingly, no one is taking credit. The paper’s authors have been monitoring the amount of CFCs in the atmosphere for a long time, mostly by catching a bunch of air every once in a while and seeing what kinds of molecules react with what’s in the air. And they noticed that around 2012, the rate of decline of CFCs slowed, suggesting a 25% increase in emissions.

They didn’t just automatically accuse people of violating the Montreal Protocol, though. They considered other potential CFC sources before concluding that someone is making them. They wondered if, maybe, buildings with a lot of CFCs in them were being demolished, or if the way CFCs leave the atmosphere had changed, or if CFCs were trapped in ice that’s melting because of climate change.

But there’s so much unexpected CFCs that demolished buildings can’t explain things. And also, none of the other possibilities they considered fit the observations, because it isn’t just that CFC emissions in general have increased. All of the new emissions are a specific kind of CFC called CFC-11 or trichlorofluoromethane.

And we don’t know anything other than human production that would single out a single. CFC like that. So the scientists had to conclude that humans are making and releasing new CFCs again, in defiance of the Montreal Protocol.

But air in the atmosphere is so thoroughly mixed that, right now, no one can say where the CFC-11 is coming from. There are strong hints that it might be from eastern Asia, based on when winds brought the new CFCs toward an observatory in Hawaii. But that’s about all they can say.

Getting a better idea of the source might require something like measurements from planes, or closer analysis of any other chemicals that have also been increasing in the same way, which might provide clues as to who’s to blame. For now, all we can say confidently is that someone is making new CFCs again, despite how harmful we know they are. If you’re watching...stop!

And if you’re not releasing new CFCs into the atmosphere, thank you for that, and for watching SciShow! If you want to keep up to date on all the latest news in science, make sure to click on that subscribe button and check out all of our news episodes that we post every single Friday. Also, if you’re curious about the full history of CFCs, you might want to watch the episode we did last year over on SciShow Space that’s all about that.