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Tattooing goes way back in human history. We’ve been inking ourselves for thousands of years.

And yet, exactly how the ink sticks around has remained somewhat of a mystery. The rough idea was that either some skin cells or immune cells absorb ink particles, and then basically never die. Unless it’s been a long time or they’re blasted apart by lasers.

But a new study in The Journal of Experimental Medicine is here to set the record straight: it is immune cells that suck up the ink. But they die all the time, and new ones take over the ink hoarding. At least, that’s what research in mice indicates.

When you’re getting tattooed, tiny needles poke holes that let the ink into your second layer of skin, called the dermis. Previous studies have looked at tattooed skin under a microscope. But when you’re looking at a bunch of ink-filled blobs, it’s tough to learn specifics — like which cells suck up the ink, how long they live, or what happens to the ink when they die.

So this team took a different approach. They used genetically modified mice that let the researchers flip a genetic switch and kill off macrophages, the immune cells that travel around your body gobbling up foreign substances and other debris. And they used machines that can sort cells by really specific characteristics that a microscope can’t see, like what proteins are on the outside.

That way, they could dig deep into which cells were there, and which had ink in them. Through these experiments, they discovered that a unique kind of dermal macrophage is probably involved. Specifically, it’s a kind of melanophage, which are cells that ingest melanin, the pigment responsible for tanning and variations in skin color.

Unlike other kinds of macrophages, they don’t carry their contents away to be destroyed—they stay put and hold onto that pigment. And when the researchers gave their mice green tail tattoos, they found that melanophages were picking up the ink. When the researchers killed these cells off, the ink hung out in the goo between cells.

Then, as the melanophages were replaced with new ones from the blood, they sucked up the same ink. To see if this kind of turnover is normal, they grafted tattooed skin from one mouse to another. And sure enough, while all the ink-containing cells were from the graft donor at the start, after about 6 weeks, they were mostly from the recipient instead.

Of course, this was work done in mice, so it may not perfectly model human tattooing. But our skin does contain similar melanophages. And the findings could explain why laser tattoo removal is so tricky.

The goal is to use light to break the ink pigments into smaller chunks and kill the cells holding them. But before all the ink is whisked away by your immune system, nearby melanophages might absorb it again, helping your tattoo stick around — whether you want it or not. Genetic engineering is a bit of a theme this week, from modified mice to plants.

Crops are among the thirstiest organisms on Earth, guzzling up to 90% of the world’s available fresh water. And, like, we can’t just stop growing them, because we need to eat. Not to mention, we’re staring down a future of widespread droughts thanks to climate change.

So this huge water problem is one that genetic engineers have been trying to solve. And this week we may have taken one step forward. A new study published in Nature Communications describes a single gene that can be manipulated to make plants more tolerant to drought.

The new finding comes from an international research project called Realizing Increased. Photosynthetic Efficiency or RIPE. It's a huge collaboration by seven institutions and dozens of scientists.

They’re studying the remaining mysteries of photosynthesis, the process by which plants use light energy to produce sugars and oxygen from carbon dioxide and water. And now, they’ve discovered that increasing the amount of one protein—called Photosystem. II Subunit S or PsbS—creates water-conserving plants.

PsbS is found in all crop plants because it’s one of about 30 proteins in Photosystem II, the machinery that captures light energy. When plants absorb more light than they need, PsbS helps dissipate some of the absorbed energy as heat. So when there’s a whole bunch of PsbS, more light energy is diverted.

The plants are basically tricked, so they aren’t getting ready to photosynthesize as much, and need less of the other ingredients. Plants take in carbon dioxide through little pores on their leaves and stems called stomata, which can open or close to let in different amounts. And these pores let water evaporate as a side effect.

But with more PsbS tricking the plant, stomata will stay partially closed, preventing some. CO2 uptake /and/ water loss. To figure all this out, the researchers took tobacco plants and engineered them to produce more or less PsbS.

They tested four overproducing and two underproducing varieties against non-engineered plants by growing them in fields, where there’s a little less control than greenhouses or labs. And they found that the PsbS overproducers lost an average of 25% less water than the non-engineered tobacco plants. Even with partially closed stomata, the engineered plants had plenty of CO2 because there’s a lot more of it in the atmosphere than there used to be… thanks to us.

The main downside the researchers found was that half of their engineered plants were less productive than the normal ones — they were shorter, had less mass, or had smaller or fewer leaves. But half weren’t, and they weren’t entirely sure why. And they note that their experiment was done with lots of available water.

But if they’d simulated a drought, they expect the genetically modified tobacco would perform even better. Now, they aren’t going to unleash these plants on the world anytime soon. This is mostly just proof-of-concept that manipulating one gene found in a lot of plants can make water conservation possible.

In the future, the RIPE researchers hope to engineer plants that can thrive in dry, nutrient-poor soils. But until these plants are made and tested there’s no way to know how successful or revolutionary they’ll be. And that’s how science works.

It takes a lot of clever people working together to learn, experiment, and solve problems — both in the classroom and out in the field. Our friends at Skillshare have asked us to spend the time we would normally dedicate to talking about their classes to instead talk about a great YouTube creator you might not have heard of as part of their Skillshare Spotlight program. Alex Nickel is a 16-year-old nerdfighter who makes educational content on his channel Technicality.

If you love learning, well-timed jokes about your favorite fandoms, and the occasional, but very good, pun, check out Technicality. There’s a link to his most recent video in the description where you can learn more about Technicality and a special offer from Skillshare if you’re just hearing about them for the first time too. ♪.