Protecting blood stem cells from the sun’s UV rays

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Most people never question why human blood stem cells live in the bone marrow, a place known as the stem cell “niche.” But it’s actually a strange arrangement when you think about it. Bones provide our bodies with structure, so why do they also produce our blood?

Zebrafish study

This week, Harvard researchers reported in Nature that blood stem cells may live in bone marrow to protect them from the DNA‑damaging effects of UV radiation. They uncovered this by studying zebrafish, whose blood stem cells reside not in bones but in the kidneys. While examining these cells under a microscope, first author Fredrich Kapp, MD, noticed that a layer of other cells covering the kidney was blocking his view—and that observation led to new insights.

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In a zebrafish larva (illustration above), a dark umbrella formed by pigmented cells (white arrows point to these black spots in box, left) in the kidney protects vulnerable stem cells from damaging UV light. Right image is a closeup of the box. Scale bars equal 100 micrometers (left) and 50 micrometers (right). Credit: F. Kapp et al./Nature 2018

Melanocytes

The cell layer turned out to be melanocytes, which produce melanin—the pigment that gives skin its color and protects cells from UV‑induced DNA damage. In a press release, Kapp described his moment of insight:

“The shape of the melanocytes above the kidney reminded me of a parasol, so I thought, do they provide UV protection to blood stem cells?”

To test this idea, the team compared normal zebrafish with mutants lacking the melanocyte layer. As Kapp predicted, fish without melanocytes had fewer blood stem cells. Even flipping normal fish upside down and exposing their kidneys to UV light reduced their stem cell numbers.

The story gets even more interesting in frogs, which are closer to humans evolutionarily. As tadpoles grow legs, their blood stem cells migrate from the melanocyte‑covered kidney to the bone marrow—an even stronger shield against UV radiation. Senior author Leonard Zon highlighted the importance of this finding:

“We now have evidence that sunlight is an evolutionary driver of the blood stem cell niche. As a hematologist and oncologist, I treat patients with blood diseases and cancers. Once we understand the niche better, we can make blood stem cell transplants much safer.”

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