Shielding haematopoietic stem cells from the sun

Haematopoietic stem cells lie in a protected microenvironment, in a species-specific location, and these niches provide signals to modulate their quiescence and expansion. How these cells have come to require such distinct and specific microenvironments during evolution is unknown. Leonard Zon and colleagues find that haematopoietic stem cells in zebrafish, for which the niche is located above the kidney marrow lie below an umbrella of melanocytes that provides protection against DNA damage induced by ultraviolet irradiation. They also see similar umbrellas in several aquatic animals and propose that during the transition to terrestrial life, ultraviolet light may have driven the haematopoietic stem cells to the confinement of the bone marrow, the major niche of these cells in terrestrial vertebrates.