Lineage tracing of both quiescent G0 and active Hoxb5+ LT-HSCs that actively contribute to homeostatic mouse hematopoiesis.

Every day, our bodies produce billions of new blood cells — red blood cells to carry oxygen, white blood cells to fight infection, and platelets to help blood clot. All of them ultimately come from a rare population of cells in the bone marrow called long-term hematopoietic stem cells (LT-HSCs). These are the "master cells" of the blood system: they can renew themselves indefinitely and give rise to every other type of blood cell. Scientists have long wanted to know exactly which cells keep this process running under normal, day-to-day conditions — without any special stress, injury, or medical treatment like chemotherapy. The trouble is that LT-HSCs live alongside other closely related cells — short-term stem cells and "multipotent progenitors" (MPPs) — that look and behave similarly, making them hard to tell apart and track individually. To solve this, researchers used a genetic marker called Hoxb5, which is found almost exclusively on true LT-HSCs (roughly 1 in every 100,000 bone marrow cells) but not on the closely related short-term stem cells or MPPs. By activating a genetic "tag" specifically in these Hoxb5-marked cells, the team could follow, over time, exactly which descendant cells came from them — like putting a tracking dye on a single family line and watching where its members end up. The results confirmed that MPPs are indeed the offspring of LT-HSCs: MPPs only became labeled several months after the LT-HSCs did, and MPPs were never labeled without their parent LT-HSCs also being labeled. This confirms that LT-HSCs are the main long-term source sustaining normal blood production in the body. Interestingly, only a portion of LT-HSCs picked up the genetic tag right away. The researchers discovered why: many LT-HSCs are "quiescent," meaning they're in a resting, non-dividing state. In these resting cells, the DNA-editing process used to apply the tag started but didn't fully complete — leaving behind a kind of unfinished cut in the DNA. Only when these dormant cells eventually woke up and began dividing did the tagging process finish, causing more LT-HSCs to appear labeled later on. This delayed group was mostly made up of "myeloid-biased" stem cells — a subtype that leans toward producing myeloid cells (like white blood cells involved in fighting infection) rather than an even mix of blood cell types. The researchers also found that LT-HSCs surge in number when the body is under blood-related stress. And consistent with what's known about aging — where blood production naturally shifts to favor myeloid cells — the myeloid-biased stem cells specifically expanded more than other types after six months of tracking. Together, these findings clarify which stem cells are truly responsible for sustaining blood production over time, and shed light on how resting stem cells behave differently from active ones — insights that could inform future research into blood disorders, aging, and stem cell-based therapies.