Hematopoietic stem and progenitor cells, usually abbreviated HSPCs, are the rare cells in your body responsible for producing every type of blood cell you have, from oxygen-carrying red blood cells to infection-fighting white blood cells to the platelets that stop a cut from bleeding. They live primarily in the bone marrow, and a healthy adult makes hundreds of billions of new blood cells every day from this small reservoir of parent cells. What makes HSPCs remarkable is their dual ability: they can copy themselves to maintain the pool, and they can mature into specialized blood cell types as needed. That combination is the foundation of bone marrow transplants, an expanding frontier in gene therapy, and a growing area of cancer research.
Where HSPCs Come From
HSPCs do not simply appear in the bone marrow at birth. During embryonic development, the first definitive blood cells emerge from the walls of major blood vessels in a process called the endothelial-to-hematopoietic transition, or EHT. Certain endothelial cells that line the embryo’s vasculature activate a blood-cell gene program, physically round up, break their connections to neighboring vessel-wall cells, and become blood-forming cells instead. This process has been observed across vertebrate species and is considered one of the most conserved developmental events in blood biology.1PubMed Central. Endothelial-to-haematopoietic transition: an update on the process of making blood
Recent single-cell mapping of human embryos has added detail to this picture. A specialized subset of endothelial cells, called hemogenic endothelium, is the direct precursor of HSPCs capable of long-term blood production. Signaling pathways, including one involving fibroblast growth factor (FGF), appear to be tuned differently in the embryo compared to what happens when scientists try to recreate the process using lab-grown stem cells. Specifically, FGF signaling seems to quiet down in the embryo’s hemogenic endothelium during EHT, but it stays active in lab-derived cells, which may explain why generating true transplantable HSPCs in a dish remains so difficult.2Stem Cell Reports. Single-cell mapping of human hematopoietic stem cell origin and endothelial-to-hematopoietic transition The zebrafish, whose transparent embryos allow scientists to watch blood formation in real time, has been central to working out these steps, because the molecular machinery of its blood development is closely shared with mammals.3PubMed Central. The zebrafish: A fintastic model for hematopoietic development and disease
The Bone Marrow Niche
Once established, HSPCs settle in the bone marrow, but they do not float around randomly. They occupy specialized neighborhoods, or niches, that control whether the cells stay dormant, divide, or move into the bloodstream. One of the most striking findings in the field is that the most quiescent, long-lived HSPCs cluster near small arterioles in the inner bone surface region. These tiny arteries are wrapped by a specific type of support cell, NG2-positive pericytes, that actively keep HSPCs in a sleeping state. When researchers depleted these pericytes in mice, HSPCs woke up and began cycling, and the marrow lost its reservoir of long-term blood-forming cells.4PubMed Central. Arteriolar niches maintain haematopoietic stem cell quiescence
Chemical signals reinforce this arrangement. A molecule called CXCL12 acts as an anchor, holding HSPCs in the marrow. Cholinergic nerve signals, the same kind of signaling used by the parasympathetic nervous system, boost CXCL12 production in marrow support cells by activating a nicotinic receptor. Under stress, like after chemotherapy or severe blood loss, this cholinergic pathway helps preserve a reserve pool of stem cells so the body can recover.5Nature Communications. Cholinergic signals preserve haematopoietic stem cell quiescence during regenerative haematopoiesis
Physical properties of the niche matter too. The stiffness of the surrounding tissue, the shape of the space, and mechanical forces like blood-flow shear stress all influence HSPC behavior. In lab experiments, stiffer surfaces pushed human HSPCs toward forming more diverse colony types compared to soft surfaces, suggesting that the marrow’s physical architecture is not just structural but actively instructive.6PubMed Central. Biomechanical cues as master regulators of hematopoietic stem cell fate Engineers are now trying to incorporate these physical cues into culture systems that grow HSPCs outside the body.7Stem Cell Research & Therapy. The physical microenvironment of hematopoietic stem cells and its emerging roles in engineering applications
How HSPCs Decide What to Become
For decades, textbooks depicted blood cell development as a tidy branching tree: an HSPC divides, and the daughter cells step through a fixed series of intermediate progenitor stages before becoming a red blood cell, a neutrophil, a platelet, and so on. That model has been substantially revised. Single-cell studies now show that differentiation is less like climbing down a staircase and more like drifting through a cloud. Individual cells gradually accumulate signals that tilt them toward a lineage, rather than snapping cleanly from one defined state to the next.8PubMed Central. Hematopoiesis Lineage Tree Uprooted: Every Cell Is a Rainbow
Even among cells we call HSPCs, there is real internal diversity. Some HSPCs are biased toward making platelets, some lean toward red blood cells, and others favor immune cells. These preferences appear to be stable and inherited when the cell divides. Researchers have identified surface markers that distinguish platelet-biased human HSPCs from the broader pool, and they have shown that a biomimetic culture system mimicking the bone marrow niche can selectively expand this subpopulation.9Nature Communications. Expansion of human megakaryocyte-biased hematopoietic stem cells by biomimetic Microniche Single-cell multi-omic profiling confirms that these distinct subpopulations are not artifacts of measurement but correspond to real differences in gene activity, protein expression, and the chemical marks on their DNA.10University of Oxford. Characterising haematopoietic stem cell heterogeneity using single-cell multi-omic sequencing
The Epigenetic Layer
The fact that HSPCs can hold a lineage bias without actually turning on the genes for that lineage is an epigenetic phenomenon. Chemical modifications on DNA and on the histone proteins that package it act like bookmarks, keeping certain genes accessible for quick activation without switching them on yet. HSPCs with different lineage biases carry distinct patterns of these bookmarks, especially at regulatory regions called enhancers.11PubMed Central. Epigenetic regulation of hematopoietic stem cell fate These epigenetic patterns also govern self-renewal and aging. As HSPCs accumulate epigenetic changes over a lifetime, their balance of lineage outputs can shift, which contributes to the well-known skewing of blood production toward certain cell types in older adults.12PubMed. Epigenetic Mechanisms: Role in Hematopoietic Stem Cell Lineage Commitment and Differentiation
A Peculiar Metabolism
Most cells in your body rely on mitochondria to burn fuel with oxygen. HSPCs are different. In their resting state, they depend heavily on glycolysis, a less efficient energy pathway that does not require oxygen, even though they could, in theory, use their mitochondria. An enzyme called pyruvate dehydrogenase kinase (Pdk) actively blocks the flow of glycolytic byproducts into mitochondria, enforcing this metabolic preference. When researchers knocked out Pdk2 and Pdk4 in mice, HSPCs lost their quiescence and their ability to sustain long-term blood production after transplantation.13PubMed Central. Regulation of glycolysis by Pdk functions as a metabolic checkpoint for cell cycle quiescence in hematopoietic stem cells
The reason for this metabolic austerity seems to be damage control. Mitochondrial respiration generates reactive oxygen species as a byproduct, and those molecules can damage DNA and proteins. By keeping mitochondrial activity low, resting HSPCs protect their genomes over years of dormancy. But the picture is not purely one-sided: HSPCs do maintain some mitochondrial activity and keep their mitochondrial membranes charged, and the transcription factor FOXO3 is essential for managing this baseline mitochondrial function.14PubMed Central. Mitochondrial metabolism in hematopoietic stem cells requires functional FOXO3 When an HSPC receives the signal to differentiate, it flips a metabolic switch, ramping up mitochondrial oxidative phosphorylation to meet the energy demands of rapid cell division.15PubMed Central. Metabolic Regulation of Hematopoietic Stem Cells Because glycolysis and the mitochondrial fuel cycle are not directly connected in HSPCs the way they are in most cells, amino acids and fatty acids serve as alternative fuel pipelines to feed the mitochondria when needed.
Circadian Rhythms and Nerve Signals
HSPC behavior is not constant throughout the day. The autonomic nervous system relays circadian timing signals from the brain’s master clock to the bone marrow, synchronizing stem cell activity with light and dark cycles. Sympathetic nerve fibers release norepinephrine in rhythmic pulses that drive HSPCs out of the marrow and into the blood at predictable times, while cholinergic (parasympathetic-type) signals counterbalance this by promoting retention and quiescence.16PubMed Central. The Autonomic Nervous System Pulls the Strings to Coordinate Circadian HSC Functions This circadian oscillation has practical implications: the timing of blood draws, stem cell harvests, and even chemotherapy may all be influenced by these daily rhythms, though translating this into routine clinical protocols is still a work in progress.
Moving HSPCs Out of the Marrow
For transplantation, doctors often need HSPCs circulating in the patient’s or donor’s bloodstream rather than tucked inside the bone marrow. The standard method is to inject G-CSF, a growth factor that triggers a chain of events: neutrophil enzymes degrade CXCL12 in the marrow, weakening the anchor that holds HSPCs in place, and the concentration gradient that normally points inward reverses, coaxing the cells out into the blood.17PubMed. G-CSF induces stem cell mobilization by decreasing bone marrow SDF-1 and up-regulating CXCR4 This process, called mobilization, replaced surgical bone marrow harvest as the dominant collection method for many transplant settings.
Once collected, HSPCs must find their way back to the marrow when infused into a patient. This homing process involves a cascade of interactions: selectins on blood vessel walls slow the cells down, chemokine gradients pull them toward the marrow, and integrins lock them in place once they arrive.18PubMed Central. Molecular mechanisms underlying adhesion and migration of hematopoietic stem cells Whether transplanted cells engraft successfully depends heavily on how well this sequence plays out, and researchers are studying ways to enhance each step.19PubMed Central. Homing and Engraftment of Hematopoietic Stem Cells Following Transplantation: A Pre-Clinical Perspective
Growing HSPCs in the Lab
A persistent bottleneck in HSPC medicine is quantity. Cord blood, for example, contains HSPCs that are immunologically flexible and useful for patients who lack a matched donor, but a single cord blood unit often does not contain enough cells for an adult transplant. Over the past two decades, researchers have developed small-molecule compounds that can expand HSPCs outside the body while preserving their stem cell properties. Several of these compounds have entered early clinical trials with encouraging safety profiles.20PubMed Central. Development and clinical advancement of small molecules for ex vivo expansion of hematopoietic stem cell
More recent approaches combine multiple small molecules into cocktails. One optimized three-compound cocktail has been shown to robustly increase HSPC yield from both cord blood and peripheral blood samples.21PubMed. Robust Expansion of Hematopoietic Stem Cells Ex Vivo Using Small Molecule Cocktails Epigenetic modulators represent another promising strategy: compounds like valproic acid and nicotinamide can push HSPCs to divide without losing their identity as stem cells.22PubMed. Ex Vivo Expansion of Cord Blood Hematopoietic Stem and Progenitor Cells The long-term goal is to make a single cord blood donation sufficient for any adult patient, or even to manufacture transplant-ready cells from a small blood draw.
Gene Therapy Through HSPCs
Because HSPCs replenish the blood system for life, correcting a genetic defect in a patient’s own HSPCs can, in principle, cure a blood disease permanently. The most visible early success came in sickle cell disease: a patient treated with a lentiviral vector that inserted an antisickling version of the beta-globin gene into their HSPCs maintained high levels of the therapeutic protein fifteen months later, with no recurrence of pain crises and correction of the disease’s biological markers.23PubMed. Gene Therapy in a Patient with Sickle Cell Disease Gene-editing approaches that directly correct the sickle mutation in HSPCs, rather than adding a new gene, have also shown promise in preclinical work.24Science Translational Medicine. Development of β-globin gene correction in human hematopoietic stem cells as a potential durable treatment for sickle cell disease
These therapies are not risk-free. Preparing a patient’s marrow to accept gene-modified HSPCs traditionally requires harsh chemotherapy conditioning. And follow-up studies of gene therapy patients have found a small but statistically significant increase in driver mutations in HSPCs after treatment, rising from about one in a thousand cells before therapy to roughly one in 125 afterward within three years. Normal aging contributes to such mutations, but the pace observed after gene therapy was faster than aging alone would explain.25Nature Medicine. Clonal selection of hematopoietic stem cells after gene therapy for sickle cell disease Whether these mutations lead to clinical problems over decades is still being watched.
Gentler Ways to Prepare the Marrow
One of the biggest barriers to wider use of HSPC gene therapy and transplantation is the toxicity of conditioning regimens. Standard drugs like busulfan destroy existing marrow cells to make room for new ones, but they also cause severe side effects and frequently cause infertility. A new class of agents called antibody-drug conjugates (ADCs) aims to solve this by targeting only the cells that need to be cleared. An ADC directed against CD117, a surface marker on HSPCs, achieved greater than 99% depletion of marrow stem cells in primates with a single dose, matched the engraftment efficiency of busulfan, and critically preserved fertility and caused minimal off-target toxicity.26Nature Communications. Fertility-preserving myeloablative conditioning using single-dose CD117 antibody-drug conjugate in a rhesus gene therapy model
A parallel approach targets CD45, a marker found on all blood cells. ADCs carrying a potent cytotoxic payload against human CD45 completely cleared human HSPCs in mouse models without damaging non-blood tissues, and the cleared marrow successfully accepted both gene-corrected and donor HSPCs.27Molecular Therapy. Anti-CD45 PBD-based antibody-drug conjugates are effective targeted conditioning agents for gene therapy and stem cell transplant If these targeted conditioning agents prove safe in humans, they could make HSPC-based therapies accessible to patients who currently cannot tolerate the treatment’s preparatory phase.
Aging, Clonal Hematopoiesis, and Cancer
As you age, your HSPCs accumulate mutations, just like every other dividing tissue. Occasionally, a mutation gives one HSPC a growth advantage, and its descendants gradually crowd out other stem cell clones. The result is that a large share of your circulating blood cells descend from a single mutant stem cell. This phenomenon, called clonal hematopoiesis, is common in older adults and has been linked not only to blood cancers but also to increased cardiovascular risk.28PubMed Central. Clonal hematopoiesis in human aging and disease
When clonal hematopoiesis becomes fully malignant, leukemia stem cells drive the disease. In acute myeloid leukemia, these cancer stem cells share some surface markers and metabolic features with normal HSPCs, which makes them hard to target selectively. Distinguishing malignant stem cells from their healthy counterparts remains a central challenge in leukemia treatment and relapse prevention.29PubMed Central. Acute Myeloid Leukemia Stem Cells: Origin, Characteristics, and Clinical Implications
How HSPCs Respond to Infection
HSPCs are not passive bystanders during infection. When exposed to signals from pathogens, short-term HSPCs and multipotent progenitor cells can produce substantial amounts of cytokines, the signaling molecules normally associated with mature immune cells. They do this through the same inflammatory signaling pathway that immune cells use.30Cell Stem Cell. Identification of Diverse Cytokine Production by Hematopoietic Stem and Progenitor Cells This means HSPCs do not just supply new immune cells; they participate directly in the immune response.
But not all HSPCs react to infection equally. During malaria infection in mice, researchers recently identified a rare subpopulation of HSPCs that senses the inflammatory environment yet stays functionally intact. These cells are not shielded from the inflammation; they actively detect it but maintain a stem-cell-associated metabolic profile that protects their long-term function. This reserve pool may be the reason the blood system can bounce back even after severe infections.31Blood Journal. Rare Hematopoietic Stem Cells resilient to infection-induced stress sense yet withstand inflammation Understanding how these resilient cells maintain their identity under stress could eventually inform strategies for protecting the blood system during chemotherapy or chronic inflammatory diseases.