The Origin of Blood Stem Cells
The body’s supply of blood and immune cells originates from hematopoietic stem cells (HSCs), first generated during embryonic development from a specialized subset of endothelial cells known as hemogenic endothelium. This involves a cellular transformation called the endothelial-to-hematopoietic transition (EHT). During EHT, endothelial cells, which line blood vessels, change their shape and function to become blood-forming cells.
This transformation is prominent in the aorta-gonad-mesonephros (AGM) region. Within the floor of the main embryonic artery, the dorsal aorta, select endothelial cells change. Imaging studies have visualized these cells rounding up, detaching from the vessel wall, and budding into the circulation as new hematopoietic cells, releasing the precursors that will populate the entire blood system.
Once detached, these cells are considered precursors, not yet fully mature HSCs. They form clusters, known as intra-aortic hematopoietic clusters (IAHCs), attached to the inside of the aorta before being swept away by blood flow. This budding process is a step in definitive hematopoiesis, the wave of blood production that establishes the lifelong, self-renewing pool of HSCs.
Molecular Identity of Hemogenic Endothelium
The ability to generate blood stem cells is restricted to a specialized population of endothelial cells with a unique molecular profile. Hemogenic endothelial cells are programmed for this function by transcription factors, proteins that control gene expression and cell identity. The most recognized transcription factor for the endothelial-to-hematopoietic transition (EHT) is Runx1.
Runx1 is a primary regulator of EHT. Studies where the Runx1 gene is inactivated show a complete failure to produce definitive HSCs, demonstrating its direct involvement in the transition. The activation of Runx1 initiates a genetic cascade that turns off endothelial-specific genes while activating genes required for a hematopoietic identity.
The timing of Runx1 expression is controlled by signaling pathways, with the Notch pathway being a prominent contributor. Notch signaling is involved in determining cell fate, including the specification of arteries. For EHT to proceed, Notch signaling must first be active to establish an arterial identity in the endothelial cells, but its activity must then be precisely modulated. A decrease in Notch signaling in Runx1-expressing cells permits them to detach and complete their transformation.
Developmental Timing and Significance
The generation of blood stem cells from hemogenic endothelium occurs within a narrow window of embryonic development. In human embryos, this process begins in the aorta-gonad-mesonephros (AGM) region at approximately four to six weeks of gestation. The equivalent process in mouse embryos happens around embryonic day 10.5. This timing is coordinated with other developmental milestones to establish the hematopoietic system when needed.
The function of hemogenic endothelium during this period establishes the body’s lifelong source of hematopoietic stem cells (HSCs). While earlier, “primitive” waves of blood formation occur in the yolk sac, the “definitive” HSCs from the AGM are capable of self-renewal. These cells generate all blood and immune cell lineages for the organism’s lifespan and represent the foundation of the adult hematopoietic system.
After forming in the aortic endothelium, these immature HSCs enter the circulatory system and migrate to sites suitable for their expansion and maturation. The first destination is the fetal liver, which becomes the primary site of blood production for the remainder of gestation. Just before birth, HSCs migrate again to their final destination in the bone marrow, where they reside and function throughout adult life.
Therapeutic Potential and Research
The process of creating blood from embryonic blood vessels holds promise for medicine. Researchers are focused on replicating the endothelial-to-hematopoietic transition (EHT) in the laboratory to generate functional, patient-specific hematopoietic stem cells (HSCs). This research uses pluripotent stem cells (PSCs), such as induced pluripotent stem cells (iPSCs), which can be created from a patient’s own cells.
The primary strategy involves coaxing PSCs to differentiate into hemogenic endothelium and then guiding them through EHT to produce HSCs. This is achieved by exposing the cells to a specific cocktail of transcription factors. Studies have identified combinations of factors, including ERG, RUNX1, and several HOXA proteins, that can convert PSC-derived hemogenic endothelium into HSC-like cells capable of engrafting in animal models.
Generating HSCs in the lab could revolutionize treatments for conditions requiring bone marrow transplants, such as:
- Leukemia
- Lymphoma
- Inherited blood disorders like sickle cell anemia
- Fanconi anemia
This would solve the problem of finding a compatible bone marrow donor, as HSCs could be generated from the patient’s own cells. For genetic disorders, techniques like CRISPR/Cas9 could be used to correct the mutation in the patient’s cells before they are converted into healthy HSCs.
While progress has been made, creating HSCs fully equivalent to those produced in the embryo remains a challenge. The cells generated in vitro show differences in gene expression and may not engraft as robustly as cord blood HSCs. Future research will focus on refining the combination of transcription factors and culture conditions to better mimic the embryonic microenvironment.