Hematopoietic stem cells (HSCs) are immature cells that can develop into all types of blood cells, including red blood cells, white blood cells, and platelets. They are the source for the continuous production of blood, a process called hematopoiesis. The body produces over 500 billion new blood cells daily, and this constant replenishment by HSCs is necessary for a healthy blood and immune system.
The Core Properties of Hematopoietic Stem Cells
A defining feature of hematopoietic stem cells is their capacity for self-renewal. When an HSC divides, it can create at least one daughter cell that is identical to the original stem cell. This process ensures the population of HSCs is not depleted over a lifetime, maintaining a consistent reservoir of these foundational cells.
The other primary characteristic of these cells is their ability to differentiate. Differentiation is the process through which an HSC commits to a specific developmental path, maturing into a specialized blood cell. This multipotent capability allows them to give rise to every type of blood cell, ensuring the body can generate the diverse range of cells required for oxygen transport, immune defense, and clotting.
The balance between self-renewal and differentiation is carefully regulated. Most HSCs in an adult are in a state of quiescence, or dormancy, dividing infrequently. This controlled activity prevents exhaustion of the stem cell pool while meeting the daily demand for new blood cells. Specific signals can activate these quiescent cells, prompting them to divide and produce the necessary cell types in response to injury or infection.
The Role of Hematopoietic Stem Cells
HSCs differentiate into red blood cells, or erythrocytes. These cells are responsible for transporting oxygen from the lungs to the rest of the body’s tissues. They contain a protein called hemoglobin, which binds to oxygen molecules. The constant supply of new red blood cells replaces old or damaged ones, ensuring that tissues receive a continuous oxygen supply.
HSCs also generate all types of white blood cells, known as leukocytes, which are components of the immune system. These cells are broadly categorized into lymphoid and myeloid lineages. Lymphoid cells include T cells and B cells, which provide targeted immune responses, while myeloid cells include neutrophils that fight bacterial infections and macrophages that engulf cellular debris.
The third cell type produced by HSCs are platelets, or thrombocytes. These are not complete cells but rather small fragments derived from large bone marrow cells called megakaryocytes. Platelets are instrumental in blood clotting. When a blood vessel is injured, platelets gather at the site and stick together to form a plug, which helps to stop bleeding and begin the healing process.
Sources of Hematopoietic Stem Cells
In adults, the primary location of hematopoietic stem cells is the bone marrow, specifically the red marrow in the core of large bones. This spongy tissue contains specialized microenvironments, or niches, that support and regulate the HSCs. These niches provide signals to control whether the stem cells remain quiescent, self-renew, or differentiate.
HSCs can also be found circulating in peripheral blood, although in much smaller numbers than in bone marrow. It is possible to increase the number of these circulating stem cells using specific medications. This process, known as mobilization, coaxes the HSCs to move from the bone marrow into the bloodstream, where they can be collected more easily.
A third source is umbilical cord blood, which remains in the placenta and umbilical cord after birth. This blood is a rich source of young, potent hematopoietic stem cells. Cord blood can be collected at birth and cryopreserved, or frozen, for future use. These cells are a valuable resource for transplantation.
Medical Applications
A primary medical use for hematopoietic stem cells is in transplantation procedures, often called bone marrow or stem cell transplants. These transplants are used to replace a person’s unhealthy blood-forming system with healthy cells.
This procedure can treat cancers that affect the blood and bone marrow, such as leukemias and lymphomas. High-dose chemotherapy or radiation used to eliminate cancer cells also destroys the patient’s own HSCs, making a transplant necessary to restore blood production.
Transplants are also employed to treat bone marrow failure syndromes, like aplastic anemia, where the body’s own bone marrow is unable to produce enough new blood cells. A transplant from a healthy donor can re-establish a functional blood-forming system. The transplanted HSCs migrate to the recipient’s bone marrow and begin the process of hematopoiesis.
HSC transplants can be a therapeutic option for certain inherited disorders. These include some genetic immune deficiencies and metabolic diseases. By replacing the patient’s defective stem cells with healthy ones, the procedure can correct the underlying genetic problem in the blood system, allowing the body to produce normal, functional blood and immune cells.