Stem cells possess a remarkable ability to develop into many different cell types, forming the foundation for the body’s tissues and organs. Among these, hematopoietic stem cells (HSCs) are specialized “master cells” of the blood system. These cells are responsible for generating all types of blood cells, ensuring a constant supply for the body’s diverse needs. Understanding their functions and applications offers insight into how the body maintains its health and how certain diseases are treated.
Understanding Hematopoietic Stem Cells
Hematopoietic stem cells are defined by two distinct characteristics: self-renewal and multipotency. Self-renewal refers to their capacity to divide and produce more HSCs, maintaining a continuous pool of these foundational cells throughout life.
Multipotency describes their ability to differentiate into all functional types of blood cells, including red blood cells, various white blood cells, and platelets. HSCs give rise to more specialized progenitor cells that commit to specific blood lineages. These properties are essential for the body’s overall health and blood system regeneration.
The Process of Blood Cell Production
Hematopoiesis is the continuous process by which hematopoietic stem cells form all blood cellular components. In a healthy adult, this process occurs primarily within the bone marrow, where approximately 10 billion to 100 billion new blood cells are produced daily to maintain stable levels in the peripheral circulation. This constant renewal is necessary because many mature blood cells have short lifespans and are regularly consumed or damaged while performing their roles.
HSCs differentiate into various blood cell types, each with specific functions. Red blood cells, or erythrocytes, are responsible for transporting oxygen from the lungs to tissues throughout the body and carrying carbon dioxide back to the lungs for exhalation. White blood cells, including granulocytes (neutrophils, eosinophils, basophils) and lymphocytes (T-cells, B-cells, natural killer cells), form the immune system and defend against infections and foreign invaders. Platelets, also known as thrombocytes, are small cell fragments that play a role in blood clotting and wound repair. This production ensures the body can respond to demands, such as increased immune cell production during an infection.
Medical Uses of Hematopoietic Stem Cells
Hematopoietic stem cells are widely used in therapeutic applications, particularly through hematopoietic stem cell transplantation (HSCT), often referred to as bone marrow transplant. This procedure involves administering healthy HSCs to patients whose bone marrow is dysfunctional or depleted due to disease or intensive treatments like chemotherapy. The goal is to replace diseased or damaged bone marrow with healthy cells, rebuilding the patient’s blood and immune systems.
HSCT is a standard treatment for various blood cancers, including leukemias, lymphomas, and multiple myeloma. It also treats non-cancerous conditions such as aplastic anemia, a disorder where the bone marrow fails to produce enough blood cells, and certain genetic disorders like sickle cell disease. In these cases, the patient’s immune system is often suppressed with chemotherapy or radiation before transplantation to eradicate diseased cells and prevent rejection of the new cells.
Transplants can be either autologous or allogeneic. Autologous transplants use the patient’s own stem cells, which are collected before high-dose chemotherapy and then reinfused, eliminating the risk of rejection. Allogeneic transplants use stem cells from a compatible donor, offering a “graft-versus-tumor” effect where the donor’s immune cells help fight any remaining cancer cells. Donor cells are sourced from a matched sibling or an unrelated donor found through registries.
Where Hematopoietic Stem Cells Are Found and How They Are Collected
Hematopoietic stem cells used for transplantation are primarily obtained from three sources: bone marrow, peripheral blood, and umbilical cord blood. Bone marrow has traditionally been the main source, with HSCs residing within the spongy tissue inside large bones. For collection, a procedure called bone marrow harvest is performed, typically from the hip bone (pelvis) while the donor is under general anesthesia. This method is invasive but provides a rich source of cells.
Peripheral blood is another common source for HSC collection. In this method, donors receive medication, such as growth factors, to stimulate the bone marrow to release HSCs into the bloodstream. The cells are then collected from the circulating blood using a process called apheresis, which is similar to a regular blood donation. This procedure is less invasive than bone marrow harvest and has become a preferred method for many donors.
Umbilical cord blood, collected from the umbilical cord and placenta immediately after birth, is also a valuable source of HSCs. Cord blood is rich in stem cells and can be stored in public or private banks for future use, offering a readily available option for transplantation, especially for pediatric patients or those with specific genetic needs. The collection process for cord blood is straightforward and poses no risk to the mother or baby.