The haematopoietic system is the network within the body responsible for creating all cellular components of blood. This continuous process, known as haematopoiesis, ensures a steady supply of new blood cells to replace old ones, which have limited lifespans. It involves specialized organs and tissues, primarily the bone marrow, but also includes the spleen, lymph nodes, and liver at different stages of life. This system supports various bodily functions, from oxygen transport to immune defense and blood clotting.
Haematopoietic Stem Cells: The Foundation
Haematopoietic stem cells (HSCs) are the foundation of the haematopoietic system, capable of generating all types of blood cells. These cells possess two defining properties: self-renewal and multipotency. Self-renewal means that when an HSC divides, at least one daughter cell remains an HSC, maintaining the stem cell pool and ensuring a continuous supply.
Multipotency refers to their ability to differentiate into various specialized blood cell types. In adults, HSCs primarily reside in the red bone marrow, found within the cavities of most bones. Smaller quantities of HSCs can also be found in the peripheral blood circulation and umbilical cord blood.
The hematopoietic microenvironment contains non-hematopoietic cells like stromal cells, microvascular components, and nerve elements that interact with HSCs. This environment provides signals and factors that influence HSC behavior, including their dormancy during stable conditions and proliferation during regenerative demands. HSCs constitute a small fraction, approximately 0.02%, of the total bone marrow cells, yet they are responsible for producing hundreds of billions of new blood cells daily.
The Journey of Blood Cell Formation
The process of haematopoiesis begins with haematopoietic stem cells differentiating into precursor cells that become specific blood cell types. This differentiation journey involves multiple stages of cell division and maturation, guided by various growth factors and signals within the hematopoietic microenvironment. All mature blood cells have a relatively limited lifespan, necessitating constant replacement.
Red blood cells, or erythrocytes, are produced through a process called erythropoiesis. Their main function is to transport oxygen from the lungs to the body’s tissues and carry carbon dioxide back to the lungs for exhalation, a function facilitated by the protein hemoglobin. Red blood cells circulate for about 120 days before being replaced.
White blood cells, or leukocytes, are formed through leukopoiesis and are a diverse group of cells that constitute the body’s immune system. They defend against pathogens, fight infections, and aid in inflammatory responses. White blood cells include lymphocytes (T cells and B cells), neutrophils, eosinophils, basophils, and monocytes, each with specialized roles in immune defense.
Platelets, or thrombocytes, are produced through thrombopoiesis from large bone marrow cells called megakaryocytes. These small cell fragments are important for blood clotting, forming a plug at injury sites to stop bleeding. Platelets have a shorter lifespan, around 9 to 12 days.
Haematopoietic System’s Role in Health
A healthy haematopoietic system supports overall bodily function, ensuring the continuous production of blood cells. When this system malfunctions, it can lead to a range of conditions affecting the number or function of blood cells. For example, disorders of red blood cells can result in anemias, characterized by reduced oxygen delivery to tissues.
Malfunctions in white blood cell production or function can lead to immune deficiencies, making the body vulnerable to infections. Abnormal platelet counts or function can cause bleeding disorders or excessive clotting. The hematopoietic system can also be affected by certain cancers, such as leukemias and lymphomas, which involve the uncontrolled proliferation of abnormal blood cells, often originating in the bone marrow or lymphoid tissues. These conditions can disrupt normal blood cell production and infiltrate other organs.
Haematopoietic cells are used in medical treatments, particularly in hematopoietic stem cell transplantation (HSCT), often referred to as bone marrow transplantation. This procedure involves administering healthy HSCs to patients whose bone marrow is dysfunctional or depleted due to disease or intensive treatments like chemotherapy or radiation. HSCT can be autologous, using the patient’s own stem cells, or allogeneic, using stem cells from a compatible donor. This therapy is used to treat various conditions, including certain blood cancers, bone marrow failure syndromes, and some genetic disorders.