- Bone marrow is a soft, spongy tissue found within the hollow interior of certain bones, such as the pelvis, sternum, vertebrae, and long bones like the femur and humerus.
- It plays a critical role in hematopoiesis, the process of blood cell production, as well as in immune function and fat storage.
- Bone marrow exists in two primary forms: red marrow, which is hematopoietically active, and yellow marrow, which is primarily composed of fat cells and serves as an energy reserve.
- In infants and young children, most bone marrow is red and highly active, but with age, much of it gradually converts to yellow marrow in long bones, while red marrow remains concentrated in flat bones and the axial skeleton.
- The most vital function of bone marrow is the production of blood cells, including red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). This process occurs in the red marrow, which contains a complex microenvironment of hematopoietic stem cells (HSCs), stromal cells, blood vessels, and extracellular matrix components. HSCs are multipotent cells capable of giving rise to all blood cell types, maintaining a lifelong supply of new cells to replace those lost through aging or injury. The red marrow also produces immune cells such as lymphocytes and monocytes, which play crucial roles in defending the body against infection and disease.
- Yellow marrow, while less active in hematopoiesis, is important for fat storage and energy metabolism. It can also revert to red marrow under certain physiological stress conditions, such as severe blood loss or chronic anemia, to boost blood cell production. Both types of marrow are highly vascularized and innervated, allowing for efficient transport of newly formed cells into circulation and responsiveness to hormonal and neural signals.
- Bone marrow is also an essential component of the immune system, particularly through its role in producing and maturing B lymphocytes, which are responsible for antibody production. It also supports the development of other immune cell lineages that originate in the marrow and migrate to secondary lymphoid organs like the spleen and lymph nodes. The bone marrow’s intricate network of supportive stromal cells and cytokines provides a nurturing environment for immune cell education and maturation.
- In clinical medicine, bone marrow has major implications in both diagnosis and treatment. Diseases such as leukemia, lymphoma, myeloma, aplastic anemia, and various inherited blood disorders originate or affect bone marrow function. Bone marrow biopsies are often used to evaluate the status of hematopoiesis and to detect malignancies or marrow failure. Furthermore, bone marrow transplantation (also known as hematopoietic stem cell transplantation) is a life-saving treatment for many of these conditions. In such procedures, a patient’s diseased or damaged marrow is replaced with healthy stem cells, either from a donor (allogeneic) or from the patient’s own previously collected cells (autologous).
- In recent years, bone marrow-derived stem cells have also become the focus of regenerative medicine. Research into the plasticity of mesenchymal stem cells (MSCs), a population of non-hematopoietic stem cells found in bone marrow, has opened new possibilities for tissue engineering and therapy for conditions ranging from osteoarthritis to cardiovascular disease.
