Human bone marrow is a soft, spongy tissue found within bone cavities. It serves as the body’s blood cell production center, accounting for about 4% of total body weight in adults. This tissue continuously produces cells that circulate in our blood, performing functions from oxygen delivery to fighting infections.
The Anatomy and Types of Bone Marrow
Bone marrow is located within the spongy (cancellous) portions of bones, particularly in flat bones (hip bone, sternum, skull, ribs, vertebrae, shoulder blades) and the ends of long bones (femur, tibia, humerus). The tissue is highly vascular, rich with numerous blood vessels and capillaries.
There are two primary types of bone marrow: red and yellow. Red bone marrow is characterized by its reddish color and is primarily composed of hematopoietic tissue, actively producing blood cells. Yellow bone marrow is predominantly made up of adipose (fatty) tissue, giving it a yellow appearance, and functions in fat storage.
At birth, all bone marrow is red, indicating widespread blood cell production. As a person ages, a significant portion of the red marrow gradually converts to yellow marrow, particularly in the shafts of long bones. In adults, approximately half of the bone marrow remains red, concentrated in the central skeleton, while yellow marrow is more dominant in the peripheral appendicular skeleton. However, yellow marrow can revert to red marrow if there is an increased demand for blood cells, such as after significant blood loss.
The Vital Role of Bone Marrow
The primary function of red bone marrow is hematopoiesis, the process of producing all types of blood cells from hematopoietic stem cells. These multipotent stem cells, located in specialized regions, differentiate into a wide array of blood cell types. The body produces an estimated 100 billion blood cells daily to replace old ones and meet ongoing demands.
Red blood cells (erythrocytes) are produced through erythropoiesis, transporting oxygen from the lungs to tissues and carrying carbon dioxide back for exhalation. White blood cells (leukocytes) are produced through leukopoiesis. These diverse cells defend the body against infections and destroy abnormal cells, including neutrophils, basophils, eosinophils, monocytes, and lymphocytes, each with a specific immune role.
Platelets (thrombocytes) are small cell fragments formed from megakaryocytes in the bone marrow; their production is called thrombopoiesis. These cells are important for blood clotting, forming a seal at sites of injury to prevent excessive bleeding. Hematopoiesis is tightly regulated and can escalate production to meet increased demands, such as during an infection, ensuring a constant supply of necessary blood components.
When Bone Marrow Goes Wrong
Disrupted bone marrow function can lead to various conditions affecting blood cell production and overall health. One category is bone marrow failure, like aplastic anemia, where the marrow does not produce enough healthy blood cells. This results in a deficiency of red blood cells, white blood cells, and platelets, causing symptoms such as fatigue, shortness of breath, and increased susceptibility to infections.
Other conditions involve the overproduction of specific blood cells, such as in myeloproliferative neoplasms (MPN). Examples include chronic myelogenous leukemia (CML), with an overproduction of white blood cells (granulocytes), polycythemia vera (excess red blood cells), and essential thrombocythemia (overproduction of platelets). These conditions can cause various symptoms, including fatigue, bone pain, and an enlarged spleen or liver.
Certain cancers, like multiple myeloma, originate in the bone marrow. Here, cancerous plasma cells accumulate, crowding out healthy blood cells and producing abnormal proteins. This impairs the body’s ability to fight infections and can lead to bone pain, kidney problems, and anemia.
Myelodysplastic syndromes (MDS) are another group of disorders where the bone marrow produces abnormal blood cells that do not mature properly, sometimes leading to a deficiency in healthy blood cells.
Bone Marrow Transplants
Bone marrow transplants, also known as hematopoietic stem cell transplants, replace diseased or damaged bone marrow with healthy stem cells. This treatment is often used for patients with certain blood cancers (leukemia, lymphoma, multiple myeloma) or conditions like aplastic anemia where the bone marrow is not functioning correctly. The procedure typically involves suppressing the recipient’s immune system with chemotherapy or radiation before infusing the healthy stem cells.
There are two main types of stem cell transplants: autologous and allogeneic. In an autologous transplant, the patient’s own healthy stem cells are collected, stored, and returned after high-dose chemotherapy or radiation. This approach eliminates the risk of incompatibility between donor cells and the recipient’s body. Autologous transplants are commonly used for conditions like Hodgkin’s lymphoma, non-Hodgkin’s lymphoma, and multiple myeloma.
An allogeneic transplant involves receiving stem cells from another person, a donor. Donors can be related (e.g., a matched sibling) or unrelated, found through a national registry based on tissue type matching (HLA typing). Umbilical cord blood can also be a source. The goal is for transplanted healthy stem cells to engraft in the recipient’s bone marrow and begin producing new, healthy blood cells, often providing a graft-versus-cancer effect where donor cells help eliminate remaining cancer cells.