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Bone marrow is the soft spongy tissue that lies within the hollow interior of long bones. In adults, marrow in large bones produces new blood cells. Bone marrow forms around 4% of total body weight (around 2.6 kg in a healthy adult).
There are two types of bone marrow:
There are a number of blood vessels and capillaries traversing through the marrow making it a very vascular organ.
At birth and in early childhood most of the marrow is red. As a person ages more and more of it is converted to the yellow type. About half of adult bone marrow is red.
Red blood cells (erythrocytes) carry oxygen to the tissues.
Platelets or thrombocytes (derived from megakaryocytes) help prevent bleeding and aid in clotting of blood.
Granulocytes (neutrophils, basophils and eosinophils) and macrophages (collectively known as myeloid cells) fight infections from bacteria, fungi, and other parasites. They also remove dead cells and remodel tissue and bones.
B-lymphocytes produce antibodies, while T-lymphocytes can directly kill or isolate invading cells.
RBC live for around 170 days and rest are shorter lived and need to be replenished continuously. An average human requires approximately one hundred billion new hematopoietic cells each day. This is performed by the Hematopoietic Stem Cells (HSCs).
Around the central bore of the bone or the central sinus lie the Mesenchymal stem cells. These cells have the capacity to form various cells of the body including osteoblasts (that form bones), chondrocytes (that form cartilage), myocytes (that form muscles) and other cells. Apart from this there are the endothelial stem cells that form blood vessels.
Certain diseases of the bone marrow like leukemia, multiple myeloma, myelodysplastic syndrome (MDS), pancytopenia, anemia etc. require examination of the bone marrow tissue. This is called bone marrow aspiration or bone marrow biopsy. A needle is used to withdraw samples of the marrow from within the bone. This is often a very painful process.
Bone marrow is suppressed with the use of cancer chemotherapy. This leads to severe drop in production of RBCs (leading to anemia), WBCs (leading to increased risk of life threatening infections) and platelets (leading to risk of bleeding tendencies).
With advent of medical science it is possible now to transplant the bone marrow in diseased individuals. This process has shown success in a number of cancer patients.