Bone marrow is a highly cellular structure present within the hollow cavities of hard bone tissue. Bone marrow (Picture 1) is of 2 types, red bone marrow (produces blood cells) and yellow bone marrow (fatty tissue). The nature of bone marrow in different parts of the body changes with age. During childhood, bone marrow in all bones is red. In adulthood, the bone marrow cells in long bones of hand and leg become non-functional and are replaced by fat cells to form yellow bone marrow. The only bones to carry red bone marrow throughout life, are the vertebrae (back bones), sternum (breast bone), hip bone, and skull bones. Thus, any disease of bone marrow in adults is first seen in these bones !
Picture 1: Types of bone marrow in cross section of a bone
(Source: Wikimedia Commons)
Bone marrow cells are highly functional and continuously divide and give rise to the different cells present in blood. This includes red blood cells, white blood cells, and platelets. The functions performed by each cell are
- Red Blood Cells (RBC)- transfer of oxygen from lungs to body tissues.
- White Blood Cells (WBC) – fighting infections by production of various types of cells (T Lymphocytes), chemicals, and antibodies (specific proteins against micro-organisms).
- Platelets – clotting of blood after any injury to prevent blood loss.
Thus, changes in bone marrow cause a widespread change in the composition of blood leading to various diseases, like
- anemia – decrease in red blood cells
- polycythemia – increase in red blood cells
- recurrent infections – decrease in white blood cells
- leukemia – increase in white blood cells
- clotting defects – decrease in platelets
- hypercoagulable conditions – increased platelets
The bone marrow receives feedback from the flowing blood and controls the number of cells produced for a particular cell type. Overproduction of a particular cell type, especially that forced by disease, like leukemia, suppresses production of other cell types leading to corresponding symptoms.Bone marrow cells are highly sensitive to radiation due to their constantly dividing nature. Radiation therapy for any cancer or disease causes a substantial loss of bone marrow cells. This condition is called aplastic anemia, in which there is a generalized decrease in the number of cells present in blood (cytopenia). In the absence of adequate quantities of bone marrow or during increased requirement of blood cells, the body can stimulate conversion of yellow bone marrow to red bone marrow in an adult. This leads to presence of red bone marrow at unusual sites in an adult.
Abnormal functioning of bone marrow cells causes a wide spectrum of diseases. The diseases in which there is an increase in the blood cells are called myeloproliferative disorders. They are many times precursors of blood cancers or leukemias. The diseases causing a decrease in numbers of blood cells or production of abnormal blood cells are called myelodysplastic disorders. Both myeloproliferative as well as myelodysplastic disorders are associated with defective immune functioning. This clearly underlines the fact, that appropriate number and type of cells are of utmost importance in smooth functioning of the immune system.
Thus, bone marrow can be considered an organ, which precisely controls and regulates the composition of blood, and consequently it maintains the functions performed by the various cells present in blood !