By Dr Ananya Mandal, MD
The bone marrow is a soft spongy tissue within the bones. It is the seat of production of hematopoietic cells. These cells include red blood cells or erythrocytes, white blood cells like granulocytes and myelocytes and platelets like thrombocytes.
The sites of bone marrow location include the sternum (middle of the chest), pelvis (hip bone), and femur (thigh bone).
The bone marrow contains stem cells that are primitive cells capable of turning into any desired cell in the body. As needed, the stem cells differentiate to become a particular kind of cell - a white blood cell, red blood cell, or platelet. From the bone marrow only the mature cells are released into the blood stream. Apart from the stem cells the bone marrow contains supporting fibrous tissues as well.
Main bone marrow problems
Diseases of the bone marrow may lead to an abnormality in the production of any of the mature blood cells, or their precurosor or predecessor immature cells. The main types of problems with the bone marrow include:
increased production of one type of cell - this packs up the marrow with one type of cells and decreases the production of the other cell types
increase in one cell line because the cells do not die at normal times
producing immature cells that do not mature or function properly
producing fragile cells that die easily or producing less number of cells
increased growth of the supporting fibrous tissue network leading to formation of abnormal cells and decreased numbers of cells.
lack of iron making RBC formation difficult
spread of other diseases to the bone marrow
Diseases of the different cell types
Cell types and functions:
White blood cells (WBCs)
WBCs are of five different types:
neutrophils (also called granulocytes)
These play different roles in infection prevention and protection of the body. For example, Neutrophils, basophils, and eosinophils kill and digest bacteria. Monocytes kill bacteria but also are produced more rapidly than the neutrophils and tend to be longer lived.
Lymphocytes are of two types B and T. T cells distinguish between own and foreign and B cells that circulate in the blood, produce antibodies - proteins that attach to specific antigens from the invading bacteria or viruses.
Red blood cells (RBCs)
RBCs are disc shaped small cells without a nucleus. They carry iron in a heme protein called hemoglobin. Hemoglobin allows RBCs to carry oxygen to tissues throughout the body.
Platelets or thrombocytes
These are small parts of large cells called megakaryocytes. Platelets circulate in blood and help in clotting process to plug holes in leaking blood vessels and to help activate other clotting factors.
Diseases and disorders of the bone marrow
The diseases and disorders of the bone marrow include Leukemia, Myelodysplastic Syndrome, Myeloproliferative disorders and so forth.
Leukemia is a cancer of the white blood cells that can affect any of the five WBC types. The cancer affects a line of cell that begins to replicate non-stop clogging the bone marrow and decreasing production of other cells.
The resulting leukemic cloned cells do not function normally. They do not fight infections.
Patients with leukemia may have frequent infections, anemia, bleeding, bruising, night sweats, and bone and joint pain.
The spleen that normally filters the blood and gets rid of old cells, may become enlarged. Lymph nodes that house the WBCs may also enlarge.
Blood picture shows immature cells from the bone marrow called blast cells. These are released due to excess production within the bone marrow.
Myelodysplastic Syndrome (MDS)
MDS is a group of diseases where there is abnormal bone marrow cell production. There are not enough normal blood cells being made. This leads to anemia, bleeding and risk of infections.
MDS syndromes are classified by how the cells in the bone marrow and blood smear look under the microscope. This includes anemias that are resistant to treatment, those that are inherited or genetic and a complex form of MDS. Over time, MDS tends to progress to acute myeloid leukemia.
Myeloproliferative disorders (MPD)
“Myelo” means bone marrow and MPD signifies proliferation of the bone marrow. These are a group of diseases.
There is overproduction of a precursor (immature form) of a marrow cell. This results in release of the immature forms of other precursors as well that are released in blood as blast forms when the body requires them.
The bone marrow in MPD shows a mixture of cells in various stages of maturity.
This is a condition where there is loss or suppression of production of RBCs. This may be due to a defect in the stem cell producing them or due to an injury to the bone marrow environment.
The injury may result from exposure to chemicals such as benzene, radiation, or certain drugs or may be genetic e.g. Fanconi's anemia or associated with a viral infection with parvo virus.
Iron deficiency anemias
Iron deficiency anemias lead to the formation of deformed and smaller RBCs released from the marrow. These are pale and small and are called microcytic RBCs.
Anemias may also be caused by deficiency or dysfunction of erythropoietin, a chemical produced by the kidneys that stimulates RBC production.
Other diseases and disorders of the bone marrow
Other diseases and disorders of the bone marrow include:
Disorders of plasma cells or Plasma cell dyscrasia – There is overproduction of one clone of a B lymphocyte and its antibody protein.
Lymphomas and other cancers that spread into the marrow and affect cell production.
Thrombotic thrombocytopenic purpura – This results in the reduction of platelet production and bleeding tendencies.
Unexplained cytopenia – This results in decrease in production of all cell types.
Other causes include small cell tumors of childhood, Mast cell disease, Disseminated granulomatous disease, Primary amyloidosis, Metabolic bone disease etc.
Bone marrow depression may be caused due to cancer chemotherapy, bone marrow transplantation and cancer radiation therapy.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)