• Bone Marrow is the soft, highly vascular and flexible connective tissue within bone cavities which serve as the primary site of new blood cell production or hematopoiesis.
• In adult humans, bone marrow is primarily located in the ribs, vertebrae, sternum, and bones of the pelvis.

• The majority of the cell types involved in the immune system is produced from a common hemopoietic stem cell (HSC).
• HSC are found in the fetal liver, fetal spleen and neonate and adult bone marrow.
• Bone marrow is the primary source of pluripotent stem cells that give rise to all hemopoietic cells (blood cells) including lymphocytes.
• As a part of the lymphatic system, it is the major organ for B cell maturation and gives rise to the precursor cells of the thymic lymphocytes.
• The thymus and the bone marrow are primary lymphoid organs as T and B cells must first undergo maturation in these organs/tissues before migrating to the secondary lymphoid tissues, such as the spleen, lymph nodes and mucosa associated lymphoid tissues (MALT).
• Staring from the last months of fetal development when bone marrow becomes the dominant site of hemopoiesis (blood cell formation), the great majority of cells involved in mammalian immunity are derived from precursors in the bone marrow

Types of Bone Marrow

There are two categories of bone marrow tissue: red marrow and yellow marrow. Most of the bone marrow during birth to early adolescence is red marrow while the red marrow is replaced with yellow with age.

Red Marrow

In adults, red marrow is confined mostly to skeletal system bones of the skull, pelvis, spine, ribs, sternum, shoulder blades, and near the point of attachment of the long bones of the arms and legs. Not only does red marrow produce blood cells, but it also helps to remove old cells from circulation. Other organs, such as the spleen and liver, also filter aged and damaged blood cells from the blood. Red marrow contains hematopoietic stem cells that produce two other types of stem cells: myeloid stem cells and lymphoid stem cells. These cells develop into red blood cells, white blood cells, or platelets. (See, bone marrow stem cells).

Yellow Marrow

Yellow marrow consists primarily of fat cells. It has poor vascular supply and is composed of hematopoietic tissue that has become inactive. Yellow marrow is found in spongy bones and in the shaft of long bones. When blood supply is extremely low, yellow marrow can be converted to red marrow in order to produce more blood cells.

Structure of Bone Marrow

The structure of bone marrow constitutes of hematopoietic tissue islands and adipose cells surrounded by vascular sinuses interspersed within a meshwork of trabecular bone.

• The bone marrow is composed of both cellular and non-cellular components and structurally be divided into vascular and non-vascular regions.
• The non-vascular section of bone marrow is composed of hemopoietic cells of various lineages and maturity, packed between fat cells, thin bands of bony tissue (trabeculae), collagen fibers, fibroblasts and dendritic cells. This is where hematopoiesis takes place.
• The vascular section contains blood vessels that supply the bone with nutrients and transport blood stem cells and formed mature blood cells away into circulation.
• Ultrastructural studies show hemopoietic cells cluster around the vascular sinuses where they mature, before they eventually are discharged into the blood.
• Lymphocytes are found surrounding the small radial arteries, whereas most immature myeloid precursors are found deep in the parenchyma.

Bone Marrow Stem Cells

Red bone marrow contains hematopoietic stem cells that produce two other types of stem cells: myeloid stem cells and lymphoid stem cells. These cells develop into red blood cells, white blood cells, or platelets.

Myeloid Stem Cells

Develop into red blood cells, platelets, mast cells, or myeloblast cells. Myeloblast cells develop into granulocyte and monocyte white blood cells.

Red Blood Cells—also called erythrocytes, these cells transport oxygen to body cells and deliver carbon dioxide to the lungs.

Platelets—also called thrombocytes, these cells develop from megakaryocytes (huge cells) that break into fragments to form platelets. They aid in the blood clotting process and tissue healing.

Myeloblast Granulocytes (white blood cells)—develop from myeloblast cells and include neutrophils, eosinophils, and basophils. These immune cells defend the body against foreign invaders (bacteria, viruses, and other pathogens) and become active during allergic reactions.

Monocytes—these large white blood cells migrate from blood to tissues and develop into macrophages and dendritic cells. Macrophages remove foreign substances, dead or damaged cells, and cancer cells from the body by phagocytosis. Dendritic cells aid in the development of antigen immunity by presenting antigenic information to lymphocytes. They initiate primary immune responses and are commonly found in the skin, respiratory tract, and gastrointestinal tract.

Mast Cells—these white blood cell granulocytes develop independently from myeloblast cells. They are found throughout body tissues, particularly in the skin and lining of the digestive system. Mast cells mediate immune responses by releasing chemicals, such as histamine, stored in granules. They aid in wound healing, blood vessel generation, and are associated with allergic diseases (asthma, eczema, hay fever, etc.)

Lymphoid Stem Cells

Develop into lymphoblast cells, which produce other types of white blood cells called lymphocytes. Lymphocytes include natural killer cells, B lymphocytes, and T lymphocytes.

Natural Killer Cells—these cytotoxic cells contain enzymes that cause apoptosis (cellular self-destruction) in infected and diseased cells. They are components in the body's innate immune response protecting against pathogens and tumor development.

B Cell Lymphocytes—these cells are important for adaptive immunity and long lasting protection against pathogens. They recognize molecular signals from pathogens and produce antibodies against specific antigens.

T Cell Lymphocytes—these cells are active in cell-mediated immunity. They help to identify and destroy damaged, cancerous, and infected cells.

Function of Bone Marrow

• The bone marrow gives rise to all of the lymphoid cells that migrate to the thymus and mature into T cells, as well as to the major population of conventional B cells.
• B cells mature in the bone marrow and undergo selection for non-self before making their way to the peripheral lymphoid tissues.
• Since the bone marrow constitutes of the hemopoietic cells derived from multipotential stem cells, they not only give rise to all of the lymphoid cells found in the lymphoid tissue, but also to all of the cells found in the blood.
• Platelets, which are crucial for the blood clotting process, are formed from bone marrow just like other blood cells.
• Yellow marrow is actively involved in lipid storage.