Bone Marrow and Blood Cell Development

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What Is Bone Marrow?

Bone Marrow Broken Finger
This colored scanning electron micrograph (SEM) is showing the internal structure of a broken finger bone. Here, the periosteum (outer bone membrane, pink), compact bone (yellow) and bone marrow (red), in the medullary cavity, can be seen. STEVE GSCHMEISSNER/Science Photo Library/Getty Images

Bone marrow is the soft, flexible connective tissue within bone cavities. A component of the lymphatic system, bone marrow functions primarily to produce blood cells and to store fat. Bone marrow is highly vascular, meaning that it is richly supplied with a large number of blood vessels. There are two categories of bone marrow tissue: red marrow and yellow marrow. From birth to early adolescence, the majority of our bone marrow is red marrow. As we grow and mature, increasing amounts of red marrow is replaced by yellow marrow. On average, bone marrow can generate hundreds of billions of new blood cells every day.

Bone Marrow Structure

Bone marrow is separated into a vascular section and non-vascular sections. The vascular section contains blood vessels that supply the bone with nutrients and transport blood stem cells and mature blood cells away from the bone and into circulation. The non-vascular sections of the bone marrow are where hematopoiesis or blood cell formation occurs. This area contains immature blood cells, fat cells, white blood cells (macrophages and plasma cells), and thin, branching fibers of reticular connective tissue. While all blood cells are derived from bone marrow, some white blood cells mature in other organs such as the spleen, lymph nodes, and thymus gland.

Bone Marrow Function

The major function of bone marrow is to generate blood cells. Bone marrow contains two main types of stem cells. Hematopoietic stem cells, found in red marrow, are responsible for the production of blood cells. Bone marrow mesenchymal stem cells (multipotent stromal cells) produce the non-blood cell components of marrow, including fat, cartilage, fibrous connective tissue (found in tendons and ligaments), stromal cells that support blood formation, and bone cells.

  • 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.

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Bone Marrow Stem Cells

Blood Cell Development
This image show the formation, development, and differentiation of blood cells. OpenStax, Anatomy & Physiology/Wikimedia Commons/CC BY 4.0

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.

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Bone Marrow Disease

Lymphocytes in Hairy Cell Leukemia
Hairy cell leukemia. Colored scanning electron micrograph (SEM) of abnormal white blood cells (B-lymphocytes) from a patient suffering from hairy cell leukemia. These cells show characteristic hair-like cytoplasmic projections and ruffles on their surfaces. Leukemia is a blood cancer in which the blood-producing tissue in bone marrow produces excessive numbers of immature white blood cells, as seen here, which impair the function of normal blood cells. The immune system is thus weakened. Prof. Aaron Polliack/Science Photo Library/Getty Images

Bone marrow that becomes damaged or diseased results in low blood cell production. In bone marrow disease, the body's bone marrow is not able to produce enough healthy blood cells. Bone marrow disease may develop from marrow and blood cancers, such as leukemia. Radiation exposure, certain kind of infections, and diseases including aplastic anemia and myelofibrosis can also cause blood and marrow disorders. These diseases compromise the immune system and deprive organs and tissues of the life giving oxygen and nutrients they need.

A bone marrow transplant may be performed in order to treat blood and marrow diseases. In the process, damaged blood stem cells are replaced by healthy cells obtained form a donor. The healthy stem cells can be obtained from the donor's blood or bone marrow. Bone marrow is extracted from bones located in places such as the hip or sternum. Stem cells may also be obtained from umbilical cord blood to be used for transplantation.

Sources:

  • Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005. Chapter 1, Blood and the cells it contains. Available from: (http://www.ncbi.nlm.nih.gov/books/NBK2263/)
  • What Is a Blood and Marrow Stem Cell Transplant? National Heart, Lung and Blood Insitute. Updated (11/15/11). Available at: http://www.nhlbi.nih.gov/health/health-topics/topics/bmsct/
  • National Cancer Institute: PDQ® Chronic Myelogenous Leukemia Treatment. Bethesda, MD: National Cancer Institute. Date last modified (08/07/3213). Available at: http://cancer.gov/cancertopics/pdq/treatment/CML/Patient. Accessed (09/25/2013>.