The Anatomy of Bone Marrow

Stem cells produce blood cells and platelets in the bone marrow

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Bone marrow is a spongy organ that fills the center of various bones of your body. It is where stem cells produce red and white blood cells and platelets. Without bone marrow, you couldn't move oxygen through your body or fight infections, and blood wouldn't clot.

Bone marrow under the microscope

Steve Gschmeissner / Science Photo Library / Getty Images


Bones provide the primary support and structure for the body, but they also do much more. They have a vital role in maintaining the body's mineral composition and protect vital organs from harm. Bones also house bone marrow, which helps to produce a number of blood cell types that are vital to healthy body function.


There are six main types of bones:

  • Long
  • Short
  • Flat
  • Sesamoid
  • Irregular
  • Sutural

Within these general bone types, there are two different bone structures—cortical and trabecular bones. About 80% of the bones in the body are cortical bones. These bones are the most strong and dense, but play a minor role in metabolism.

Trabecular bones are just 20% of the bones in the body but perform a host of metabolic functions. Bone marrow is found within trabecular bones.

Bone marrow is considered the fourth largest organ in the body by weight, accounting for 4% to 5% of a person's total body weight.

Bone marrow itself is a spongy, jelly-like material found in the center of bones, in a space called the medullary cavity. The cavity is surrounded and protected by a hard layer called the periosteum, which must be punctured or penetrated during a bone marrow biopsy.


Bone marrow is a widely distributed substance in the body, and can be found in all bone cavities at birth,. By the teen years, however, bone marrow is primarily found in axial bones found in the:

  • Chest bones
  • Ribs
  • Spinal bones
  • Collar bone
  • Shoulder blades
  • Skull
  • Pelvic bones
  • Parts of the femur and humerus bones of the legs


Bone marrow serves a crucial function for the body, producing bone marrow stem cells and blood products. The process of the bone marrow creating red blood cells, white blood cells, and platelets is called hematopoiesis. There are two main types of bone marrow, and they each perform specific roles.

Red Bone Marrow

Red bone marrow, also called myeloid tissue, is made up of fibrous tissue that contain hematopoietic cells, or blood-forming stem cells. All red blood cells and platelets in adults are formed within red bone marrow, as well as 60% to 70% of white blood cells.

The remainder of early white blood cells, called lymphocytes, begin formation in red bone marrow and then become fully mature in other parts of the body like the thymus, spleen, and lymph nodes.

A Blood Cell Powerhouse

Bone marrow replaces about 1% of the body's red blood cells daily—creating more than 220 million red blood cells each day. Other blood cells are created here, too, and they must each be replaced regularly. White blood cells have a lifespan of hours to days, platelets must be replaced after about 10 days, and red blood cells can last for roughly 120 days.

Yellow Bone Marrow

Yellow bone marrow is fattier and is home to mesenchymal or marrow stromal cells. These are the stem cells that produce the body's connective tissues like fat, cartilage, muscle, and bone cells.

Yellow bone marrow also stores fat and nutrients for red bone marrow to use and to maintain body functions. If the body is stressed, like during an infection or severe blood loss, yellow bone marrow can transform into red bone marrow and take over its function.

Associated Conditions

Bone marrow has a lot of important functions in the body, and when there is a problem with the production of or function carried out by bone marrow, the effects are widespread. Here are some of the problems that can arise within, and because of problems with, bone marrow:

  • Leukemia: Leukemia is a cancer of the blood, where the bone marrow produces abnormal white cells.
  • Aplastic anemia: In this disease, the bone marrow does not produce red blood cells.
  • Myeloproliferative disorders: These include chronic myelogenous leukemia (CML), polycythemia vera, primary myelofibrosis, essential thrombocytopenia, chronic neutrophilic leukemia, and chronic eosinophilic leukemia. These diseases all impact the production of white blood cells, red blood cells, or platelets.
  • Lymphoma: Lymphoma is a cancer of the immune cells that begins in the lymphocytes.


A number of laboratory tests can be used to provide a general picture of your bone marrow health.

Blood Testing

The primary test that can paint a picture of bone marrow function is a complete blood count (CBC). A CBC will provide a number of red blood cells, white blood cells, platelets, and a number of other specialized blood cells.

This test may also include a reticulocyte count, which measures how often your bone marrow is releasing new red blood cells.

Bone Marrow Aspiration

Bone marrow can also be tested directly. This is done by performing a bone marrow aspiration. During bone marrow aspiration, a long hollow needle is inserted into a bone, typically the hip bone, and marrow is extracted.

Your healthcare provider will numb the area before performing the procedure, but you may still feel pain in that area for a few days after the test.

Bone Marrow Biopsy

Bone marrow aspiration and biopsies usually happen together. You healthcare provider may use two needles, or reposition the same needle. Along with aspirating bone marrow for testing, a biopsy is performed by removing a small piece of bone that contains marrow for additional testing.

If you have a bone marrow aspiration and/or biopsy done, the marrow can be used for a number of tests.

  • Florescence in situ hybridization (FISH): This test examines the chromosome makeup of your bone marrow. It can be used to identify abnormal cells and determine how effective treatments for bone marrow diseases are.
  • Flow cytometry: This test can examine bone marrow cells for specific antibody features.
  • Immunophenotyping: This test that can identify different types of blood cells within a bone marrow sample. It can help find antigen markers on cell surfaces and be used to identify antibodies.
  • Karyotype tests: This test identifies the order, number, and appearance of chromosomes in a bone marrow sample.
  • Polymerase chain reaction: This is a highly sensitive test that examines biomarkers in blood or bone marrow cells. It can be used to detect cancer cells where other tests have failed.
7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  5. Genetic and Rare Diseases Information Center. Chronic myeloproliferative disorders.

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By Rachael Zimlich, BSN, RN
Rachael is a freelance healthcare writer and critical care nurse based near Cleveland, Ohio.