A Complete Guide to Your Bone Marrow

Medically reviewed by Kashif J. Piracha, MD

In 1868, Drs. Ernst Neumann and Giulio Bizzozero first proposed that bone marrow—the soft tissue inside certain bones of the body—is where blood is made. Since then, research on bone marrow has evolved and experts learned that your bone marrow is a sponge-like tissue that houses stem cells. Your stem cells produce three major types of blood cells: platelets, red blood cells, and white blood cells.

When your bone marrow functions properly, it plays a vital role in several body functions—like delivering oxygen to cells and fighting off infection or disease. But when you develop certain health conditions, like leukemia (a type of blood cancer) or anemia (a lack of healthy red blood cells), it may be a sign that your bone marrow isn't functioning as normal.

Anatomy of the Bone Marrow

The skeleton, which accounts for about 15% of your body weight, consists of several different types of bones. You have two specific bone structures called cortical and trabecular. Cortical bones represent 80% of your bone mass and are tougher and denser than other bones. Trabecular bones are softer, fibrous, and house bone marrow. Bone marrow is one of your body’s largest organs. In adults, bone marrow alone makes up about 5% of your body weight.

Structure

Bones consist of three types of cells: osteocytes (make up mature and hard bones), osteoblasts (cells that turn into bone), and osteoclasts (which break down and recycle bone material). Bone marrow occupies a space inside your bones called the medullary cavity. This is surrounded by a fibrous tissue (called periosteum) with blood vessels that run through it.

There are two types of bone marrow:

• Red bone marrow: Red bone marrow (also called myeloid tissue) is a tissue that contains hematopoietic cells (or cells that help produce blood). These stem cells are pluripotent, meaning they can turn into different kinds of blood cells. At birth, all of your bone marrow is red. As you get older, the amount of red bone marrow you have drops to about half.

• Yellow bone marrow: This type of bone marrow is fattier and consists of mesenchymal cells, which produce connective tissues in the body, like tendons, fat, or cartilage. As you get older, this type of bone marrow gradually replaces red bone marrow during a process called conversion.

Location

When you're born, all your bones contain bone marrow. By adolescence, only the axial bones have it. Your axial bones include:

• Sternum (breast bone)

• Ribs

• Neck and spine bones (vertebrae)

• Clavicles (collar bones)

• Scapulae (shoulder blades)

• Pelvis

• Skull

Around the age of 25, red bone marrow is primarily found in the sternum, shoulder blades, clavicles, pelvis, and spine. The rest of your bones develop yellow bone marrow as you age.

What Does the Bone Marrow Do?

While your bone marrow is responsible for protecting your body and organs, it also plays a role in your metabolism (how your cells use energy), immune function, and hematopoiesis—or, the production of blood cells and cell components. Red and yellow bone marrow both have separate functions.

Red Bone Marrow Function

The fibrous tissue that makes up red bone marrow is rich in hematopoietic stem cells. In the bone marrow, these cells form into several types of blood cells and have different functions, such as:

• Oxygen delivery: Red blood cells (RBCs) deliver oxygen from the lungs to the body and bring carbon dioxide back. In adults, all RBCs form in red bone marrow, which produces approximately two billion RBCs a day.

• Immune function: The hematopoietic cells can also turn into white blood cells (WBCs), which fight off infection and help repair damaged cells. Red bone marrow produces most of your body’s WBCs. This type of bone marrow also produces lymphocytes, which are early (or "younger") white blood cells that mature in the lymph node or other parts of the body.

• Repair: Specialized cells called megakaryocytes form into platelets in the red bone marrow. Platelets form blood clots to stop bleeding, making them critical for repairing your body from damage and injury.

Yellow Bone Marrow Function

Yellow bone marrow is the storehouse of mesenchymal stem cells, which are cells that turn into cartilage, muscle, fat (adipocytes), bone (osteocytes), and other tissues in the body. These are critical for growing, repairing, and maintaining your body. Yellow bone marrow also helps with metabolism, stores energy for your cells to use, and regulates nutrition for red bone marrow, bones, and organs throughout your body.

If your body experiences severe distress, such as having a disease or undergoing extreme blood loss, yellow bone marrow can turn into red bone marrow to increase blood production. This is called reconversion.

Associated Health Conditions

Given its vital role in many important functions, bone marrow diseases or disorders can have a serious effect on your overall health. The following conditions often occur either due to problems with the stem cells themselves or the process of producing them:

• Leukemia: With leukemia (a type of blood cancer), the bone marrow produces abnormal blood cells. More often, this is a cancer of white blood cells from either the lymphoid or myeloid lineage.

• Aplastic anemia: This rare condition occurs when your bone marrow doesn’t produce enough red blood cells. With aplastic anemia, there isn’t enough oxygen in your blood, which may cause dizziness and fatigue, among other symptoms.

• Polycythemia vera: This condition is a myeloproliferative disorder, meaning the bone marrow produces too many cells. With polycythemia vera, there’s an excess of red blood cells that cause your blood to thicken.

• Thrombocythemia: Another myeloproliferative disorder, thrombocythemia occurs when the marrow makes too many megakaryocytes, which produce blood platelets.

• Myelofibrosis: This type of cancer occurs when the bone marrow becomes replaced by scar tissue. This leads to inadequate blood cell production which can often lead to complications.

• Lymphoma: Lymphoma is a cancer of the lymph system (which is part of the immune system) and affects your lymph nodes.

Diagnostic Testing

Healthcare providers rely on blood tests and laboratory tests to assess bone marrow. A complete blood count (CBC) is the most common and calculates your total red and white blood cell, platelet, and hemoglobin content. The reticulocyte count is another test that tracks immature red blood cells in the bone marrow.

If your provider suspects something wrong with your bone marrow, they can also take samples of your bone and bone marrow to send it in for testing. A common diagnostic test is a bone marrow aspiration which uses a long needle to collect liquid from your bone marrow in the hip bone. Your provider may also recommend a bone marrow biopsy to remove a small amount of bone that contains bone marrow.

A bone marrow aspiration and biopsy are often done side by side, and pathologists (specialists in identifying diseases) employ several clinical tests to assess these samples, including:

• Fluorescence in-situ hybridization (FISH): Maps genetic material in a sample, detects abnormal cells, and checks if treatments for underlying health conditions are working

• Flow cytometry: Checks the presence or absence of certain proteins and helps diagnose or assess the severity of blood cancers

• Immunophenotyping: Provides detailed information about specific blood cells to detect some conditions

• Karyotype test: Assesses the chromosomes in bone marrow, which lets healthcare providers tailor treatment

• Polymerase chain reaction (PCR): Tracks biomarkers (biological signs) of specific cancers, helping providers to diagnose these conditions when other tests aren't effective

Tips for Keeping Your Bone Marrow Healthy

To help preserve the health and function of your bone marrow, consider adopting the following strategies:

• Taking certain vitamin supplements or seeking dietary sources of vitamins B9, B12, C, D, K, calcium, and iron

• Eating more protein, such as fish, beans, and lean meats

• Consuming whole grains, vegetables, and fruit

• Reducing your intake of packaged or processed foods or foods with added sugars or salts

• Incorporating strength training exercises to help improve the strength of your bones

A Quick Review

Bone marrow is a spongy-like substance found in certain bones of your body. This material produces stem cells and plays an important role in your blood cell production and immune system function. Some conditions, however, can affect how well your bone marrow functions. These conditions include leukemia, lymphoma, and anemia. Getting treatment for these conditions and adopting lifestyle habits like eating a nutritious diet can help preserve the health of your bone marrow.

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