Bone marrow is the tissue that produces blood cells. It is the soft, spongy material inside the cavities of bones, and in adults the active, blood-forming part sits mainly in the pelvis, spine, ribs, breastbone, and skull. Every day it produces blood cells by the hundreds of billions: red cells to carry oxygen, white cells to fight infection, and platelets to stop bleeding.
In my practice, I find that once patients understand how this tissue works, their blood test results and diagnoses start to make sense. Most hematologic conditions are, at heart, a problem of too much, too little, or abnormal output from the marrow.
What Is Bone Marrow?
Bone marrow fills the spaces inside spongy (trabecular) bone and the central canal of long bones. It is made up of blood-forming cells, fat cells, a framework of supporting cells called stroma, and a rich network of thin-walled blood vessels called sinusoids. New blood cells pass through the sinusoid walls to enter the circulation.
A closer look at bone marrow tissue function and clinical implications shows why disruption at any level of this system, from stem cells to the supporting stroma, can lead to recognizable blood disease.
Red marrow and yellow marrow
There are two types. Red marrow is the active, blood-forming tissue. Yellow marrow is mostly fat and acts as an energy store and a reserve that can convert back to red marrow when the body needs more blood cells, for example after severe blood loss.
| Feature | Red marrow | Yellow marrow |
|---|---|---|
| Main content | Blood-forming cells and stem cells | Fat cells |
| Main job | Hematopoiesis (blood cell production) | Energy storage and reserve capacity |
| Location at birth | Nearly all bones | Very little |
| Location in adults | Pelvis, vertebrae, ribs, sternum, skull, ends of femur and humerus | Shafts of long bones in arms and legs |
| Can it change? | Gradually replaced by fat with age | Can revert to red marrow under stress |
How Bone Marrow Produces Blood Cells
Blood production, called hematopoiesis, begins with hematopoietic stem cells (HSCs). These rare cells can both renew themselves and give rise to every type of blood cell. They live in specialized “niches” in the marrow, where signals from stromal cells and blood vessels keep them in balance.
As stem cells divide, their offspring commit to one of two main branches. The myeloid branch produces red cells, platelets, neutrophils, monocytes, eosinophils, and basophils. The lymphoid branch produces B cells, T cells, and natural killer cells.
The three main production lines
- Erythropoiesis makes red blood cells, driven by the kidney hormone erythropoietin. Red cells live about 120 days.
- Leukopoiesis makes white blood cells. Neutrophils circulate for only hours to a few days, so the marrow must produce them continuously.
- Thrombopoiesis makes platelets, which are fragments shed from large marrow cells called megakaryocytes. Platelets live about 7 to 10 days.
The red bone marrow adjusts its output constantly. After bleeding, erythropoietin rises and red cell production increases; during infection, signals from the immune system boost neutrophil output within days.
Why Bone Marrow Matters for Health
Because blood cells are short-lived, the body depends on the marrow working every single day. A healthy adult CBC reflects this steady output.
| Blood cell | Typical adult reference range | Main role |
|---|---|---|
| Hemoglobin | Men about 13.5–17.5 g/dL; women about 12.0–15.5 g/dL | Oxygen transport |
| White blood cells | About 4,000–11,000 per microliter | Fighting infection |
| Platelets | About 150,000–450,000 per microliter | Blood clotting |
Exact ranges vary slightly between laboratories. The marrow is also a key immune organ: B lymphocytes mature there, and T cell precursors are made there before moving to the thymus. Long-lived plasma cells, which make antibodies, often settle back in the marrow.
Disorders That Affect Bone Marrow
Several conditions impair the marrow’s ability to produce healthy cells. The major groups include:
- Leukemia: a cancer in which abnormal white cells multiply and crowd out normal production.
- Aplastic anemia: marrow failure, usually immune-mediated, leading to low counts of all cell types.
- Myelodysplastic syndromes (MDS): disorders in which the marrow makes poorly formed, dysfunctional cells.
- Multiple myeloma: a cancer of plasma cells within the marrow that can also damage bone.
- Myeloproliferative neoplasms: conditions in which the marrow overproduces one or more cell types.
Symptoms follow from which cells are affected. Low red cells cause tiredness and breathlessness; low white cells cause repeated infections; low platelets cause easy bruising and bleeding. More detail on these bone marrow disorders is available in our in-depth guide.
How Bone Marrow Is Tested
The first step is almost always a complete blood count (CBC) and a blood smear. If these suggest a marrow problem, a direct sample is taken. Bone marrow aspiration draws out liquid marrow to examine individual cells, while a biopsy removes a small core of bone to show the marrow’s architecture and cellularity.
Both are usually done together from the back of the hip bone under local anesthetic, and the procedure typically takes less than half an hour. Our guide to bone marrow aspiration and biopsy explains the steps in detail. Samples may also go for flow cytometry, chromosome studies, and genetic testing, which help classify disease precisely. For background on what the pathologist is looking at, see our overview of the composition and function of bone marrow.
Treatment and Management
Treatment depends on the specific disorder:
- Chemotherapy and targeted drugs for leukemia, myeloma, and some MDS.
- Immunosuppressive therapy for aplastic anemia.
- Stem cell (bone marrow) transplant, which replaces diseased marrow with healthy stem cells.
- Supportive care, including transfusions, antibiotics, and growth factors.
Untreated marrow failure can lead to life-threatening infection, severe anemia, and bleeding, so timely diagnosis matters.
Frequently Asked Questions
Which bones contain the most active bone marrow?
In adults, the pelvis, vertebrae, ribs, sternum, and skull hold most of the active red marrow. This is why marrow samples are usually taken from the back of the hip bone.
Can bone marrow regenerate?
Yes. Stem cells allow the marrow to recover after blood loss, infection, or chemotherapy, often within weeks. Recovery can be incomplete if the stem cells themselves are damaged.
Is bone marrow the only place blood cells are made?
After birth, the marrow is the main site. Before birth, the liver and spleen also make blood cells, and in some severe marrow diseases they can resume this role, called extramedullary hematopoiesis.
Does donating bone marrow harm the donor’s own supply?
No. Donors give only a small fraction of their marrow or circulating stem cells, and the body replaces it over the following weeks.
Key Takeaways
- Bone marrow is the soft tissue inside bones and the main organ that produces blood cells after birth.
- Red marrow makes blood; yellow marrow stores fat and can convert back when demand rises.
- Hematopoietic stem cells give rise to red cells, white cells, and platelets.
- Persistent tiredness, frequent infections, or easy bruising warrant a CBC and a conversation with your doctor.
