Bone marrow is the soft tissue inside your bones that makes nearly all of your blood cells. In adults it is found mainly in the pelvis, spine, ribs, breastbone, and skull, and those same sites are where doctors go when they need a sample. Its clinical importance is simple: when the marrow fails, is invaded by cancer, or is scarred, every blood count can fall at once, and examining or replacing the marrow becomes central to diagnosis and cure.
As a specialist in haematology, I am often asked where to get bone marrow, both for testing and for transplant. This guide covers where marrow lives, what it does, and why it matters so much in medicine.
What Is Bone Marrow and What Does It Do?
Bone marrow fills the hollow spaces inside bones and is the main site of hematopoiesis, the continuous production of blood cells. At its core are hematopoietic stem cells, rare cells that can both renew themselves and mature into every blood cell type.
A healthy adult marrow produces around 200 billion red blood cells a day, alongside large numbers of white cells and platelets. This output has to be high because the cells are short-lived: red cells last about 120 days, platelets about 7 to 10 days, and neutrophils only hours to a day in the circulation.
| Cell made in the marrow | Main job | Typical lifespan in circulation |
|---|---|---|
| Red blood cells | Carry oxygen | About 120 days |
| Neutrophils (white cells) | Fight bacterial infection | Hours to about a day |
| Lymphocytes (white cells) | Immune memory and antibody production | Weeks to years |
| Platelets | Start blood clotting | About 7 to 10 days |
Beyond blood production, the marrow also houses immune cells, stores iron, and provides a supportive “niche” of stromal cells, blood vessels, and signaling molecules that keep stem cells healthy.
Where Is Bone Marrow Found? Red and Yellow Marrow
Marrow comes in two forms. Red bone marrow is the active, blood-forming tissue. Yellow bone marrow is mostly fat cells, though it can convert back to red marrow when the body needs more blood, for example after major blood loss or in chronic hemolytic anemia.
At birth nearly all marrow is red. During childhood, red marrow in the long bones of the arms and legs is gradually replaced by yellow marrow. By adulthood, active marrow is concentrated in the central skeleton.
| Feature | Red marrow | Yellow marrow |
|---|---|---|
| Main content | Blood-forming stem and precursor cells | Fat cells |
| Adult locations | Pelvis, vertebrae, ribs, sternum, skull, ends of femur and humerus | Shafts of long bones |
| Role | Daily blood cell production | Energy store; reserve that can revert to red marrow |
Sources of Bone Marrow in Clinical Practice
“Where to get bone marrow” has two answers depending on the purpose.
For diagnosis
Bone marrow aspiration and biopsy are usually taken from the posterior iliac crest, the back of the hip bone. It is close to the skin, rich in red marrow, and away from vital organs. The sternum can be used for aspiration alone in adults, and the tibia is sometimes used in infants.
For transplantation
Blood-forming stem cells for transplant come from three sources:
- Bone marrow harvest: marrow is drawn from both hip bones under general anesthesia.
- Peripheral blood stem cells: growth-factor injections push stem cells out of the marrow into the blood, where a machine collects them. This is now the most common source for adult transplants.
- Umbilical cord blood: stem cells saved from the placenta and cord after birth, useful when no matched adult donor is available.
What Goes Wrong: Bone Marrow Disorders
Many factors can disrupt bone marrow function and lead to hematological disorders. Broadly, the marrow can fail to produce, produce abnormally, or be crowded out.
- Marrow failure: aplastic anemia, where stem cells are damaged, often by an autoimmune attack.
- Clonal disorders: leukemias, myelodysplastic syndromes, and myeloproliferative neoplasms. Mutations in genes such as JAK2, CALR, or MPL drive many myeloproliferative neoplasms.
- Scarring: in primary myelofibrosis, fibrous tissue replaces normal marrow, and blood production shifts to the spleen, which enlarges.
- Infiltration: myeloma, lymphoma, or spread from solid cancers occupying marrow space.
- External damage: chemotherapy, radiation, certain chemicals such as benzene, and some infections.
- Nutritional causes: vitamin B12, folate, or iron deficiency impairing cell production.
Symptoms follow the missing cells. Low red cells cause fatigue and breathlessness, low neutrophils bring frequent infections, and low platelets cause easy bruising or bleeding. Fevers, night sweats, and weight loss suggest a malignant process.
How Doctors Assess the Marrow
The starting point is a complete blood count (CBC) and a blood film. If these point to a marrow problem, a marrow sample is examined under the microscope and with specialized tests.
- Flow cytometry identifies abnormal cell populations by their surface markers.
- Cytogenetics and molecular testing detect chromosome changes and gene mutations.
- Imaging such as MRI can show marrow infiltration or replacement across the skeleton.
A normal adult marrow has fewer than 5% blast cells, and its cellularity is roughly 100 minus the patient’s age as a percentage.
Treatment and Why Marrow Matters Clinically
Treatment depends on the cause: vitamin or iron replacement for deficiencies, immunosuppression for many cases of aplastic anemia, targeted drugs such as JAK inhibitors for myeloproliferative neoplasms, and chemotherapy for leukemia. Transfusions support patients while the marrow recovers.
When the marrow itself must be replaced, stem cell transplantation is used. An autologous transplant uses the patient’s own stored cells to rescue the marrow after high-dose chemotherapy. An allogeneic transplant uses a donor’s cells and adds an immune effect against residual cancer. It can cure some leukemias and marrow failure syndromes.
Research into the marrow microenvironment, and into gene therapy and gene editing for inherited disorders, is steadily widening what can be treated.
Key Takeaways
- Bone marrow makes red cells, white cells, and platelets from a small pool of stem cells.
- In adults, active red marrow sits mainly in the pelvis, spine, ribs, sternum, and skull.
- Diagnostic samples usually come from the back of the hip; transplant stem cells come from marrow, blood, or cord blood.
- Marrow disorders often show up as low blood counts with fatigue, infections, or bleeding.
- See a doctor if these symptoms persist or a blood test shows unexplained abnormal counts.
Frequently Asked Questions
Which bones contain the most bone marrow?
In adults, the pelvis holds the largest share of active red marrow, followed by the vertebrae, ribs, sternum, and skull. This is why the hip bone is the standard site for marrow sampling and donation.
Can bone marrow regenerate after donation?
Yes. The marrow removed during donation is only a small fraction of the total, and the body replaces it within a few weeks. Donors do not lose their ability to make blood.
Is yellow marrow useless?
No. Yellow marrow stores fat as an energy reserve and can convert back into blood-forming red marrow when demand rises sharply, such as after severe blood loss.
What is the difference between bone marrow and stem cells?
Bone marrow is the whole tissue, including fat, support cells, and developing blood cells. Hematopoietic stem cells are a tiny subset within it, and they are the cells that actually rebuild the blood system after a transplant.
