Raw bone marrow is the living, unprocessed tissue inside the cavities of bones where blood cells are made. It is a soft mix of blood-forming hematopoietic stem cells, developing blood cells, fat cells and a supporting network of stromal cells and blood vessels. In adults, active marrow sits mainly in the pelvis, spine, ribs, sternum and skull, and it produces every red cell, white cell and platelet the body needs.
For clinicians, understanding bone marrow in its native state underpins the diagnosis and treatment of many hematological disorders, from anemia to haematologic malignancies. This guide covers its structure, function, sampling and the disorders that affect it.
What Is Raw Bone Marrow?
The term describes marrow as it exists in the body or as first drawn out, before it has been processed, filtered or separated into components. Marrow makes up a meaningful share of body weight in adults, and it is one of the most productive tissues we have: a healthy adult makes on the order of 200 billion red blood cells a day.
Marrow comes in two main forms. Red marrow is the active tissue that produces blood cells. Yellow marrow is mostly fat. At birth nearly all marrow is red; with age, yellow marrow gradually replaces it in the long bones. Under heavy demand, such as severe hemolysis, yellow marrow can partly convert back to red.
| Feature | Red marrow | Yellow marrow |
|---|---|---|
| Main content | Hematopoietic cells, stroma, vessels | Fat cells (adipocytes) |
| Main adult sites | Pelvis, vertebrae, sternum, ribs, skull, ends of femur and humerus | Shafts of long bones |
| Function | Blood cell production | Energy store; reserve for blood production |
| Change with age | Decreases | Increases |
Structure and Function
Hematopoietic stem cells are rare but can both renew themselves and give rise to all blood lineages. They produce myeloid progenitors, which become red cells, platelets, granulocytes and monocytes, and lymphoid progenitors, which become B cells, T cells and natural killer cells.
These cells live in specialized niches. Stromal cells, osteoblasts lining the bone, endothelial cells of the marrow sinusoids and the extracellular matrix all send signals that control whether a stem cell rests, divides or differentiates. Growth factors guide each lineage: erythropoietin from the kidney drives red cell production, thrombopoietin from the liver drives platelet production, and colony-stimulating factors drive white cell production. The healthy balance of these signals explains normal bone marrow function.
Platelets come from large marrow cells called megakaryocytes, which shed platelets directly into the sinusoids. The biology of platelet production is an active field of research, including how megakaryocytes sense their physical surroundings.
Sampling and Examining Marrow
Marrow is examined when the blood count or film cannot explain a problem. The standard procedure is bone marrow aspiration and biopsy, usually from the posterior iliac crest under local anesthesia. It takes about 20 to 30 minutes.
- Aspirate: liquid marrow drawn into a syringe. It is smeared on slides to assess cell shape and proportions, and sent for flow cytometry, cytogenetics and molecular tests.
- Trephine biopsy: a small core of bone and marrow. It shows cellularity, architecture, fibrosis and infiltration by abnormal cells.
Normal marrow cellularity falls with age. A rough rule is that the percentage of marrow occupied by blood-forming cells is about 100 minus the patient’s age, so a 60-year-old would be expected to have roughly 40 percent cellularity. A normal myeloid-to-erythroid (M:E) ratio is roughly 2:1 to 4:1. Blasts normally make up less than 5 percent of nucleated cells.
| Test on marrow | What it shows |
|---|---|
| Morphology (aspirate smear) | Cell types, maturation, dysplasia, blast percentage |
| Trephine histology | Cellularity, fibrosis, infiltration, architecture |
| Flow cytometry | Surface markers and clonal populations |
| Cytogenetics and FISH | Chromosome changes, e.g. translocations |
| Molecular testing (e.g. NGS) | Gene mutations that guide diagnosis and prognosis |
| Iron stain | Iron stores and ring sideroblasts |
How samples are handled matters as much as how they are taken. Our guide to bone marrow samples covers anticoagulant choice and sample quality in more detail.
Disorders That Affect the Marrow
Bone marrow disorders fall into a few broad groups:
- Marrow failure: aplastic anemia, often immune-mediated, and inherited syndromes such as Fanconi anemia.
- Clonal myeloid disorders: myelodysplastic syndromes (MDS), myeloproliferative neoplasms and acute myeloid leukemia (AML).
- Lymphoid malignancies: acute lymphoblastic leukemia, lymphomas that involve the marrow, and multiple myeloma.
- Infiltration: metastatic cancer, granulomas from infection, or fibrosis that crowds out normal cells.
- Nutritional causes: vitamin B12 or folate deficiency, which produce a megaloblastic marrow.
Clinically, marrow disease shows itself through cytopenias: anemia causes fatigue and breathlessness, low neutrophils bring infections, and low platelets cause bruising and bleeding. Some disorders instead raise counts, as in the myeloproliferative neoplasms.
Clinical Uses of Marrow
Beyond diagnosis, marrow is a therapy. Hematopoietic stem cell transplantation (HSCT) replaces diseased or damaged marrow with healthy stem cells. Stem cells may be collected directly from the marrow by repeated aspiration from the pelvic bones under anesthesia, or, more often today, from the bloodstream after mobilization with growth factors. Donors may be the patient themselves (autologous) or a matched donor (allogeneic).
HSCT is a cornerstone for many acute leukemias, severe aplastic anemia and selected inherited disorders. Supportive care, including transfusion and growth factors, remains essential for many patients, and targeted drugs, such as tyrosine kinase inhibitors in chronic myeloid leukemia, have changed the outlook for several marrow cancers. For a broader view of the field, see our guide to hematology.
Key Takeaways
- Raw bone marrow is the unprocessed, living tissue where all blood cells are made.
- Red marrow is active; yellow marrow is mostly fat and expands with age.
- Aspiration and trephine biopsy provide complementary information.
- Cellularity, M:E ratio and blast percentage are core parameters in marrow reports.
- Marrow is both a diagnostic window and, through transplantation, a treatment.
Frequently Asked Questions
Where is bone marrow sampled from?
In adults the posterior iliac crest at the back of the pelvis is the usual site because it is safe and rich in red marrow. The sternum can be used for aspiration only, and in infants the tibia is sometimes used.
Does a bone marrow biopsy hurt?
Local anesthetic numbs the skin and bone surface, but most people feel a brief deep pressure or pulling sensation during aspiration. Soreness at the site for a few days is common and usually eases with simple painkillers.
Why does marrow cellularity fall with age?
Fat gradually replaces active marrow throughout life. This is a normal change, which is why cellularity is always interpreted against the patient’s age.
Is eating animal bone marrow good for blood health?
Culinary bone marrow is mostly fat and is not a treatment for any blood disorder. A balanced diet with adequate iron, vitamin B12 and folate is what supports healthy blood production.
