Bone marrow
The soft tissue inside our bones is the “factory” of our blood. This topic explains where the marrow is, how it is built, what happens to it in leukemia, and why examining it matters so much.
01Short answer
02Simple explanation
From the outside, bone looks solid, but inside many bones look like a sponge. The soft, reddish tissue that fills the spaces of this sponge is the bone marrow. You can think of it as a blood factory hidden inside the bones, working day and night.
In children, almost all marrow is red and makes blood. With age, the marrow in the middle of the long bones gradually turns into yellow marrow — mostly fatty tissue. In adults, blood-forming red marrow remains in the hip bones, breastbone, vertebrae, ribs, skull and the ends of the thigh and upper-arm bones.
The marrow is not just stem cells. It contains blood cells at every stage of maturity, fat cells, support (stromal) cells and many fine blood vessels. Mature cells pass through these vessels into the blood.
In leukemia, abnormal cells multiply in the marrow and take over the factory floor. Normal blood cells are left with less space and fewer “raw materials”, so the blood may end up with fewer red cells, working white cells and platelets. This is called bone marrow failure.
The good news is that the marrow has a remarkable ability to recover. When treatment reduces the leukemia cells, the normal marrow cells usually start working again, and blood counts begin to rise within a few weeks. During this time doctors check blood tests often and give supportive care, such as transfusions and infection protection, when needed.
03Scientific explanation
Bone marrow fills the central canal of the long bones and the spaces of spongy (trabecular) bone. It has two main components: the hematopoietic compartment (stem cells, progenitors and maturing blood cells) and the stroma — fibroblasts, fat cells, mesenchymal stromal cells, osteoblasts, endothelial cells and the surrounding matrix. The stroma forms the “niche” for stem cells and uses signals to regulate their resting, dividing and maturing.
After birth and in early childhood, red marrow is present in all bones; with age it is replaced by yellow (fatty) marrow in the shafts of the long bones. In adults, red marrow remains mainly in the axial skeleton — skull, vertebrae, ribs, sternum, clavicles and pelvis — and in the upper ends of the femur and humerus. Marrow cellularity (the ratio of blood-forming tissue to fat) falls with age, and the pathologist interprets results in light of the person’s age.
Two main procedures are used for diagnosis. In aspiration, liquid marrow is drawn up through a needle; this sample is used for smears (a myelogram), flow cytometry, cytogenetics (karyotype, FISH) and molecular tests (PCR, NGS). In a trephine biopsy, a small core of bone and marrow is removed to assess the marrow’s structure, cellularity, fibrosis and the pattern of involvement. The sample is usually taken from the back of the hip bone. When the marrow is very fibrotic or tightly packed, no liquid can be drawn (a “dry tap”) — this is seen, for example, in hairy cell leukemia.
In acute leukemias, blasts may occupy most of the marrow; classically, blasts making up ≥20% of the marrow or blood is one of the main criteria for acute leukemia, although modern classifications (WHO 2022, ICC 2022) allow the diagnosis with a lower blast percentage for certain genetically defined subtypes. After treatment, the marrow is examined again: morphologic remission is usually defined by blasts below 5% together with recovery of blood counts, and more sensitive methods look for measurable residual disease (MRD).
04What happens in the body?
Red and yellow marrow
Red marrow makes blood; it gets its colour from the many red cells and their precursors it contains. Yellow marrow mainly stores fat and acts as an energy reserve. Together, red and yellow marrow weigh about 2.5 kilograms in the body. Red marrow dominates in childhood, and the share of yellow marrow grows with age.
How does leukemia change the marrow?
Leukemia cells fill the marrow spaces, crowd out normal stem cells and progenitors, and disturb their environment. In some people this causes bone pain. In the blood, signs of anemia, a tendency to infections and bleeding appear — because the factory cannot turn out enough healthy product.
What does a marrow test show?
A marrow sample shows whether leukemia is present, what type it is, what its genetic features are and how extensive it is. Several tests are run on the same sample: microscopy, flow cytometry, and chromosome and molecular analyses. During treatment, repeat tests help show whether remission has been reached and whether residual disease remains.
Recovery after treatment
Chemotherapy affects normal marrow cells as well as leukemia cells, so blood counts drop for a while. Usually within a few weeks the marrow “fills back in” and healthy cells start moving into the blood again. This period is different for everyone; your care team will follow it closely through blood tests.
05From marrow sample to treatment plan
A marrow examination happens in four steps and answers three key questions for treatment.
06Aspiration and trephine biopsy: the differences
| Feature | Aspiration (liquid marrow) | Trephine biopsy (core of bone and marrow) |
|---|---|---|
| What is taken? | A small amount of liquid marrow | A small core of tissue a few millimetres across |
| What does it show well? | The appearance and number of individual cells | Marrow structure, cellularity, fibrosis |
| Which tests? | Smear, flow cytometry, karyotype, FISH, PCR, NGS | Histology, immunohistochemistry |
| What does it feel like? | A brief, sharp “pulling” sensation is possible | Pressure and a dull ache are possible |
| Time to results | Some results within days; genetic tests take longer | Usually several days or more |
| When is it especially important? | In almost every leukemia diagnosis | When no aspirate can be drawn (“dry tap”) and when fibrosis is suspected |
* Both procedures are often done at the same time, through the same small skin puncture. Timings depend on the laboratory and the test; ask your doctor about preparation and pain relief.
07Key terms
- Bone marrow
- The soft, spongy tissue inside bones; red marrow makes blood.
- Red marrow
- Marrow that produces blood cells.
- Yellow marrow
- Marrow made mostly of fat that does not produce blood.
- Stroma
- The marrow’s supporting tissue: fat cells, fibroblasts and other cells, and vessels.
- Cellularity
- The ratio of blood-forming tissue to fat in the marrow; it falls with age.
- Aspiration
- Drawing up liquid marrow through a needle.
- Trephine biopsy
- Removing a small core of bone and marrow.
- Myelogram
- A percentage count of the different cells in a marrow smear.
- Fibrosis
- An increase in fibrous tissue in the marrow; it can make aspiration difficult.
- Bone marrow failure
- The marrow being unable to make enough healthy blood cells.
08Key points
- Bone marrow is the soft tissue inside some bones; red marrow makes blood, yellow marrow is mostly fat.
- In adults, red marrow is found mainly in the pelvis, spine, breastbone, ribs and skull.
- The marrow makes billions of new blood cells every day.
- Leukemia usually starts in the marrow and crowds out normal blood production.
- Aspiration assesses the cells; a trephine biopsy assesses the marrow’s structure.
- Several tests are run on one sample: microscopy, flow cytometry and genetic analyses.
- The marrow recovers well; after treatment it usually starts working again within a few weeks.
09Frequently asked questions
Is bone marrow the same as the spinal cord?
No — this is a very common mix-up. Bone marrow is the blood-forming tissue inside bones. The spinal cord is part of the nervous system and runs inside the spinal canal. A bone marrow test does not touch the spinal cord; the sample is usually taken from the hip bone.
Does a marrow test hurt?
The skin and the surface of the bone are numbed with a local anaesthetic. Many people describe pressure as the needle enters the bone, and a brief, sharp pulling sensation during aspiration, which usually lasts a few seconds. There may be some soreness for a few days afterwards. If you are anxious, ask in advance about sedation or pain relief.
Why is the test repeated?
During treatment, a marrow test shows whether leukemia cells are decreasing, whether remission has been reached and whether a very small amount of disease remains (MRD). This information is important for planning the next stage of treatment. Your doctor decides when and how often it is needed.
Can the marrow “run out”?
Healthy marrow constantly renews itself. After intensive treatment, recovery takes time, and supportive care is important during this period. Very rarely the marrow does not recover well; if so, doctors look for the cause and offer other options. Only your care team can assess your own situation.
If my blood count is normal, do I still need a marrow test?
Sometimes, yes. A blood test does not fully reflect what is happening in the marrow: for example, a marrow sample may be needed to assess how deep a remission is or to measure MRD. In some cases the tests can be done on blood instead. The decision depends on the type of leukemia and the stage of treatment.
Does bone pain always mean leukemia in the marrow?
No. Bone and joint pain has many causes, and most of them have nothing to do with leukemia. In leukemia, such pain can be linked to the marrow being packed with cells. Tell your doctor about any new or worsening pain.
10Questions for your doctor
Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.
What percentage of my bone marrow is leukemia cells?
When will the bone marrow test need to be repeated?
What can be done to reduce pain during a bone marrow test?
How does the state of my marrow affect the choice of treatment?
12Sources
- Cleveland Clinic. Bone Marrow: What It Is & Function
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: bone marrow
- National Cancer Institute (NCI), SEER Training. Structure of Bone Tissue
- American Cancer Society. Blood Cells and Bone Marrow — Anatomy Gallery
- American Cancer Society. What Is Acute Myeloid Leukemia (AML)? — normal bone marrow, blood and lymph tissue
- MedlinePlus (U.S. National Library of Medicine). Bone marrow aspiration
- MedlinePlus (U.S. National Library of Medicine). Bone marrow biopsy
- MedlinePlus (U.S. National Library of Medicine). Bone Marrow Diseases
- National Cancer Institute (NCI). Adult Acute Myeloid Leukemia Treatment (PDQ®) — Patient Version
- National Cancer Institute (NCI). Hairy Cell Leukemia Treatment (PDQ®) — Health Professional Version
- American Society of Hematology (ASH). Blood Basics