Yellow bone marrow is the fat-rich marrow that fills the hollow central shafts of the long bones in adults, such as the femur, humerus, and tibia. Its main job is storing fat as an energy reserve, but it also acts as a backup blood factory: when the body needs more blood cells, yellow marrow can convert back into active red marrow. Changes in the balance between the two types show up in aging, anemia, marrow disorders, and osteoporosis.
In hematology, one question I hear often is “where is yellow bone marrow found?” This guide covers its location, what it does, how it changes over a lifetime, and why clinicians pay attention to it.
What Is Yellow Bone Marrow?
Yellow marrow is one of the two types of bone marrow. It is made mainly of adipocytes (fat cells), which give it its yellow color, supported by a network of connective tissue and blood vessels.
Its counterpart, red bone marrow, is where hematopoiesis (blood cell production) takes place. Red marrow is packed with developing red cells, white cells, and platelet-producing megakaryocytes, and it owes its color to hemoglobin in the red cell precursors.
| Feature | Red marrow | Yellow marrow |
|---|---|---|
| Main content | Blood-forming cells | Fat cells |
| Main function | Produces red cells, white cells, and platelets | Stores fat; reserve capacity for blood production |
| Adult location | Spine, pelvis, sternum, ribs, skull, and the upper ends of the femur and humerus | Shafts of long bones and the bones of the hands and feet |
| Blood supply | Rich | Less extensive |
| Appearance on T1-weighted MRI | Darker (intermediate signal) | Bright, similar to fat under the skin |
Where Is Yellow Bone Marrow Found?
At birth, virtually all marrow is red, because a growing child needs enormous blood cell output. Over childhood and adolescence, red marrow is gradually replaced by yellow marrow in a predictable pattern, starting in the hands and feet and moving from the ends of the limbs toward the trunk.
By early adulthood, roughly half of the total marrow space is yellow. In adults, yellow marrow occupies:
- The medullary cavity (central shaft) of long bones such as the femur, tibia, humerus, radius, and ulna
- Most of the small bones of the hands and feet
- The far (distal) ends of the long bones
Red marrow remains in the central skeleton: the vertebrae, pelvis, sternum, ribs, and skull. That is why marrow samples in adults are taken from the back of the hip bone rather than from a limb.
Functions of Yellow Marrow
Yellow marrow is not simply inert padding. Its known roles include:
- Energy storage: the fat is a reserve the body can draw on.
- Reserve blood production: under sustained demand, yellow marrow can reconvert to red marrow.
- Stem cell reservoir: it contains mesenchymal stem cells, which can develop into bone, cartilage, and fat cells.
- Metabolic signaling: marrow fat is now recognized as an active tissue that interacts with bone and blood-forming cells.
Disturbances in the balance between red and yellow marrow can reflect or contribute to disease, which is why radiologists and hematologists look closely at the pattern.
What Shifts the Balance Between Red and Yellow Marrow
Aging is the main driver of conversion from red to yellow marrow. Other factors push the balance one way or the other.
Factors that increase yellow marrow:
- Advancing age
- Osteoporosis, in which marrow fat tends to increase as bone is lost
- Radiation therapy to a region, which can replace red marrow with fat in the treated bones
- Aplastic anemia, where blood-forming cells are lost and fat fills the space
- Prolonged immobility
Factors that cause reconversion to red marrow:
- Chronic anemia, particularly hemolytic anemias such as sickle cell disease and thalassemia
- Significant blood loss
- Living at high altitude
- Endurance athletics and heavy smoking in some people
- Treatment with blood cell growth factors
Severe malnutrition, as in anorexia nervosa, can cause an unusual change called gelatinous transformation, in which both fat and blood-forming cells are replaced by a gel-like material.
Clinical Relevance and Diagnosis
Yellow marrow itself causes no symptoms. What matters clinically is what its pattern reveals about red marrow and blood production. A marrow that is too fatty for a person’s age may point to marrow failure, and hematological disorders such as aplastic anemia typically cause fatigue, infections, and easy bruising from low counts.
At the other extreme, when leukemia, lymphoma, myeloma, or cancer spread from elsewhere infiltrates the marrow, the normal fatty signal in the long bones is replaced. This is a common MRI finding that prompts further testing.
- MRI: the best imaging test for marrow composition, because fat appears bright on T1-weighted images.
- Complete blood count: an indirect measure of whether the marrow is producing enough cells.
- Bone marrow biopsy: directly shows cellularity, the proportion of marrow space occupied by blood-forming cells rather than fat. A rough bedside rule is that normal cellularity is about 100 minus the patient’s age, as a percentage.
Yellow marrow in long bones is also where fat emboli can originate after a fracture of the femur or pelvis. For more on marrow structure and disease, see our bone marrow guide.
Key Takeaways
- Yellow bone marrow is fat-rich marrow found mainly in the shafts of adult long bones.
- All marrow is red at birth; conversion to yellow marrow is a normal part of aging.
- Yellow marrow stores energy and can revert to red marrow when blood demand rises.
- Too much or too little marrow fat for age can signal marrow failure, infiltration, or chronic anemia.
- MRI and marrow biopsy are the main tools for assessing marrow composition.
Our overview of hematologic disorders explains the conditions that alter marrow in more detail.
Frequently Asked Questions
Can yellow bone marrow turn back into red marrow?
Yes. When the body needs more blood cells for a sustained period, such as with chronic hemolytic anemia or major blood loss, yellow marrow can reconvert to red marrow. Reconversion generally happens in the reverse order of the original conversion, starting closest to the trunk.
Is a fatty bone marrow on MRI a bad sign?
Not usually. Fatty marrow in the long bones of an adult is expected, and marrow fat increases with age. It becomes a concern only if the marrow is much fattier than expected for age together with low blood counts, which may need further investigation.
Does yellow bone marrow make any blood cells?
Under normal conditions, very few. Its value is as a reserve that can be recruited when demand rises. The bulk of adult blood production takes place in the red marrow of the central skeleton.
Can yellow marrow be used for stem cell transplants?
Blood stem cells for transplant come from red marrow or, more commonly, from the bloodstream after mobilization. Yellow marrow is a source of mesenchymal stem cells, which are studied in bone and cartilage repair but are not used to rebuild blood production.
