Bone marrow comes in two colors, red and yellow. Red bone marrow makes blood cells and gets its color from the hemoglobin in developing red cells. Yellow bone marrow is mostly fat. A newborn’s marrow is almost all red, and in an adult roughly half is red and half is yellow.
That is the whole short answer. Below I cover where each type sits in an adult skeleton, what it looks like, why the balance shifts with age, and what an unusual marrow color can mean when a hematologist looks at a biopsy sample.
What Bone Marrow Is Made Of and What It Looks Like
Bone marrow is the soft tissue that fills the hollow centers of bones and the spaces within spongy (trabecular) bone. In an adult it weighs roughly 2.5 kg, just under 6 lb, which makes it one of the larger organs in the body even though you never see it.
Both kinds of marrow share the same basic framework: a mesh of supporting cells called the stroma, threaded with thin-walled blood vessels called sinusoids. What sits in that mesh decides the color.
- Red marrow is soft, spongy and jelly-like, similar to thick blood. It is packed with hematopoietic stem cells and the red cells, white cells and platelet-forming megakaryocytes they produce.
- Yellow marrow is greasy and pale, much like other body fat. It is made mainly of fat cells (adipocytes) with far fewer blood vessels.
Why Red Marrow Is Red and Yellow Marrow Is Yellow
Red marrow is red for the same reason blood is. Developing red blood cells fill up with hemoglobin, the iron-containing protein that carries oxygen, and hemoglobin is red. Red marrow also has a dense network of sinusoids full of circulating blood, which deepens the color.
Yellow marrow is yellow because it is mostly fat. Fat tissue has a natural yellowish tint, partly from carotenoid pigments stored in fat cells. It also has fewer blood vessels and very little hemoglobin-rich tissue to redden it.
Red vs Yellow Bone Marrow: Side by Side
| Feature | Red bone marrow | Yellow bone marrow |
|---|---|---|
| Color | Dark red to reddish-brown | Pale yellow |
| Main contents | Hematopoietic stem cells and developing blood cells | Fat cells (adipocytes) and mesenchymal stem cells |
| Main job | Makes red cells, white cells and platelets (hematopoiesis) | Stores fat as an energy reserve; backup for blood production |
| Location in adults | Skull, sternum, ribs, vertebrae, pelvis, shoulder blades, collarbones, upper ends of the femur and humerus | Shafts of long bones (femur, tibia, humerus, forearm bones) and bones of the hands and feet |
| Share of marrow | Nearly 100% at birth; about 50% in adults | Almost none at birth; about 50% in adults, more in old age |
| Blood supply | Rich | Sparse |
| Can it change? | Converts to yellow with age | Can revert to red under heavy demand |
Where Red Bone Marrow Is Found in Adults
In adults, red marrow lives mostly in the axial skeleton, the bones along the body’s central axis. If you picture a skeleton and shade the active blood-making areas, you would shade:
- Skull (the flat bones of the cranium)
- Sternum (breastbone)
- Ribs
- Vertebrae (spine)
- Pelvis (hip bones), the largest single reservoir
- Shoulder blades (scapulae) and collarbones (clavicles)
- The upper ends of the thigh bone (femur) and upper arm bone (humerus)
Everything else, especially the long shafts of the arm and leg bones and the small bones of the hands and feet, is filled mainly with yellow marrow. This is why a bone marrow aspiration in adults is taken from the back of the pelvic bone (the posterior iliac crest). It is a large, reachable store of red marrow.
What Each Type of Marrow Does
Red marrow runs hematopoiesis, the production of all blood cells. It turns out billions of new cells every day to replace red cells that last about 120 days, platelets that last about 7 to 10 days, and neutrophils that survive only hours to days in the blood. If you want the detail on how this red marrow blood formation works, we cover it separately.
Yellow marrow is not just padding. It stores fat as an energy reserve and contains mesenchymal stem cells, which can become bone, cartilage or fat cells. It also acts as a reserve: under heavy demand, yellow marrow can switch back to blood production.
How Marrow Color Changes With Age
| Age | What the marrow looks like |
|---|---|
| Birth | Nearly all red; every bone makes blood |
| Early childhood | Conversion to yellow begins in the fingers and toes, then the shafts of long bones |
| Around age 7 | Yellow marrow clearly established in the middle of the long bones |
| Late teens to about 25 | Adult pattern reached: red marrow mainly in the axial skeleton and upper femur and humerus |
| Adulthood | About half red, half yellow |
| Older age | Yellow share keeps rising, even in the spine and pelvis |
Conversion runs from the extremities toward the center of the body, and from the middle of each long bone toward its ends. It is a normal process, not a sign of disease.
Can yellow marrow turn back to red?
Yes. This is called reconversion. It happens when the body needs more blood cells than the existing red marrow can supply, for example in chronic hemolytic anemias like thalassemia or sickle cell disease, after heavy blood loss, or at high altitude. It runs in the reverse order: axial skeleton first, then long bones. Radiologists know this pattern so they don’t mistake reconverted marrow for a tumor on MRI.
What an Abnormal Marrow Color Can Mean
You can’t see your own marrow, but when a hematologist collects a sample, its appearance gives early clues before microscopy results arrive. In a healthy adult, the proportion of marrow space taken up by blood-forming cells (the cellularity) is roughly 100 minus the person’s age, as a percentage.
- Pale, fatty or watery: the marrow may be hypocellular, as in aplastic anemia, where blood-forming tissue has been replaced by fat.
- Deep red and packed: a hypercellular marrow, seen in polycythemia vera, chronic hemolysis or leukemia, where cells crowd out the fat spaces.
- White, gray or gritty: may point to myelofibrosis, where scar tissue replaces marrow. These samples are often a “dry tap”, meaning no liquid can be drawn.
- Dark or brownish: can reflect marrow necrosis, seen with severe sickle cell crises, overwhelming infection or cancer infiltration.
On MRI, yellow marrow looks bright on T1-weighted images and red marrow looks darker. That contrast lets doctors spot marrow replacement without a needle.
How to Keep Your Bone Marrow Healthy
- Eat enough of the nutrients marrow needs to make cells: iron, vitamin B12 and folate.
- Avoid smoking, and keep alcohol moderate. Heavy drinking suppresses blood-cell production.
- Limit exposure to marrow toxins such as benzene and unnecessary radiation.
- Stay active; weight-bearing exercise supports the bones that house the marrow.
- Take unexplained abnormal blood counts seriously, and get them followed up.
When to See a Doctor
See a doctor if you have symptoms that can come from marrow that isn’t working properly:
- Persistent tiredness, breathlessness or pale skin (possible anemia)
- Easy bruising, nosebleeds or bleeding gums (low platelets)
- Frequent or unusually severe infections (low white cells)
- Unexplained bone pain, fevers or night sweats
- Abnormal results on a complete blood count (CBC)
Frequently Asked Questions
Where is red bone marrow found in adults?
Mainly in the skull, sternum, ribs, vertebrae, pelvis, shoulder blades and collarbones, plus the upper ends of the femur and humerus. The shafts of long bones hold mostly yellow marrow.
Why is red bone marrow red?
Because it is full of developing red blood cells loaded with hemoglobin, the red, iron-containing oxygen carrier. Its rich blood supply adds to the color.
At what age does red marrow start turning yellow?
Conversion starts in early childhood, first in the hands and feet. By about age 7 the shafts of the long bones hold clear yellow marrow, and the adult pattern is reached by the mid-20s.
How much bone marrow does the body have?
An adult has roughly 2.5 kg (just under 6 lb) of bone marrow in total, split about half red and half yellow.
What does yellow bone marrow do?
It stores fat as an energy reserve, holds mesenchymal stem cells that can form bone, cartilage and fat, and can switch back to red, blood-making marrow when the body needs it.
Can marrow color alone diagnose leukemia?
No. A packed, deep-red sample can raise suspicion, but a leukemia diagnosis needs microscopy, flow cytometry and genetic tests.
For the bigger picture, see our bone marrow guide.
