The formation of bone marrow happens before birth. Blood production starts in the embryo’s yolk sac, moves to the liver, and settles in the hollow centers of developing bones during the second trimester. By birth, bone marrow is the body’s main blood factory, and it stays that way for life. This article walks through how marrow forms, how it changes with age, and what happens when the process goes wrong.
What Is Bone Marrow?
Bone marrow is the soft, spongy tissue inside bones. It comes in two forms. Red marrow is active tissue that makes red blood cells, white blood cells, and platelets. Yellow marrow is mostly fat and makes few blood cells, though it can convert back to red marrow when the body needs more blood.
The output is remarkable. Healthy marrow makes roughly 2 million red blood cells every second, replacing cells that reach the end of their 120-day lifespan. For more on its structure, see our article on the composition and function of bone marrow, and our bone marrow guide for the bigger picture.
How Bone Marrow Forms Before Birth
Blood cell production, called hematopoiesis, moves through several sites during development. Each site takes over as the embryo grows and the next organ matures.
| Stage of development | Main site of blood production | What happens |
|---|---|---|
| Weeks 3 to 8 of gestation | Yolk sac | Blood islands produce the first, primitive red cells |
| About week 6 to the third trimester | Liver (with a smaller role for the spleen) | Blood stem cells multiply; the liver becomes the main source |
| From the second trimester | Bone marrow | Stem cells seed the new marrow cavities |
| Birth | Nearly all bones | Marrow throughout the skeleton is red and active |
| Adulthood | Axial skeleton | Red marrow persists in the pelvis, spine, ribs, sternum, skull, and ends of long bones |
Building the marrow cavity
Most bones start as cartilage models. During endochondral ossification, blood vessels invade the cartilage, bone-forming cells lay down mineral, and bone-resorbing cells called osteoclasts hollow out the center. The space left behind is the marrow cavity.
Once the cavity has a blood supply and supporting tissue, blood stem cells migrate in from the fetal liver and take up residence. This close partnership is why the skeletal system plays a direct role in blood cell production.
Stem Cells and the Marrow Niche
The key cell is the hematopoietic stem cell (HSC). It can renew itself and also give rise to every blood cell type. HSCs split into two broad families: the myeloid line (red cells, platelets, and most white cells) and the lymphoid line (lymphocytes).
Stem cells do not work alone. They live in a specialized niche made of stromal cells, bone-lining cells, blood vessel cells, fat cells, and a supporting protein framework. The niche releases signaling molecules that tell stem cells when to rest, divide, or mature.
Hormones fine-tune output. Erythropoietin, made by the kidneys when oxygen is low, boosts red cell production. Thrombopoietin from the liver drives platelet production, and growth factors called colony-stimulating factors increase white cells during infection.
How Marrow Changes With Age
At birth, almost all marrow is red. From early childhood, red marrow in the arms and legs is gradually replaced by yellow, fatty marrow, working from the middle of the long bones toward the body. By adulthood, active marrow is concentrated in the central skeleton, which is why marrow samples are usually taken from the back of the hip bone.
In older age, the proportion of fat in marrow keeps rising. Healthy older adults still make enough blood, but their marrow has less reserve during stress such as bleeding, infection, or chemotherapy.
If demand rises sharply, as in severe hemolytic anemia, yellow marrow can reconvert to red. In some conditions the liver and spleen restart blood production, called extramedullary hematopoiesis, which can enlarge those organs.
When Marrow Formation or Function Goes Wrong
Normal bone marrow function underpins oxygen delivery, immunity, and blood coagulation. Problems in the marrow cause a wide range of hematological disorders:
- Anemia: too few red cells or too little hemoglobin, causing fatigue and breathlessness. Our anemia guide covers causes, including reduced red blood cell production.
- Leukemia: a cancer of blood-forming cells in which abnormal cells crowd out normal production.
- Aplastic anemia: the marrow fails and becomes empty of blood-forming cells, lowering all three cell types.
- Myelodysplastic syndromes and myelofibrosis: the marrow produces faulty cells or becomes scarred.
Low platelets from marrow failure can cause easy bruising and bleeding disorders, while low white cells lead to frequent infections.
How marrow problems are diagnosed
A complete blood count and blood smear come first. If they suggest a marrow problem, a bone marrow aspiration and biopsy takes a small sample, usually from the back of the hip bone under local anesthetic. It shows how full the marrow is, which cells are present, and whether abnormal cells or scarring are present.
Treatment options
Treatment for bone marrow disorders depends on the cause. Options include replacing iron, vitamin B12, or folate; erythropoiesis-stimulating agents; immunosuppressive therapy for aplastic anemia; and chemotherapy for leukemia. When marrow is severely damaged, a hematopoietic stem cell transplant replaces it with healthy donor or stored stem cells, which travel to the marrow cavity and rebuild blood production.
When to See a Doctor
See a doctor if you notice signs that your marrow may not be keeping up:
- Persistent tiredness, pale skin, or breathlessness on mild exertion
- Frequent or unusually severe infections
- Easy bruising, nosebleeds, bleeding gums, or tiny red spots on the skin
- Unexplained fevers, night sweats, weight loss, or bone pain
A simple blood count is the right first step and often gives quick answers.
Frequently Asked Questions
When does bone marrow start making blood?
Bone marrow begins producing blood cells during the second trimester of pregnancy. Before that, the yolk sac and then the fetal liver do the work. By birth, marrow is the main site.
Which bones contain red marrow in adults?
Adults keep red marrow mainly in the pelvis, spine, ribs, sternum, skull, and the upper ends of the thigh and arm bones. The shafts of long bones hold mostly yellow, fatty marrow.
Can bone marrow regenerate?
Yes. Marrow has a large capacity to recover after blood loss or many chemotherapy treatments, because stem cells can multiply to restore output. Severe or repeated damage can exhaust that capacity, which is when transplant may be considered.
Is yellow marrow useless?
No. It stores energy as fat and supports the skeleton, and it can turn back into red marrow when the body needs extra blood production.
Key Takeaways
- Blood production moves from the yolk sac to the liver and then into bone marrow during fetal life.
- Marrow cavities form as developing bone is hollowed out and seeded by stem cells.
- Stem cells depend on a supportive niche and hormones such as erythropoietin.
- Red marrow retreats to the central skeleton with age, and disorders of marrow cause anemia, infection, and bleeding.
