Blood cells are formed inside the red bone marrow, the soft, spongy tissue found in the center of certain bones. In adults, roughly half of all bone marrow remains the active “red” type responsible for producing every red blood cell, white blood cell, and platelet circulating in your body. The other half has converted to yellow marrow, which is mostly fat and largely inactive.
Where Red Marrow Is Found in Adults
Not every bone in your skeleton makes blood cells. Red marrow concentrates in flat bones and the spongy ends of some long bones. The major sites include the hip bones (pelvis), sternum (breastbone), ribs, vertebrae (spine), shoulder blades, and skull. The spongy ends of the femur (thigh bone), tibia (shinbone), and humerus (upper arm bone) also retain active marrow.
At birth, virtually every bone contains red marrow. As you grow, much of it gradually converts to yellow marrow filled with fat cells. By adulthood, active blood production has retreated to the locations listed above. The hip bones hold the largest single reserve of red marrow, which is why doctors almost always sample from the back of the hip (the posterior iliac crest) when they need to examine marrow directly. In rare cases, marrow can be drawn from the breastbone, and in infants younger than about 18 months, the lower leg bone is sometimes used instead.
How Blood Cells Develop
Every blood cell traces back to a single cell type: the hematopoietic stem cell. These stem cells are rare, but they’re self-renewing, meaning they can copy themselves indefinitely while also producing offspring that specialize into mature blood cells. The process from stem cell to finished blood cell is called hematopoiesis.
A stem cell’s first major decision is which of two pathways to follow. One pathway, the lymphoid lineage, produces the immune cells known as lymphocytes. These include T cells (which coordinate immune attacks and kill infected cells), B cells (which make antibodies), and natural killer cells (which destroy virus-infected or cancerous cells).
The other pathway, the myeloid lineage, is more diverse. It generates red blood cells, which carry oxygen; platelets, the tiny cell fragments that form clots; and several types of white blood cells involved in frontline defense. Neutrophils, the most abundant white blood cells, swarm to infection sites. Monocytes leave the bloodstream and mature into macrophages, larger cells that engulf bacteria and debris. Eosinophils and basophils play roles in allergic responses and parasite defense.
Each step along these pathways narrows what the cell can become. A stem cell can become anything. Its immediate descendants can become several things. By the final stages, the cell is locked into one identity and released into the bloodstream. Your marrow produces hundreds of billions of new blood cells every day to replace those that wear out or are consumed in immune responses.
Blood Cell Formation Before Birth
During fetal development, blood cell production doesn’t start in the bones. It migrates through several organs before settling in the marrow. The timeline unfolds in overlapping waves.
The first blood cells appear in the yolk sac, the small nutrient structure attached to the early embryo. This phase lasts roughly seven weeks. Around week 10 of gestation, blood-forming cells migrate to the fetal liver, which becomes the dominant production site for the middle trimester. The spleen and thymus contribute during this period as well. By about 16 weeks of gestation, production begins shifting to the bone marrow, which gradually takes over and becomes the permanent site by birth.
When the Body Makes Blood Cells Elsewhere
In healthy adults, blood cell formation happens exclusively in the bone marrow. But when marrow fails or can’t keep up with demand, the body can reactivate production in other organs, most commonly the liver and spleen. This fallback process is called extramedullary hematopoiesis, meaning blood formation outside the marrow.
The conditions that trigger it are serious. Myelofibrosis, a disease in which scar tissue gradually replaces normal marrow, is one of the most common causes. Certain inherited blood disorders, particularly thalassemia and sickle cell disease, can also push the body into extramedullary production because the marrow can’t produce enough functional red blood cells on its own. Cancers that infiltrate and crowd out normal marrow tissue can have the same effect. In these situations, the liver and spleen enlarge as they take on a workload they haven’t handled since fetal life.
Why Marrow Location Matters Clinically
When doctors suspect a blood disorder, they often need to examine the marrow directly. A bone marrow biopsy involves inserting a needle into a bone to withdraw both liquid marrow and a small core of tissue. The back of the hip is the preferred site because it holds a large, accessible pocket of red marrow close to the skin surface, and the procedure carries lower risk there than at other locations. The breastbone is used occasionally for aspiration (drawing out liquid only), but biopsies are not performed there because the bone is thin and vital organs lie just beneath it.
Understanding which bones contain active marrow also matters in cancer treatment. Radiation therapy aimed at marrow-rich areas can suppress blood cell production, leading to low counts of red cells, white cells, or platelets. Treatment planning takes these locations into account to minimize damage to the body’s blood-forming capacity while still targeting the disease.