Bone marrow is the soft, spongy tissue inside your bones that produces nearly all of your blood cells. It churns out roughly 2 million red blood cells every second, along with white blood cells and platelets, making it one of the most active organs in the body. Your body holds just under 6 pounds of it, and it comes in two types: red marrow, which makes blood cells, and yellow marrow, which stores fat.
Where Bone Marrow Is Found
Bone marrow sits in the hollow center of most bones and in the spongy ends of bones like those in your hips, spine, ribs, and sternum. In texture, it resembles a thick jelly. At birth, nearly all of it is red marrow, actively producing blood cells. By around age seven, yellow marrow begins replacing red marrow in the long bones of the arms and legs. This conversion follows a predictable pattern: in the thighbone, for instance, the shaft converts first (between ages 1 and 10), followed by the ends closer to the knee (ages 10 to 20), with an adult distribution settling in by age 24.
In adults, red marrow concentrates in flat bones like the pelvis, skull, sternum, ribs, and vertebrae, plus the very ends of long bones. Yellow marrow fills most of the shafts of long bones. This matters clinically because when doctors need a marrow sample, they typically go to the back of the hip bone, where red marrow is still abundant.
How It Makes Blood Cells
The process of creating blood cells, called hematopoiesis, starts with a single type of master stem cell that can become any blood cell your body needs. These stem cells divide into increasingly specialized cells, branching down two main paths. One path produces red blood cells (which carry oxygen), platelets (which help blood clot), and several types of white blood cells that fight bacteria and parasites. The other path produces a different set of immune cells, including the ones responsible for making antibodies and killing virus-infected cells.
What makes the master stem cells special is their ability to copy themselves indefinitely. Every cell further down the chain loses that ability and can only keep specializing until it becomes a mature blood cell ready to enter the bloodstream. This self-renewing quality is what makes stem cells so valuable in medicine.
Two Types of Stem Cells
Bone marrow contains two distinct families of stem cells that do very different jobs. The first group, hematopoietic stem cells, creates all blood cell types. These are the cells involved in bone marrow transplants and the cells damaged by leukemia and other blood cancers.
The second group, mesenchymal stem cells, has nothing to do with blood. These cells can develop into bone, cartilage, muscle, and fat cells. They also help regulate inflammation by releasing signaling molecules that calm or activate immune responses. Together, the two stem cell types form a shared environment inside the marrow where each supports the other’s function.
Red Marrow vs. Yellow Marrow
Red marrow gets its color from the enormous volume of red blood cells being produced inside it. It’s the active, blood-forming tissue and contains a dense network of blood vessels that newly made cells enter before circulating through the body.
Yellow marrow is mostly fat. It serves as an energy reserve and houses mesenchymal stem cells that can become bone, cartilage, or fat cells. Yellow marrow isn’t useless filler, though. In emergencies like severe blood loss, your body can convert yellow marrow back into red marrow to ramp up blood cell production. This flexibility is one reason the body maintains both types throughout life.
What Happens When Marrow Malfunctions
Because bone marrow is responsible for producing all blood cells, problems with it tend to show up as abnormal blood counts. In leukemia, the marrow produces large numbers of defective white blood cells that crowd out healthy cells. In aplastic anemia, the marrow fails to make enough red blood cells, leading to fatigue, shortness of breath, and increased infection risk. In myeloproliferative disorders, the marrow overproduces certain white blood cells.
Cancers that originate elsewhere in the body can also spread to the bone marrow, disrupting its function. Doctors use marrow exams to diagnose these conditions, monitor treatment response, stage cancers, check iron levels, and investigate unexplained fevers. There are two types of marrow exams: aspiration, which draws out a liquid sample, and biopsy, which removes a small solid core of tissue. Both are typically done from the back of the hip bone and are often performed together.
Bone Marrow Transplants
When bone marrow is severely damaged by disease or by the chemotherapy and radiation used to treat cancer, a transplant can replace it with healthy stem cells. There are two main approaches.
In an autologous transplant, your own stem cells are collected before treatment, stored, and then returned to your body afterward. The advantage is that your body won’t reject its own cells, and recovery tends to involve fewer serious complications. The risk is that collected cells could contain traces of cancer. This approach is used most often for lymphoma and a blood cancer called myeloma.
In an allogeneic transplant, stem cells come from a donor whose tissue type closely matches yours. The donor cells are cancer-free, and they bring their own immune system, which can actually attack remaining cancer cells in your body. The tradeoff is a higher risk of complications, because the donor’s immune cells can also attack your healthy tissues. This approach is used primarily for leukemias and related bone marrow cancers, and it has traditionally been offered to younger patients who can better tolerate the side effects.
Nutrients That Support Healthy Marrow
Bone marrow’s relentless production schedule means it has high nutritional demands. Iron is essential for building the oxygen-carrying molecule in red blood cells, and iron deficiency is one of the most common reasons marrow can’t keep up with demand. B vitamins, particularly B12 and folate, are critical for the rapid cell division that hematopoiesis requires. Protein provides the building blocks for new cells, phosphorus supports the bone structure that houses the marrow, and healthy unsaturated fats contribute to the yellow marrow’s energy reserves. A diet that consistently includes these nutrients helps keep your marrow functioning at its normal, remarkable pace.