The two types of bone marrow are red bone marrow and yellow bone marrow. Red marrow produces blood cells, while yellow marrow stores fat. Both types fill the hollow centers of your bones, but they differ in composition, location, and function, and the balance between them shifts dramatically as you age.
Red Bone Marrow: Your Blood Cell Factory
Red bone marrow is where your body manufactures blood cells. It contains blood stem cells that can develop into red blood cells (which carry oxygen), white blood cells (which fight infection), and platelets (which help blood clot). This process, called hematopoiesis, runs constantly. Your body produces billions of new blood cells every day to replace old ones that wear out.
Red marrow gets its color from the high concentration of blood and blood-forming cells packed inside it. Its composition is roughly 40% fat, 40% water, and 20% protein. In adults, red marrow persists in the axial skeleton (your spine, pelvis, ribs, and skull), as well as the upper portions of the thighbones and upper arm bones. These are the bones closest to your body’s core, where a reliable blood supply keeps the marrow active.
This is why doctors almost always collect bone marrow samples from the back ridge of the hipbone. That site reliably contains active red marrow and is accessible with a needle. In some cases the front of the hip is used instead, and rarely, marrow can be drawn from the breastbone.
Yellow Bone Marrow: Fat Storage With Backup Potential
Yellow bone marrow is composed primarily of fat cells. It’s about 80% fat and 15% water, with a small amount of protein making up the rest. Yellow marrow fills the central cavities of long bones like those in your arms, legs, hands, and feet. It contains stem cells, but these are a different type than those found in red marrow. Instead of producing blood cells, yellow marrow stem cells can develop into cartilage, fat, or bone tissue.
Yellow marrow serves as an energy reserve. The fat stored there can be used by the body during periods of extreme starvation or high energy demand. But its most remarkable feature is its ability to reactivate.
How Your Marrow Changes With Age
At birth, virtually your entire skeleton contains red bone marrow. There’s no yellow marrow at all in a newborn. Starting in early childhood, red marrow gradually converts to yellow marrow in a predictable pattern: the process begins in the hands, feet, and other extremities, then slowly progresses inward toward the spine and pelvis. By roughly age 25, the conversion is complete, and you’ve reached the adult distribution where red marrow occupies only the core skeletal sites.
This means that as you grow, your body consolidates blood cell production into fewer, more centralized locations while repurposing the remaining marrow space for fat storage. A child’s skeleton is far more active in blood production across its entire structure than an adult’s.
Yellow Marrow Can Revert to Red
One of the more surprising aspects of bone marrow biology is that yellow marrow can convert back into red marrow when the body needs more blood cells. This reversal, called reconversion, happens in response to several triggers: severe anemia, significant blood loss, living at high altitude, heavy athletic training, and even cigarette smoking (which reduces oxygen delivery and forces the body to compensate by producing more red blood cells).
Reconversion follows the opposite path of the original childhood conversion. It starts in the axial skeleton and moves outward toward the extremities. Doctors sometimes intentionally trigger this process using bone marrow-stimulating medications, which are frequently given alongside chemotherapy to help patients recover their blood cell counts after treatment damages the marrow.
What Happens When Bone Marrow Malfunctions
Because red bone marrow is responsible for producing all your blood cells, diseases that affect it can have serious consequences. In leukemia, the marrow produces abnormal white blood cells that crowd out healthy ones. In aplastic anemia, the marrow fails to produce enough red blood cells, leading to fatigue, weakness, and increased vulnerability to infection. Myeloproliferative disorders push the marrow to overproduce white blood cells. Other cancers, like lymphoma, can spread into the bone marrow from elsewhere in the body and disrupt normal blood cell production.
Yellow marrow is less commonly the focus of disease, but it can be affected too. In conditions where the marrow’s fat content increases abnormally, the space available for blood cell production shrinks. Conversely, when diseases consume yellow marrow or force it to reconvert, imaging scans can pick up the changes, sometimes creating patterns that mimic cancer spread on MRI and require careful interpretation by radiologists.