Bone marrow is the soft, spongy tissue that fills the interior of bones. It comprises two primary types: red marrow and yellow marrow. Red marrow is responsible for the continuous production of all blood cells, a process called hematopoiesis. Yellow marrow is largely composed of fat cells and serves a different role, primarily related to long-term energy storage and support.
Anatomical Identity and Location
Yellow marrow is characterized by its yellowish hue, a direct result of its high lipid content. This soft, gelatinous tissue is primarily situated within the medullary cavity, the hollow central shaft of long bones in adult humans. Examples of these long bones include the femur and the humerus.
While all bone marrow is red at birth, a conversion process begins around age seven. Red marrow is progressively replaced by yellow marrow in the long bones. This conversion starts in the diaphysis, or shaft, and moves toward the ends of the bone. By adulthood, yellow marrow dominates the central cavities of the appendicular skeleton, although a thin layer of red marrow often remains at the ends of the long bones.
Cellular Composition and Primary Role
Yellow marrow is overwhelmingly dominated by adipocytes, or fat cells, which make up approximately 95% of its composition. These fat cells store energy in the form of triglycerides, making the marrow a significant metabolic reservoir. The total chemical composition is roughly 80% lipid, 15% water, and 5% protein.
Yellow marrow also contains mesenchymal stem cells (MSCs), a type of stromal cell. These MSCs are multipotent, meaning they can differentiate into various cell types, including bone cells (osteoblasts), cartilage cells (chondrocytes), and new fat cells (adipocytes). This capacity provides structural and regenerative support for the bone itself.
The Dynamic Relationship with Red Marrow
Yellow marrow is a dynamic tissue that maintains a functional connection with red marrow. Under conditions of extreme physiological need, the fatty tissue can undergo a process called reconversion, transforming back into active red (hematopoietic) marrow. This reversal increases the body’s capacity for producing new blood cells, such as red blood cells.
Reconversion is triggered by acute or chronic hematopoietic demand, which is the need for increased blood cell production. Examples include severe blood loss, chronic anemia, or sustained physiological stress experienced by high-endurance athletes. The process occurs in the reverse order of the initial conversion, typically moving from the center of the skeleton outward to the long bones.