Bone marrow is a soft, gelatinous tissue found within the cavities of bones. This dynamic tissue is a significant component of the human body, constituting approximately 4% of total body weight in an adult. Within this intricate system, yellow bone marrow is a distinct type, characterized by its yellowish appearance. It is predominantly found in the hollow interior of long bones, such as the femur and tibia, in adults.
Primary Components
Yellow bone marrow is primarily composed of fat cells, known as adipocytes, which account for 50% to 70% of its volume. These adipocytes serve as storage sites for triglycerides. The presence of these fat cells means yellow marrow has a lower cellular density compared to other marrow types.
Beyond adipocytes, yellow bone marrow also contains mesenchymal stem cells (MSCs). These multipotent cells possess the ability to differentiate into various cell types, including bone cells (osteoblasts), cartilage cells (chondrocytes), and additional fat cells. While predominantly fatty, yellow marrow also houses a sparse number of hematopoietic stem cells, though blood cell production is not its primary role. A network of blood vessels and nerves also permeates the yellow marrow, providing necessary nourishment and regulatory signals.
Yellow Marrow’s Main Purpose
The main functional role of yellow bone marrow is to serve as a long-term energy reserve for the body. It stores lipids, primarily in the form of triglycerides within its adipocytes, which can be released and utilized as an energy source when the body’s demands increase, such as during prolonged physical activity or periods of fasting.
Yellow bone marrow also exhibits a remarkable adaptive capacity. Under specific physiological demands, such as severe blood loss or anemia, it can convert back into red bone marrow. This conversion allows it to temporarily contribute to the production of new blood cells, including red blood cells, white blood cells, and platelets, to meet the body’s urgent needs.
Distinguishing Yellow from Red Marrow
Yellow bone marrow and red bone marrow have distinct primary functions. Yellow marrow functions mainly as an energy storage site due to its high fat content, while red marrow is the primary site for the production of blood cells, a process known as hematopoiesis. This difference in function is reflected in their composition.
Red marrow is rich in hematopoietic tissue, giving it a reddish color, and contains a high density of blood-forming cells. In contrast, yellow marrow’s composition is predominantly adipocytes, leading to its characteristic yellow appearance and lower cellular density. Their typical locations also vary in adults; yellow marrow is largely found in the hollow shafts of long bones, whereas red marrow is concentrated in flat bones like the sternum, pelvis, and vertebrae, as well as the ends of long bones. At birth, all bone marrow is red, but as a person ages, particularly from around age seven, red marrow is gradually replaced by yellow marrow, with approximately half of the bone marrow remaining red in adulthood.