marrow

Marrow is the soft tissue inside the medullary cavities of bones. In humans it exists in two main forms: red marrow, which is responsible for hematopoiesis (the production of red blood cells, white blood cells, and platelets), and yellow marrow, which consists largely of adipocytes and serves primarily as an energy reserve. Under certain conditions red marrow can revert to a more hematopoietically active state.

Distribution varies with age. In children, red marrow is present throughout most bones. In healthy adults, red marrow remains mainly in the axial skeleton—skull, ribs, sternum, vertebrae, and pelvis—and in the proximal ends of the femur and humerus, while the cavities of many long bones are filled with yellow marrow. Some red marrow persists in the skull and vertebrae into adulthood.

Function and microenvironment: Red marrow contains developing blood cells in various stages and supporting stromal cells that create the niche required for hematopoiesis. Yellow marrow stores fat, can provide energy and occasionally contributes a small amount of hematopoietic potential when needed.

Clinical aspects: Bone marrow can be sampled by biopsy to diagnose hematologic diseases and to assess marrow function. It is a key site for transplantation in leukemia and lymphoma. Diseases such as aplastic anemia, leukemia, and marrow infiltration by metastases disrupt hematopoiesis. In some conditions, extramedullary hematopoiesis occurs when marrow cannot meet hematopoietic demand.