Bonemarrow

Bone marrow is the soft tissue inside bones where blood cells are produced. In adult humans it exists mainly as red marrow, which is actively hematopoietic, and yellow marrow, which consists largely of adipose tissue and serves as an energy reserve. Red marrow contains developing red and white blood cells and megakaryocytes, while yellow marrow stores fat and is relatively inactive in normal conditions. In adults red marrow is concentrated in the axial skeleton—vertebrae, pelvis, sternum, ribs, and skull—as well as the proximal ends of the femur and humerus. Most long bone cavities contain yellow marrow.

Hematopoiesis arises from hematopoietic stem cells that differentiate along myeloid and lymphoid lines to produce erythrocytes (red cells), various white blood cells, and platelets. The process is supported by a specialized bone marrow stroma that includes osteoblasts, endothelial cells, fibroblasts, and adipocytes, forming niches that regulate stem cell self-renewal and lineage commitment through cytokines and growth factors.

Clinical relevance includes bone marrow examination by aspiration or biopsy, commonly from the posterior iliac crest, for diagnosis of anemia, infections, leukemia, and marrow disorders. Bone marrow transplantation or cellular therapy is used to treat certain cancers and hematologic diseases. Conditions affecting marrow function include aplastic anemia, myelodysplastic syndromes, leukemias, lymphomas, and metastases to marrow.