Bone marrow, the primary site of hematopoiesis, is found in the cavities of cancellous bones and the medullary canals of long bones. There are 2 types: red marrow (hematopoietic with abundant blood cells) and yellow marrow (predominantly filled with adipocytes). Human marrow composition changes with age. In the young, the entire bone marrow is red, as there is increased blood cell production. As age increases, there is gradual change to the yellow marrow variety. The yellow marrow can revert to red marrow when hematopoiesis is needed (e.g., anemia). The red marrow, by hematopoiesis, produces about 6 billion cells per kilogram per day. The process relies on hematopoietic cells (stem cells and progenitors) producing mature effector cells (lymphocytes, platelets, granulocytes, erythrocytes) with the aid of nonhematopoietic elements. Production is regulated by cytokines released in the bone marrow environment and feedback from target tissues. The marrow structure allows hematopoiesis to take place in the extravascular area, and after staged differentiation, blood cells are released into circulation.
