the tissue that fills the cavities of the bones in vertebrate animals and man. A distinction is made between red marrow, with a predominance of hematopoietic myeloid tissue, and yellow marrow, with a predominance of fatty tissue. Red marrow remains throughout life in the flat bones (ribs, sternum, and cranial and pelvic bones) and in the vertebrae and epiphyses of the long bones. In man, bone marrow constitutes approximately 1.5 percent of the body mass. The hematopoietic tissue in the cavities of the long bones is eventually replaced by fatty tissue and the marrow becomes yellow.
Red bone marrow is the main hematopoietic organ in man and other adult mammals. The red blood cells, granular leukocytes (neutrophils, eosinophils, basophils), blood platelets (thrombocytes), and marrow lymphocytes are produced by the red bone marrow. Bone marrow contains hematopoietic stem cells (approximately 0.1 percent of all its cells). Owing to their capacity for repeated division and development into all forms of hematopoietic and lymphoid cells, stem cells sustain hematopoiesis in the bone marrow and replace the white and red blood cells that are constantly being destroyed in the body. Bone marrow primarily consists of different series of maturing cells (erythroid, myeloid, lymphocytic megakaryocytic). All are produced and replenished by the stem cells, and some are capable of repeated division. The relative amount of maturing cells of the individual blood series and of more or less mature cellular forms of each series in bone marrow is an important characteristic of hematopoiesis. As the cells from the bone marrow mature, they enter the bloodstream. In addition to mature cells, some hematopoietic stem cells emerge from the bone marrow and migrate to other hematopoietic organs. The basis of bone marrow is reticular tissue, which forms the syncytium, in which the hematopoietic cells are distributed. The multiplication and maturation of these cells are largely dependent on their interaction with reticular tissue (which is also capable of forming bone, a phenomenon manifested during the healing of bone fractures). The rate of hematopoiesis in bone marrow can increase sharply. As a result, a substantial loss of blood cells (for example, in bleeding) or the destruction of many bone marrow cells is usually compensated for quickly. However, bone marrow (especially its stem cells) is highly sensitive to certain agents (for example, ionizing radiation). Therefore, the condition of bone marrow is one of the principal factors determining the body’s resistance to such agents.
A. IA. FRIDENSHTEIN