Health Human Disease Blog

Bone marrow in the adult occupies the verte­brae, sternum, ribs, pelvic bones, and, to a lesser extent, the long bones and skull, and comprises about 1 kg of tissue. In the child, hematopoiesis is more prominent in the long bones and skull, whereas in adulthood fatty tissue replaces the he­matopoietic marrow in these peripheral areas. In addition, liver and spleen, primary sites of he­matopoiesis in the fetus, may regain hemato­poietic acitivity in certain adult diseases, includ­ing severe anemias, skeletal marrow failure, myelofibrosis, and hematologic malignancies such as leukemia.
Normal bone marrow consists of about 50 per cent hematopoietic cells and 50 per cent fat, with the hematopoietic cells being arranged in cords around sinusoids; blood vessels are abundant (Figs. 50-1 and 50—2). The space occupied by fat increases during adulthood, although hen poietic tissue can replace it under approp: conditions at any age. The hematopoietic cell elude a small pluripotential stem cell comj ment, consisting of small cells resembling 1 phocytes, a large compartment of prolifera cells of committed lineage (the ratio of r loid:erythroid precursors is normally 3 to and a large compartment of postmitotic matu cells of both myeloid and erythroid line Smaller numbers of megakaryocytes, which ferentiate into platelets, plasma cells (which duce immunoglobulins), reticulum cells, lymphocytes are also present. It requires al one week for a stem cell to differentiate into ture daughter cells of either erythroid or myei type; the marrow additionally contains about week’s worth of mature leukocytes and erytlcytes. The circulating half-life of polymorpho­nuclear leukocytes is about 6 hours, of platelets 8 to 10 days, and of erythrocytes 120 days; thus, the granulocyte pool is released much faster than the erythrocyte pool, which is renewed at a rate of 0.8 per cent per day. Because the ratio in the periph­eral blood of granulocytes: erythrocytes: platelets is about 1:5000:100, approximately equal num­bers of these three cell types are released from the bone marrow each day.
Erythropoiesis is under the control of the hor­mone, erythropoietin, released from the kidney under the’stimulus of tissue hypoxia. A glyco­protein of molecular weight 46,000, erythropoi­etin may also be produced in the liver. It stimu­lates primitive pluripotential or stem cells into erythropoietic differentiation, acting mainly on the CFU-E (colony forming unit-erythroid) to pro­mote differentiation into proerythroblasts. The earliest recognizable red cell precursor, the eryth-Toblast, eventually gives rise to eight or more mature erythrocytes. The erythroblast is a large cell with abundant endoplasmic reticulum, which decreases in size as it matures, incorporates transferrin-bound iron via specific membrane recep­tors, and begins to form hemoglobin. In more ma­ture stages the developing erythroblasts, the basophilic, then polychromatophilic, then or-thochromic normoblasts, lose the capacity to di­vide. Their nuclei become pyknotic and are fi­nally extruded prior to release of the erythrocytes from the bone marrow. Newly released erythro­cytes, or reticulocytes, have active mitochondrial function and retain the capacity to form hemo­globin; about 25 per cent of hemoglobin is syn­thesized after these cells leave the bone marrow, but the mature erythrocyte in the circulation lacks the capacity for protein synthesis and is anu-cleate. Its energy is supplied by glycolysis.