A Comprehensive Review of Hematological Disorders

Hematopoiesis is the maturation of hematopoietic stem cells into blood cells. This process takes place in different places during the early embryo. During the third week of human embryogenesis, a group of mesenchymal cells in the yolk sac forms clusters called blood islands. These islands have central cells that differentiate to form primitive blood-forming cells, and peripheral cells that form a primitive vascular system. Some of these cells become primitive erythroblasts, the first cells to produce hemoglobin. These cells are not promyelocytes, but they do not develop into erythrocytes as do the adult bone marrow pronormoblasts. In the third month of embryonic development, stem cells migrate to the liver, which becomes the primary site of blood cell production. Hematopoiesis is also accomplished in the spleen, lymph nodes, and thymus. By the fourth month of gestation, bone marrow hematopoiesis begins, and by birth it is the main source of blood cell production. The process of hematopoiesis in bone marrow is called medullary hematopoiesis, and the process of the creation of blood cells outside the bone marrow is called extramedullary hematopoiesis. Hematopoiesis is present in almost all bones at birth. In adults, it occurs primarily in flat bones (sternum, ribs, skull, and vertebrae) and in the ends of long bones. When demand is high, the marrow might reappear in other sites. Extramedullary hematopoiesis normally diminishes after birth, but can happen in the liver or spleen in certain diseases. In the bone marrow, nutrient arteries give rise to arterioles and sinusoids, which supply the blood cell-forming cells. The marrow stroma is the site of erythropoiesis, granulopoiesis, and thrombopoiesis. The release of blood cells from marrow into circulation is a crucial step, and occurs when mature blood cells pass through the walls of the sinusoids into the blood. This is essential but not well understood.