USA Surgery Today

Hematopoiesis in the human embryo and fetus can be divided into three overlapping phases: mesoblastic, hepatic, and myeloid (1). The earliest cells of erythroid lineage have been detected at day 14 of gestation within the blood islands of the yolk sac. By day 35, erythropoiesis occurs in the liver, and granulopoiesis and thrombocytopoiesis follow. Until 6 months gestation, the liver is the major site of hematopoiesis. The myeloid phase starts in the fourth month, when hematopoiesis, including monocyte and lymphocyte production, begins in the bone marrow. The bone marrow becomes the major site of hematopoiesis. Through early childhood, most of the bone marrow space is occupied by red (hematopoietically active) marrow. With age, the distal marrow spaces slowly fill with fat.