Progenitor hematopoietic stem cells originate in which of the following?

Key Concept: By birth, marrow throughout the skeleton is hematopoietically active and hepatic hematopoiesis dwindles to a trickle, persisting only in widely scattered foci that become inactive soon after birth. Haematopoietic Organs In the human embryo, the yolk sac is the main site of haematopoiesis in the first few weeks of gestation. By about 3rd month, however, the liver and spleen are the main sites of blood cells formation and continue to do so until about 2 weeks after birth. Haematopoiesis commences in the bone marrow by 4th and 5th month and becomes fully active by 7th and 8th month, so that at birth practically all the bones contain active marrow. During normal childhood and adult life, therefore, the marrow is the only source of new blood cells. However, during childhood, there is progressive fatty replacement throughout the long bones, so that by adult life the hematopoietic marrow is confined to the central skeleton (vertebrae, sternum, ribs, skull, sacrum and pelvis) and proximal ends of femur, tibia and humerus. Even in these hematopoietic areas, about 50% of the marrow consists of fat. Non-hematopoietic marrow in the adult is, however, capable of reverting to active hematopoiesis in certain pathologic conditions. The spleen and liver can also resume their fetal hematopoietic role in certain pathologic conditions and is called extramedullary hematopoiesis.