Which organ is the primary site of hematopoiesis in the fetus before mid pregnancy?
During embryogenesis, hematopoiesis occurs in spatially and temporally distinct sites, including the extraembryonic yolk sac, the fetal liver, and the preterm marrow. The development of primitive erythroblasts in the yolk sac is critical for embryonic survival. After 7 weeks' gestation, hematopoietic progenitors are no longer detected in the yolk sac. The liver serves as the primary source of red cells from the 9th to the 24th week of gestation. The marrow becomes the major site of hematopoiesis after the 24th week of gestation and remains so throughout the remainder of fetal life. Also know: Hb Gower-1 (22) is the major hemoglobin in embryos younger than 5 weeks. Hb F (22) is the major hemoglobin of fetal life. Ref: Palis J., Segel G.B. (2010). Chapter 6. Hematology of the Fetus and Newborn. In J.T. Prchal, K. Kaushansky, M.A. Lichtman, T.J. Kipps, U. Seligsohn (Eds), Williams Hematology, 8e.
Bone
Liver
Spleen
Lung