Which organ is the primary site of hematopoiesis in the fetus before midpregnancy -

Ans. is 'b' i.e., Liver In fetus hematopoiesis occurs in three anatomic stages. Mesoblastic stage Hepatitis stage Myeloid stage o This stage of hematopoiesis begins between 10th and 14th days of gestation o It occurs in the extraembryonic structures principally in the yolk sac. o This stage of hematopoiesis essentially ceases by 10-12 week of gestation. o By 6-8 weeks of gestation, the liver replaces the yolk sac as primary site of gestation. o Hematopoiesis occurs in the liver throughout the remainder of the gestation and the liver remains the predominant hematopoietic organ,. till 20-24 weeks of the gestation. o After this the hematopoiesis begins to decrease in liver and in full term infant hepatic hematopoiesis is seen only in widely scattered small foci which soon become inactive after bih. o Hematopoiesis in the marrow stas by 4th month of gestation. o By bih, marrow throughout the skeleton is hematopoietically active and viually the sole source of blood cells. Hematopoiesis after bih Upto pubey Hematopoiesis occurs throughout the skeleton After pubey Only the veebrae, ribs, sternum, skull, pelvis and proximal epiphyseal regions of the (By the age of 18) humerus and femur retain red marrow, the remaining marrow becomes yellow, fatty an inactive. Thus, in adults only about half of the marrow space is active in hematopoiesis.