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Hematopoiesis in yolk sac start at
|C||12 th week|
Ans. A 3rd week
- Human hematopoiesis is initiated in the yolk sac during the third week of development.
- The liver serves as the primary source of red cells from the 9th to the 24th weeks of gestation.
- Hematopoietic cells are first seen in the marrow of the 10- to 11- week embryo, and they remain confined to the diaphyseal regions of long bones until 15 weeks gestation.Initially there are approximately equal numbers of myeloid and erythroid cells in the fetal marrow.
- However, myeloid cells predominate by 12 weeks gestation, and the myeloid to erythroid ratio approaches the adult level of 3 to 1 by 21 weeks gestation.
- Macrophage cells in the fetal marrow, but not in the fetal liver, express the lipopolysaccharide receptor CD14.The marrow becomes the major site of hematopoiesis after the 24th week of gestation.