From day 31, primitive red blood cells arise in the wall of the yolk sac-they are formed in blood islands with simple surrounding capillaries. The earliest primitive red blood cells contain embryonic hemoglobin, and they have a central nucleus unlike mature red blood cells. By day 74, the fetal liver will have taken over blood cell production from the yolk sac. Unlike the first primitive red blood cells, the cells produced in the fetal liver can differentiate into any of the components of fetal blood. By the end of pregnancy blood cell production also occurs within the bone marrow.

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  • Category: Pregnancy and childbirth