ABSTRACT
Fetal erythropoiesis involves coordinated activation of a series of specific genes within erythroid cells, production of hematopoietic growth factors by the fetus that influence erythropoietic activity, changes in the sites of active erythropoiesis with advancing gestational age, and regulation to maintain the hemoglobin concentration within a discrete range. This chapter summarizes state of knowledge and highlights experimental results and questions that are being actively pursued at this time. Blood cell formation occurs sequentially in the yolk sac, fetal liver and spleen, and bone marrow. The differentiated cells produced in each hematopoietic organ differ in terms of their physical size, composition and pattern of gene expression. Human fetal blood contains large numbers of hematopoietic progenitors in the circulation from 15 to 32 weeks gestation. In contrast to embryonic erythropoiesis, there is compelling evidence that fetal erythropoiesis is primarily regulated by Erythropoietin made in the fetal liver.
