HEMATOPOIESIS Circulating hematopoietic progenitor cells in first trimester fetal blood

The yolk sac and aorto-gonad-mesoneph-ros region are well recognized as the prin-cipal sites of hematopoiesis in the de-veloping embryo, and the liver is the principal site of hematopoiesis in the fetus. However, little is known about circu-lating hematopoietic stem and progenitor cells in early fetal life. We investigated the number and characteristics of circu-lating progenitors in first trimester blood of 64 human fetuses (median gestational age, 1014 weeks; range, 716-1316 weeks). CD341 cells accounted for 5.1 6 1.0 % of CD451 cells in first trimester blood, which is significantly more than in term cord blood (0.4 6 0.03%; P 5.0015). How-ever, the concentration of CD341 cells (6.6 6 2.4 3 104/mL) was similar to that in term cord blood (5.6 6 3.9 3 104/mL). The total number of progenitors cultured from unsorted mononuclear cells (MNCs) in first trimester blood was 19.2 6 2.1 3 103/mL, which is similar to that in ter