Four types of presumptive hemopoietic stem cells in the human fetal liver.

Presumptive hemopoietic stem cells in the human liver obtained from 109 embryos 28 to 49 days after ovulation and 76 fetuses between 8 and 22 weeks of ovulation were investigated by light and electron microscopy. Presumptive hemopoietic stem cells in the human embryonic liver are concluded to be a series of cells that show a variegated ultrastructure. They are classified into four subtypes (type I, II, III and IV). Presumptive hemopoietic stem cells of type I are thought to differentiate from the undifferentiated mesenchymal cells that are derived from the septum transversum. Presumptive stem cells of type I, II and III transitorily appear in the liver during the early stage of hepatic hemopoiesis, and cannot be detected in late stages. With the development of the fetus, presumptive stem cells of the type IV, however, gradually increase in number. The cells of megakaryocytic, granulocytic and erythrocytic lineages originate from the presumptive stem cells of type II in the early stage of hepatic hemopoiesis, whereas the cells of the three lineages originate from the presumptive stem cells of type IV in the late stage. The presumptive hemopoietic stem cells of type IV are surmised as corresponding to the pluripotent hemopoietic stem cells (CFU-S) in laboratory animals or pluripotent hemopoietic progenitors in human bone marrow (CFU-mix).