FETAL HEMATOPOIESIS .2. ELECTRON-MICROSCOPIC STUDIES ON HUMAN HEPATIC HEMATOPOIESIS

Hemopoiesis in the liver of 26 human embryos and fetuses at 26 days after ovulation to 30 weeks of gestation was studied by electron microscopy. The development of hepatic hemopoiesis seems to be closely correlated with the histogenetic developmental course of the liver. In the earliest stage of hepatic hemopoiesis, undifferentiated mononuclear cells, i.e. presumptive hemopoietic stem cells and transitional cells to the immature erythroid cells, are found in the intercellular spaces of the hepatocytes. With the development of the fetus, erythroid cells increase in the hepatic parenchyma, whereas the stem cells become gradually reduced in number and cannot be detected in the later stages. These stem cells are exclusively observed in the extravascular spaces and are considered to be derived from the septum transversum. The ultrastructure of hepatic erythroid cells is quite different from yolk sac-derived primitive erythroblasts and myeloid erythroblasts of the adult. Generally, hepatic erythroid cells show less discrepancy of nuclear and cytoplasmic maturation and free polyribosomes found in the cytoplasm are smaller in size and number than primitive erythroblasts. The particular cytoplasmic features found in primitive erythroblasts, and the microtubules characteristic of myeloid erythroblasts of the adult, are rarely observed in hepatic series. The primitive and hepatic series of erythroid cells seem to be independent of each other and are considered to belong to different clones. A small number of granulocytic and megakaryocytic cells are found from the earliest stages of hepatic hemopoiesis and their morphology seems to be different from that of the adult. Kupffer's stellate cells of the liver exist prior to the myeloid monoeytogenesis. © 1974 Springer-Verlag.