Immunoelectron Microscopic Study of Human Megakaryocytic Precursor Cells in Embryonal and Fetal Liver

Megakaryocytic series in human embryonal and fetal livers were studied by immunoelectron microscopy using anti Glycoprotein IIb/IIIa complex antibody. The livers were obtained from 25 embryos and fetuses from 38 days to 11 weeks of gestation. Megakaryocytic series from mature megakaryocyte （including platelet） to megakaryoblast were all positive for anti Glycoprotein IIb/IIIa. Some undifferentiated cells and macrophages revealed also positive reaction. The undifferentiated cells were regarded as progenitors of megakaryocytic cells. The megakaryocytic progenitor cells and megakaryoblast were subdivided into two subgroups （A-type and B-type）. The A-type cells were detected in the liver from 42 days of gestation. The cells were about 15μm in size, and had large nucleolus and finely dispersed chromatin. The B-type cells were detected in the liver from 50 days later, showing 7～10μm in diameter. The nucleus of the cells had obscure nucleoli and coarsely clumping chromatin. Morphologically the progenitors of A-type were quite different from B-type. These findings indicate that the ansestor of A-type megakaryocytic progenitor cells are different from those of B-type megakaryocytic progenitor cells. Consequently these data are consistent with the theory of our laboratory that presumptive hemopoietic stem cells are a series of cells consisting of four subtypes