Ultrastructural identification of the hemopoietic inductive microenvironment in the human embryonic liver.

Reciprocal interaction between the hemopoietic organ stromal cells and the cells of the granulocytic, megakaryocytic and erythrocytic series in the human liver obtained from 109 embryos 28 to 49 days after ovulation and 76 fetuses from 8 to 22 weeks of gestation were investigated by light and electron microscopy. The close association of stromal cells with immature cells of the three series was confirmed under the electron microscope and a presumptive HIM (hemopoietic inductive microenvironment) was visualized. A majority of immature erythroblasts intruded into the cytoplasm of the hepatocytes, so the presumptive hemopoietic stem cell types II and IV are undoubtedly differentiated into cells of the erythroid line by contact with hepatocytes at a certain stage of maturation. Granulopoiesis developed among the reticular cells around the ductus venosus--or large arteries in hepatic parenchyma--and the cells of the granulocytic series were enclosed by thin cytoplasmic projections of mesenchymal cells. Neither erythropoiesis nor megakaryopoiesis was noted here. Therefore the compartments composed of one or more reticular cells around the ductus venosus or large arteries seem to have a capacity to regulate the differentiation of the presumptive hemopoietic stem cells type IV into cells of the granulocytic series. This differentiation of presumptive hemopoietic stem cell types II and IV into the megakaryocytic series is belived to be induced by the presence of the microenvironments that consist of foci of a few reticular cells in the hepatic parenchyma, as immature cells of megakaryocytic lineage were encircled by the cytoplasmic projection of one or more reticular cells among hepatocytes. Erythroblastic islets are concluded to be a kind of HIM, where erythroblasts loosely adhere to the central macrophages and undergo mitoses and maturation.