Development of Lymphocytic Subpopulations and Hepatic Haematopoiesis during Human Embryonic and Fetal Periods

Abstract

At first, T and B lymphocytic subpopulations were examined for different surface markers on 5 cases of human fetal lymphoid tissues at 14 to 22 weeks of gestation (CH 15cm-29cm), and on 25 cases of human embryonic and fetal livers at 5 to 22 weeks of gestation (CR 8mm-CH 29cm).
In the second study, hepatic haematopoiesis were studied with electronmicroscope on 25 cases of livers ranging from 4 to 22 weeks of gestation. Fetal age were caluculated by fetilization age and by crown to rump or heel length. The results obtained were as followings; 1) More than 70 per cent of lymphocytes in the bone marrow and liver were null cells, in contrast to most of lymphocytes in thymus and lymph nodes were E and/or EAC rosette forming cells.2) In the liver, EAC rosette forming cells were first appeared at 6 weeks, then E rosette forming cells at 7 weeks, smIgM-cells and smIgD-cells at 10 weeks, smIgG-cells at 11 weeks and at last, smIgA-cells at 22 weeks of gestation.3) After 5 weeks of gestation, several types of undifferentiated mononuclear cells and mesenchymal cells were mainly seen in the extrasinusoidal spaces.4) The erythroid progenitor cells were first detected at 5 weeks of gestation only in the extrasinusoidal spaces and neutrophils in both the sinusoid and the extrasinusoidal spaces at the same time. After 6 weeks of gestation, Myeloblasts, lymphoid cells and megakaryoblasts were seen in both spaces and monoblasts only in the extrasinusoidal spaces.5) In hepatic erythropoiesis, several findings such as breakdown of nuclear membrane with myelinization, cytoplasmic invagination into the nucleus, nuclear extrusion to cytoplasma, binuclearity, nuclear cleft, nuclear bleb, and frequent mitotic figures were observed. These findings were similar to the cases of congenital dyserythropoiesis.
Multipotential stem cells could n ot be identified morphologically, but several undifferentiated mononuclear cells, which were presumably stem cells, could be found only at very early stage of hepatic haematopoiesis mainly in the extrasinusoidal spaces or in the hepatic cord which was directly connected with sinusoid. We could not decide whether undifferentiated mononuclear cells derived from yolk sac or from mesenchymal cells originated from septum transversum. However it is likely that the undifferentiated mononuclear cells (stem cells)come from both yolk sac and hepatic mesenchymal cells on the basis of cell appearance and their distributing course.