Differentiation of red pulp and evaluation of hemopoietic role of human prenatal spleen.

Both the development of red pulp and hemopoietic activity in spleen obtained from 62 human embryos and fetuses between 30 days and 20 weeks after ovulation were investigated light and electron microscopically. The spleen develops in the left-posterior portion of the dorsal mesogastrium at 35-40 days after ovulation. At the 8th week after ovulation, reticular cells formed a three-dimensional meshwork. Two types of reticular cells (dark and clear reticular cells) were observed in the splenic cord in the 12-13th week after ovulation. Mature hemopoietic cells, mostly of the erythroblastic series, increased in number in the extravascular spaces with the development of the fetus. However, presumptive hemopoietic stem cells or "undifferentiated mononuclear cells" (Fukuda, 1973a) did not appear in the spleen. Moreover, immature hemopoietic cells such as proerythroblasts, myeloblasts and megakaryoblasts could not be detected. Therefore, despite the occurrence of a well-developed reticular cell network, hemopoiesis was judged to have not taken place in human fetal spleen. Macrophages appeared in the spleen at the 8th week after ovulation and increased in number with the the development of the fetus. Phagocytosis of decrepit blood cells proved to be an essential function of the spleen.