ABSTRACT
For studies on the developmental stages of migrating erythroid cells and the development of sinusoid, transmission and scanning electron microscopic observations were undertaken on rat fetal liver in 13, 15, 17, 19, 21 days of gestation. The results obtained were as follows. 1. The hepatic sinusoidal endothelium were mainly non-fenestrated cell and fenestrated cell with diaphragm before 17 days of gestation, but fenestrated cells without diaphragm began to appear after 17 days of gestation. Two types of fenestrae were observed, free and clustered fenestrae, but both types were not involved in migration of erythroid cells. 2. Endothelial cell was continuous with neighboring cells by intercellular junctions between lateral cytoplasmic processes with zonula adherens, and between perinuclear cytoplasms with macula adherens. 3. After 13 days of gestation, Kupffer cells showed as matured cell morphology of irregular shape with long cytoplasmic processes into hepatic cord and perisinusoidal space. 4. Migrating erythroid cells in rat fetal liver sinusoid were mainly consisted of immature erythroblasts from proerythroblast to acidophilic erythroblast. The migration occurred through the migrating pores formed on the various sites of the endothelial cytoplasm into the hepatic sinusoidal lumen with no relation to the maturation stages of erythroblast and endothelial cell. In summary, the migration of erythroid cells in the sinusoid of rat fetal liver occurred through the invasion and migration pores transiently formed at various sites of endothelial cytoplasm, and migrating erythroid cells were mainly nucleated immature types.