an immunohistochemical and ultrastructural study on the intrahepatic biliary system of fetal and neonatal mouse *
To clarify the relationship between the hematopoietic involution of early postnatal livers and the development of the intrahepatic biliary system, fetal and neonatal mouse livers were examined through immunohistochemical observations and transmission electron microscopic observations. Bile canaliculi were formed between neighboring hepatocytes at 14 days of gestation, and the canaliculi often expanded their diameter around 1.5 to 3 times in the late fetal and early neonatal livers. Hepatocytes in early postnatal life often phagocytosed degenerating hematopoietic cell elements to form large inclusions, 3 to 5 μm in diameter, and these inclusions were phagolysosomes containing vacuolar membrane structures and myelin figures. These inclusions moved in the cytoplasm to the bile canaliculi and finally fused to the canalicular wall. As a result, the canaliculi became expanded and inclusion contents were released into the canalicular lumen. Since the wall of the canaliculi derived from phagosomes had no microvilli, enlarged canaliculi showed a different localization in microvillous distribution. In neonatal and early postnatal livers, hematopoietic cells accumulated interlobular connective tissues, especially around interlobular bile ducts, and TUNEL-positive cells appeared among the hematopoietic cell accumulation. Large TUNEL-positive inclusions could also be observed in the bile duct epithelial cells of interlobular connective tissue. The hematopoietic cell accumulations consisted of mature neutrophils and erythroblasts, and the biliary epithelial cells engulfed dying neutrophils, erythroblasts and nuclei expelled from erythroblasts to form TUNELpositive inclusions. During the involution stage of liver hematopoiesis, both hepatocytes and epithelial cells of the interlobular bile duct showed phagocytosis, and the intrahepatic biliary system appeared to have an important functional role in the removal of dying hematopoietic cell elements in the late gestational and early postnatal period. (Accepted on March 2, 2011)
