Molecular mechanism of proliferation and differentiation of hepatocyte.

肝母细胞（肝干细胞）被认为是肝细胞和胆道上皮细胞的共同前体。然而，它们的性质在很大程度上仍然未知。我们之前证明，小鼠肝母细胞表达具有EGF重复序列的细胞表面分子Dlk/Pref1，并且使用抗Dlk抗体分离的肝母细胞在体外可分化为肝细胞和胆道上皮细胞。我们发现Notch2在肝母细胞中表达，其配体Jagged-1在形成胆管的门静脉周围细胞中表达。此外，激活形式的Notch在Dlk+细胞中的表达抑制了肝母细胞向肝细胞的分化，并增强了向胆道上皮细胞的分化。这些结果与Jagged-1导致肝内胆管发育受损的Alagille综合征的发现是一致的。综上所述，Notch信号在肝母细胞分化过程中起重要作用。从胎儿肝脏分离的Dlk+肝母细胞在涂有层粘连蛋白的培养板中增殖，并建立了长期培养的可重复性。这些细胞保留了向肝细胞、胆道上皮细胞和胰腺基因表达细胞分化的能力，表明它们可能是多能内胚层祖细胞/干细胞。Dlk的表达随着肝脏的发育而下降，在成人肝脏中完全不存在。在严重损伤肝的门静脉区出现的卵形细胞被认为是成体肝干细胞。因此，我们检查了Dlk是否在卵圆细胞中表达。通过使用大鼠模型，我们发现一部分卵形细胞表达Dlk，表明卵形细胞是异质的。

Hepatoblasts (hepatic stem cells) are considered to be the common precursor for hepatocytes and biliary epithelial cells. However, their nature remains largely unknown. We previously demonstrated that mouse hepatoblasts express Dlk/Pref1, a cell surface molecule with EGF repeats, and hepatoblasts isolated by using anti-Dlk antibody were shown to differentiate to hepatocytes and biliary epithelial cells in vitro. We found that Notch2 was expressed in hepatoblasts and its ligand Jagged-1 was expressed in the cells surrounding the portal vein where bile ducts are formed. Moreover, expression of an activated form of Notch in Dlk+ cells suppressed the differentiation of hepatoblasts to hepatocytes and enhanced the differentiation to biliary epithelial cells in vitro. These results are consistent with the finding that Jagged-1 is responsible for the Alagille syndrome that exhibits impaired development of intrahepatic bile ducts. In conclusion, Notch signaling is important for the differentiation of hepatoblasts.Dlk+ hepatoblasts isolated from fetal liver proliferated in a culture plate coated with laminin and long-term culture was reproducibly established. These cells retained the ability to differentiate to hepatocytes, biliary epithelial cells and also cells with pancreatic gene expression, suggesting that they may be multipotential endodermal progenitors/stem cells.Dlk expression declines along with hepatic development and is completely absent in adult liver. Oval cells that appear in portal area of severely injured liver have been considered as adult liver stem cells. We therefore examined if Dlk is expressed in oval cells. By using a rat model we found that a subset of oval cells expressed Dlk, indicating that oval cells are heterogeneous.