Molecular mechanism of proliferation and differentiation of hepatocyte.

成肝细胞（肝干细胞）被认为是肝细胞和胆管上皮细胞的共同前体。然而，它们的性质在很大程度上仍不为人所知。我们以前证明，小鼠成肝细胞表达Dlk/Pref 1，一种具有EGF重复序列的细胞表面分子，并且通过使用抗Dlk抗体分离的成肝细胞在体外分化为肝细胞和胆管上皮细胞。我们发现Notch 2在肝母细胞中表达，其配体Jagged-1在形成胆管的门静脉周围的细胞中表达。此外，在Dlk+细胞中表达活化形式的Notch抑制成肝细胞向肝细胞的分化，并增强体外向胆管上皮细胞的分化。这些结果与Jagged-1与Alagille综合征（表现为肝内胆管发育受损）有关的发现一致。总之，Notch信号在成肝细胞分化中起重要作用。从胎肝分离的Dlk+成肝细胞在层粘连蛋白包被的培养板中增殖，并可重复地建立长期培养。这些细胞保留了分化为肝细胞、胆管上皮细胞以及具有胰腺基因表达的细胞的能力，这表明它们可能是多潜能的内胚层祖细胞/干细胞。在严重损伤的肝脏汇管区出现的卵圆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，我们发现一个子集的卵圆细胞表达Dlk，表明卵圆细胞是异质性的。

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.