Mechanism of Liver Organogenesis and Its Application to Liver

为探讨肝切除后肝窦内皮细胞(SEC)增殖的机制，采用大鼠70%肝切除模型，观察再生肝血管内皮生长因子(VEGF)及其受体的表达。肝切除后48~72h，血管内皮细胞生长因子mRNA的表达显著增加。免疫组织化学染色显示，肝切除后24小时血管内皮生长因子表达开始增加，72小时达高峰。甚至在肝切除前，Flt-1和KDR/Flk-1就在肝窦中观察到表达，但在72-120小时后有所增加。此外，肝切除后72小时分离的培养肝细胞产生的血管内皮生长因子显著增加。与肝细胞相比，SECs的增殖细胞核抗原标记指数表现出延迟和较慢的再生反应，在72小时达到峰值。这些数据有力地表明，增殖中的肝细胞分泌的VEGF可能是SEC增殖的重要刺激因子。此外，我们还重点研究了Angiopoietin-Tie系统，该系统已被证明通过调节VEGF来调节肝脏器官的形成。小鼠胚胎早期的内皮细胞包围新指定的肝脏内胚层，并划定肝芽生长到的间充质区域。在缺乏内皮细胞的Flk-1突变胚胎中，出现了肝脏规格，但在间质侵袭之前，肝脏形态发生失败。结论：血管生成的内皮细胞和新生血管在器官发生的最早阶段，在血管功能之前是至关重要的。

To clarify the mechanism of sinusoidal endothelial cell(SEC) proliferation after hepatectomy, the expression of vascular endothelial growth factor(VEGF) and VEGF-receptor of the regenerating liver were evaluated with 70% hepatectomy model in the rats. The expression of VEGF mRNA was increased markedly between 48 and 72 hr after hepatectomy. The immunohistochemical staining revealed that expression of VEGF started to increase 24 hr after hepatectomy, with a peak at 72 hr. Expression of flt-1 and KDR/flk-1 was observed along the sinusoids even before hepatectomy, but was increased between 72 and 120 hr. Furtheremore, VEGF production by cultured hepatocytes isolated 72 hr after hepatectomy was siginificantly increased. The PCNA labeling index of the SECs exhibited a delayed and slower regenerative response in comparison to the hepatocytes, teaching a peak at 72 hr. These data strongly suggest that VEGF secreted by proliferating hepatocytes may represent an important stimulator of SEC proliferation.Furthermore, we focused on the research of Angiopoietin-Tie system that has been shown to regulate liver organogenesis through the modulation of VEGF. Early endothelial cells in mouse embryo surround newly specified hepatic endoderm and delimit the mesenchymal domain into which the liver bud grows. In flk-1 mutant embryo, which lack endothelial cells, hepatic specification occurs, but liver morphogenesis fails prior to mesenchyme invasion. It is concluded that vasculogenic endothelial cells and nascent vessels are critical for the earliest stages of organogenesis, prior to blood vessel function.