Mechanism of Liver Organogenesis and Its Application to Liver

为阐明肝切除后血管内皮细胞（SEC）增殖的机制，采用70%肝切除大鼠模型，检测再生肝血管内皮生长因子（VEGF）及VEGF受体的表达。在肝切除术后48 ~ 72小时，VEGF mRNA的表达明显升高。免疫组化染色显示，肝切除术后24小时VEGF表达开始升高，72小时达到峰值。flt-1和KDR/flk-1在肝切除术前沿肝窦表达，但在72至120小时内升高。此外，肝切除术后72小时分离的培养肝细胞的VEGF产量显著增加。与肝细胞相比，SECs的PCNA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。这些数据强烈提示，增殖肝细胞分泌的VEGF可能是SEC增殖的重要刺激物。此外，我们重点研究了血管生成素- tie系统，该系统已被证明通过调节VEGF来调节肝脏器官发生。小鼠胚胎早期内皮细胞围绕在新指定的肝内胚层周围，并划定肝芽生长的间充质区域。在缺乏内皮细胞的flk-1突变胚胎中，肝脏形成，但在间质侵袭前肝脏形态发生失败。因此，血管内皮细胞和新生血管在器官发生的早期阶段至关重要，而不是血管功能。

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.