Role of endothelial cells and pericyte interdigitation in angiogenesis

内皮细胞和周细胞在血管生成中起着关键作用，血管生成部分受血管生成素(Ang)/Tie-2系统和血管内皮生长因子（VEGF）的控制。内皮细胞和周细胞间指（EPI），由周细胞的细胞质突起和相应的内皮压痕组成，经常出现在血管生成部位。大鼠皮下植入热可逆凝胶化聚合物椎间盘后，第5天毛细血管密度较低，第7天达到峰值，第10-20天下降。EPI在第5天少量出现，在第10天急剧上升至峰值，但在第20天有所下降。光镜、电镜、免疫组化和RNA原位杂交分析显示，tio2定位于内皮细胞，Ang-2定位于内皮细胞和周细胞，而Ang-1和VEGF定位于周细胞，且Ang-1在周细胞EPI处的表达最为强烈。常规、定量RT-PCR和Western blot分析显示，大鼠血清中Ang-1水平在5-7天低，10-20天升高；VEGF水平在5-10天高，20天下降。在第5 ~ 20天，Ang-2和Tie-2的表达没有变化。本研究表明血管生成阶段可能由Ang-2和VEGF启动，微血管成熟阶段可能由Ang-1和VEGF启动。此外，EPI可能作为Ang-1/Tie-2系统的途径，VEGF促进周细胞募集，维持微血管完整性。

Endothelial cells and pericytes play critical role in angiogenesis, which is controlled, in part, by the angiopoietin (Ang)/Tie-2 system and vascular endothelial growth factor (VEGF). Endothelial cell and pericyte interdigitations(EPI), consisting of cytoplasmic projections of pericytes and corresponding endothelial indentations, are frequently present at angiogenic sites. After subcutaneous implantation of a thermoreversible gelation polymer disc in rats, the capillary density was low on day 5, increased to a peak on day 7, and then decreased on days 10-20. A small number of EPI were observed on day 5, then increased sharply to a peak on day 10, but had decreased on day 20. Light and electron microscopy immunohistochemical and RNA in situ hybridization analyses revealed that Tie-2 localized at endothelial cells, and Ang-2 localized at endothelial cells and pericytes, while Ang-1 and VEGF localized at pericytes, and Ang-1 was most intensely observed at EPI of pericytes. Conventional, quantitative RT-PCR and Western blot analyses revealed that the level of Ang-1 was low on days 5-7, and then increased on days 10-20, while the level of VEGF was high on days 5-10, but had decreased on day 20. The expression of Ang-2 and Tie-2 was unchanged on days 5-20. The present study showed that the angiogenic phase might be initiated by Ang-2 and VEGF, while the microvessel maturation phase might to be initiated by Ang-1 and VEGF. Moreover, EPI might act as a pathway for the Ang-1/Tie-2 system with VEGF promoting pericyte recruitment for microvascular integrity.