Endoglin (ENG) is essential for cardiovascular development and is expressed in the heart from its earliest developmental stages. ENG expression has been reported in the cardiac crescent, endocardium, valve mesenchyme and coronary vascular endothelial cells. However, its expression in these cell types is non-uniform and the dynamic changes in ENG expression during heart development have not been systematically studied.

Using immunofluorescent staining we tracked ENG protein expression in mouse embryonic hearts aged from 11.5 to 17.5 days, and in postnatal and adult hearts. ENG is expressed in the endocardium and in venous endothelial cells throughout these developmental stages. ENG protein is down-regulated by approximately two-fold as a subset of early coronary veins reprogram to form arteries within the developing myocardium from E13.5. This two-fold higher ratio of ENG protein in veins versus arteries is maintained throughout cardiac development and in the adult heart.

ENG is also down-regulated two-fold following mesenchymal transition of endocardial cells to form cardiac valve mesenchyme, whilst expression of the pan-endothelial marker CD31 is completely lost. A subset of epicardial cells (which do not express ENG protein) delaminate and undergo a similar mesenchymal transition to form epicardially derived cells (EPDCs). This transient intra-myocardial mesenchymal cell population expresses low levels of ENG protein, similar to valve mesenchyme.

In conclusion, ENG shows dynamic changes of expression in vascular endothelial cells, endocardial cells and mesenchymal cells in the developing heart that vary according to cardiovascular cell type.

Endoglin (ENG) 对心血管发育至关重要，并且从其最早的发育阶段就在心脏中表达。已经报道在心脏新月体、心内膜、瓣膜间充质和冠状血管内皮细胞中表达ENG。然而，它在这些细胞类型中的表达是不均匀的，并且尚未系统地研究心脏发育过程中 ENG 表达的动态变化。

我们使用免疫荧光染色跟踪了 11.5 至 17.5 天的小鼠胚胎心脏以及出生后和成年心脏中的 ENG 蛋白表达。在这些发育阶段，ENG 在心内膜和静脉内皮细胞中表达。作为早期冠状静脉的一个子集，ENG 蛋白被下调大约两倍，从而在 E13.5 的发育中的心肌内形成动脉。在整个心脏发育过程和成人心脏中，静脉与动脉中 ENG 蛋白的比例高出两倍。

在心内膜细胞间质转化形成心脏瓣膜间充质后，ENG 也下调了两倍，而泛内皮标志物 CD31 的表达完全丧失。一部分心外膜细胞（不表达 ENG 蛋白）分层并经历类似的间充质转变以形成心外膜衍生细胞 (EPDC)。这种短暂的心肌内间充质细胞群表达低水平的 ENG 蛋白，类似于瓣膜间充质。

总之，ENG 显示发育中心脏中血管内皮细胞、心内膜细胞和间充质细胞的表达动态变化，这些变化随心血管细胞类型而变化。