The embryonic epicardium, originating from the proepicardial organ (PEO), provides a source of multipotent progenitors for cardiac lineages, including pericytes, fibroblasts, and vascular smooth muscle cells. Maximizing the regenerative capacity of the adult epicardium depends on recapitulating embryonic cell fates. The potential of the epicardium to contribute coronary endothelium is unclear, due to conflicting Cre-based lineage trace data. Controversy also surrounds when epicardial cell fate becomes restricted. Here, we systematically investigate expression of five widely used epicardial markers, Wt1, Tcf21, Tbx18, Sema3d, and Scx, over the course of development. We show overlap of markers in all PEO and epicardial cells until E13.5, and find no evidence for discrete proepicardial sub-compartments that might contribute coronary endothelium via the epicardial layer. Our findings clarify a number of prevailing discrepancies and support the notion that epicardium-derived cell fate, to form fibroblasts or mural cells, is specified after epithelial-mesenchymal transition, not pre-determined within the PEO.

胚胎心外膜起源于心外膜器官（PEO），为心脏谱系提供了多能祖细胞的来源，包括周细胞、成纤维细胞和血管平滑肌细胞。成人心外膜再生能力的最大化取决于胚胎细胞命运的重现。由于基于 Cre 的谱系追踪数据相互矛盾，心外膜贡献冠状动脉内皮的潜力尚不清楚。心外膜细胞命运何时受到限制也存在争议。在这里，我们系统地研究了五种广泛使用的心外膜标记物Wt1、Tcf21、Tbx18、Sema3d和Scx在开发过程中的表达。我们在 E13.5 之前显示所有 PEO 和心外膜细胞中标记物的重叠，并且没有发现离散的心外膜亚室的证据，这些亚室可能通过心外膜层贡献冠状内皮。我们的研究结果澄清了许多普遍存在的差异，并支持这样一种观点，即心外膜来源的细胞命运（形成成纤维细胞或壁细胞）是在上皮-间质转化后指定的，而不是在 PEO 内预先确定的。