Background: The origin of cardiac lymphatics from venous endothelial cells or from scattered lymphangioblasts has been discussed in the literature. We aimed to establish the stage when lymphatic vessels appear in the developing mouse heart, the location of the first lymphatics, and to define cellular phenotypes of growing lymphatics. Results: We found that scattered Lyve-1-positive cells located in the subepicardial area of developing heart expressed CD45, CD68, F4/80, or CD11b but not CD31. Prox-1+/Lyve-1+ cellular cords or vessels were found to invade 12.5–13.5-dpc hearts via two routes: from the venous pole, i.e., dorsal atrioventricular sulcus, or on the dorsal atrial surface from mediastinum and from the arterial pole, i.e., along the great arteries. The Prox-1+/Lyve-1+ vessels were located among the Prox-1+/Lyve-1− cords and among the scattered Prox-1−/Lyve-1+ cells. The Prox-1+/Lyve-1− cellular cords/tubules dominate initially at the arterial pole whereas Lyve-1+/Prox-1− cellular cords/tubules dominate initially on the venous pole, i.e., dorsal atrioventricular sulcus. The Lyve-1+/CD45+, Lyve-1+/CD11b+, Lyve-1+/F4/80+ and Lyve-1+/CD68+ cells were subsequently found to be co-opted to the wall of the developing lymphatic vessels while gaining Flk-1. Conclusions: Lymphatic primordia exhibit different cellular phenotypes and different spatiotemporal pattern on the venous pole as compared with the arterial pole of the heart. Developmental Dynamics 241:1473–1486, 2012. © 2012 Wiley Periodicals, Inc.