The development of lymphatic endothelial cells (LECs) from deep embryonic veins or mesenchymal lymphangioblasts is controversially discussed. Studies employing quail‐chick grafting experiments have shown that various mesodermal compartments of the embryo possess lymphangiogenic potential, whereas studies on murine embryos have been in favor of a venous origin of LECs. We have investigated NMRI mice from embryonic day (ED) 9.5 to 13.5 with antibodies against the leukocyte marker CD45, the pan‐endothelial marker CD31, and the lymphendothelial markers Prox1 and Lyve‐1. Early signs of the development of lymphatics are the Lyve‐1‐ and Prox1‐positive segments of the jugular and vitelline veins. Then, lymph sacs, which are found in the jugular region of ED 11.5 mice, express Prox1, Lyve‐1, and CD31. Furthermore, scattered cells positive for all of the four markers are present in the mesenchyme of the dermatomes and the mediastinum before lymphatic vessels are present in these regions. Their number increases during development. A gradient of increasing CD31 expression can be seen the closer the cells are located to the lymph sacs. Our studies provide evidence for the existence of scattered mesenchymal cells, which up‐regulate lymphendothelial and down‐regulate leukocyte characteristics when they integrate into growing murine lymphatics. Such stem cells may also be present in the human and may be the cell of origin in post‐transplantation Kaposi sarcoma. Developmental Dynamics 235:1554–1562, 2006. © 2006 Wiley‐Liss, Inc.