The maintenance of blood vessel integrity is crucial for vascular homeostasis. During development, blood vessels undergo significant growth and remodeling but how do they retain their integrity during this time? Here, Taija Mäkinen and co-workers investigate this issue and reveal a key role for platelets and lymphatic vessels during developmental vascular remodeling in mice. They first note that, between embryonic day (E) 13 and E15, developing lymphatic vessels in the mesentery unexpectedly contain red blood cells (RBCs). The authors further describe that mesenteric blood vessels undergo remodeling and maturation during this time, and that this involves a transient loss in the integrity of veins, correlating with the appearance of extravascular RBCs. Finally, the researchers show that remodeling veins also contain activated platelets, suggesting that platelets might act to prevent RBC extravasation. In line with this, they show that the mesenteric lymphatic vessels of E14 Clec1b/ embryos, which are known to exhibit impaired platelet function, contain excessive extravascular RBCs and platelets. Overall, these findings uncover novel functions for platelets and lymphatic vessels in maintaining venous integrity and clearing extravascular RBCs, respectively, during vascular remodeling; they also provide insights into how abnormalities characterized by blood-filled lymphatic vessels might arise.