Liver size is predetermined in the neonate by adding lobules at the periphery
Organs vary in size between and within species to match organismal needs1,2. Decades-old theoretical work has proposed that scaling of organs and body parts relative to the body relies on the features of energy-transport systems, the vascular system in mammals3. Yet, experimental studies on whether or how vascularization helps determine organ size have lagged behind. The mammalian liver is a remarkable example, as liver size scales proportionally with high precision between individuals4. Here, we use quantitative clonal mapping, volumetric imaging, and genetic perturbations combined with novel molecular and genetic tools to identify the temporal and spatial constraints that establish mouse liver size. We find that adult liver size is predetermined during a neonatal period when new functional units, termed lobules, are added to the organ. New lobules are vascularized by prominent sprouting angiogenesis of the hepatic vein, restricted to the periphery of the organ. When Wnt signals are ablated in the single cell-layered mesothelium at the periphery, lobule growth fails, and the organ adopts a compromised size set point. Remarkably, within a week after birth and well before hepatocyte division stops, vein sprouting rapidly declines and lobule addition concludes, setting a limit on the final liver size. These findings posit that vascularization in the neonate constrains and helps determine adult liver size. Together, these results propose a novel, vasculature-centric experimental framework for studying organ size control and scaling in mammals.