The development of early human lymphatic vessels as characterized by lymphatic endothelial markers

Lymphatic vessel development studies in mice and zebrafish models have demonstrated that lymphatic endothelial cells (LECs) predominantly differentiate from venous endothelial cells via the expression of the transcription factor Prox1. However, LECs can also be generated from undifferentiated mesoderm, suggesting potential diversity in their precursor cell origins depending on the organ or anatomical location. Despite these advances, recapitulating human lymphatic malformations in animal models has been difficult, and considering lymphatic vasculature function varies widely between species, analysis of development directly in humans is needed. Here, we examined early lymphatic development in humans by analyzing the histology of 31 embryos and three 9-week-old fetuses. We found that human embryonic cardinal veins, which converged to form initial lymph sacs, produce Prox1-expressing LECs. Furthermore, we describe the lymphatic vessel development in various organs and observe organ-specific differences. These characterizations of the early development of human lymphatic vessels should help to better understand the evolution and phylogenetic relationships of lymphatic systems, and their roles in human disease. Lymphatic vasculature varies widely in structure and function across species, limiting the translatability of lymphatic development studies in animal models to human lymphatic disease. This work provides a detailed characterization of early lymphatic development in human histological samples.Histological sections from 31 embryos and three 9-week-old human fetuses were stained for transcription factor Prox1, the master regulator of lymphatic vessel development. Human lymphatic endothelial cells emerge from embryonic veins to form initial lymphatic vessels in the trunk, with organ-specific variations in development. The development of the lymphovenous valves in human fetal embryos was analyzed. Lymphatic development differs across organs in which lymphatic vessels are widely distributed and functionally important, such as the heart, lungs, kidneys, and mesentery. Prox1 expression analysis across embryonic and fetal development allows for the comparison of human lymphatic vasculature development to other species and across different organs.