Live Imaging of Intracranial Lymphatics in the Zebrafish

Rationale The recent discovery of meningeal lymphatics in mammals is reshaping our understanding of fluid homeostasis and cellular waste management in the brain, but visualization and experimental analysis of these vessels is challenging in mammals. Although the optical clarity and experimental advantages of zebrafish have made this an essential model organism for studying lymphatic development, the existence of meningeal lymphatics has not yet been reported in this species. Objective Examine the intracranial space of larval, juvenile, and adult zebrafish to determine whether and where intracranial lymphatic vessels are present. Methods and Results Using high-resolution optical imaging of the meninges in living animals, we show that zebrafish possess a meningeal lymphatic network comparable to that found in mammals. We confirm that this network is separate from the blood vascular network and that it drains interstitial fluid from the brain. We document the developmental origins and growth of these vessels into a distinct network separated from the external lymphatics. Finally we show that these vessels contain immune cells and perform live imaging of immune cell trafficking and transmigration in meningeal lymphatics. Conclusions This discovery establishes the zebrafish as a important new model for experimental analysis of meningeal lymphatic development, and opens up new avenues for probing meningeal lymphatic function in health and disease. ### Competing Interest Statement The authors have declared no competing interest. * Non-standard Abbreviations and Acronyms : VEGF-C : Vascular Endothelial Growth Factor C FGPs : Fluorescent Granular Perithelial cells muLECs : meningeal mural Lymphatic Endothelial Cells BLECs : Brain Lymphatic Endothelial cells CSF : cerebrospinal fluid mrc1a : mannose receptor, C type 1a lyve1b : lymphatic vessel endothelial hyaluronic receptor 1b dpf : Days Post Fertilization