Discovery of Abundant Nano-scale Lymphatic-like Vessels in Brains

Abstract

As one of the most metabolically active organs, the high metabolic demands of the brain require a highly efficient system for waste clearance. However, it remains unclear whether the brain possesses such an efficient system, and if so, what are the features of the system. Over a decade ago, a glymphatic system was proposed to describe neurofluid transport through cerebrospinal fluid (CSF) and perivascular spaces (PVS), complemented by dural and meningeal lymphatic pathways. Nonetheless, it is still elusive whether transport within the CSF and PVS occurs as a structured form of flow. Moreover, unlike the abundance of blood capillaries in the brain, only a low density of lymphatic vessels has been observed. Such a low density of lymphatic vessels is unlikely to support the level of efficiency required for effective metabolic waste clearance in the brain. By combining expansion microscopic imaging with CRANAD-3, a pan-beta-amyloid fluorescence probe, we discovered abundant nano-scale lymphatic-like vessels (NLVs) in the brain parenchyma of both mouse and human. The majority of these vessels have diameters less than 1000 nanometers and show moderate positivity for several lymphatic biomarkers, including LYVE-1, Prox-1, PDPN, and VEGFR3. The NLVs coil around blood vessels and possible connections between blood vessels were observed. Notably, we also found that some NLVs travel through several layers in the cortex and display distinct orientation patterns in different cortical layers. Our discovery reveals a previously hidden vascular system, and raises the possibility that such an abundant and structured web of tubular structures could enable highly efficient means to enhance waste clearance, and could thus play a crucial role in supporting the high metabolic demands of brains.