Fig. 29. Composite of different drawings schematizing the 3D organization of the brain glymphatic system with particular reference to the hypothalamic-pituitary unit. A) Distribution by magnetic resonance imaging of paramagnetic contrast (yellow arrows) in paravascular channels of the rat hypothalamus (HYP), pituitary (Pit), and pineal (Pin) recesses following injection into the cisterna magna (adapted from ref. 30). A contingent of glymphatic flow connects the medial-basal hypothalamus to the limbic structures and olfactory area (OA) and lobe (OL), ensuring a wide interplay by local soluble materials (volume transmission); B) Structure of the glymphatic channels (adapted from ref. 206): pial arteries (Ar) coursing in the subarachnoid space are surrounded by CSF that flows in the perivascular space, termed the Virchow–Robin space. The arteriolar tunica media is isolated from the CSF by a lining of leptomeningeal cells; however, the CSF-containing Virchow–Robin spaces progressively narrow and finally disappear at the level of capillaries (red rectangle). The CSF continues its flow into perivascular spaces provided only by the extracellular matrix of the basal lamina. Astrocytic vascular endfeet expressing aquaporin-4 (AQP4) surround the entire vasculature and form the boundary of the perivascular spaces. These astrocytes may transfer information carried by solutes in the glymphatic system to surrounding glial cells and neurons, and vice versa; C) the glymphatic spaces are in continiuity with venules and veins (adapted from ref. 204), allowing for glymphatic drainage either into sinus-associated dural lymphatics and venous brain sinuses or exiting the brain into perivenular lymphatics and lymph nodes.