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Fig. 2 | Fluids and Barriers of the CNS

Fig. 2

From: Membrane transporters control cerebrospinal fluid formation independently of conventional osmosis to modulate intracranial pressure

Fig. 2

Ultrastructure of the choroid plexus luminal membrane and a schematic mathematical model domain derivation. a–c Scanning electron microcopy images of rat choroid plexus. a The typical frond-like arrangement of the epithelium dictated by the ensheathment of blood vessels in the underlying connective tissue. b At higher magnification, the abundant microvilli and the loose brush border that they form is visible. Arrows point to the terminal bulbous tip of the microvilli, also shown at higher magnification in c, which also reveals the strong interdigitation of the microvilli. d, e Transmission electron microscopy images of the epithelium show the space filling of microvilli cut perpendicularly a distance from the luminal membrane (d) and the extent of the brush border (e). Note change in diameter of microvilli from base (open arrows) to tip (filled arrows). Bars, a; 250 nm, b; 5 µm, c; 500 nm, d; 250 nm, and e; 1 µm. f Local cross-section of the choroid plexus. Luminal and basolateral membranes, as well as tight junctions are indicated. g Magnified view of the luminal epithelial cell surface showing microvilli and the inter-microvillar space, where a standing osmotic gradient might exist. h Geometric approximation of the luminal cell surface, with microvilli represented as cylinders with radius \(r\) and length \(l\), and separated by a distance of \(p\). The blue region shows one inter-microvillar space. i A single inter-microvillar space with interfaces to the four adjacent microvilli and to the base of the luminal membrane of the epithelial cell. The green arrows indicate solute flux, \({\upvarphi }\), through these interfaces. The solute flux is produced by transport mechanisms on the luminal membrane. Potential accumulation of solutes in the inter-microvillar space might yield osmotic forces that draw fluid from the choroid plexus epithelium, resulting in a CSF secretion rate of \(q\) per such space. The overall CSF production rate by this mechanism is obtained by multiplying \(q\) with the number of inter-microvillar spaces on the entire choroid plexus surface. j Circular cylindrical channel with diameter \(d\) that is hydrodynamically equivalent to the inter-microvillar space shown in (i). A one-dimensional standing osmotic gradient model was derived from this geometry

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