Choroid-epithelium

The permeability properties of cerebral capillaries are quite different from those of capillaries in other organs and tissues. Water-filled channels, which can be demonstrated in the walls of most non-nervous capillaries, are absent from cerebral capillaries. A hydrostatically promoted transcapillary flow of fluid into tissue on the arterial side, with the reverse process on the venous side (i.e. Starling-type transcapillary flow) has never been demonstrated in the brain. The central nervous system does not possess lymphatics. Cerebral capillary endothelial cells have no pino-cytotic activity. Furthermore, the central nervous system and various compartments of cerebrospinal fluid are also excluded from the extracellular fluid of the rest of the body by the tight choroid epithelium and the tight layer of arachnoid. 

Disorder characterized by atrophy ofthe choroid (the thin membrane covering most of the posterior of the eye between the retina and sclera) and degeneration of the retinal pigment epithelium resulting in night blindness. The disease is caused by mutations in Rab escort protein Repl (component A of Rab geranylgeranyl transferase). 

Melanins are prodnced in mammals in two types of cells of different developmental origin (1) the melanocytes of the skin, hair, choroids and iris and (2) the retinal pigment epithelium (RPE). Specialized organelles of the melanocytes, the melano-somes, synthesize and store eumelanins and phaeomelanins. 

The retina comprises two principal components, the non-neural retinal pigment epithelium and the neural retina. The retinal pigment epithelium is an essential component of the visual system both structurally and functionally. It is important for the turnover and phagocytosis of photoreceptor outer segments, the metabolism of retinoids, the exchange of nutrients between the photoreceptors, and the choroidal blood vessels and the maintenance of an efficient outer blood-retinal barrier. 

Wijnholds J, deLange EC, Scheffer GL, van den Berg DJ, Mol CA, van d, V et al. Multidrug resistance protein 1 protects the choroid plexus epithelium and contributes to the blood-cerebrospinal fluid barrier. J Clin Invest 2000 105(3)779-285. 

Gao B, Stieger B, Noe B, Fritschy JM, Meier PJ (1999) Localization of the organic anion transporting polypeptide 2 (Oatp2) in capillary endothelium and choroid plexus epithelium of rat brain. J Histochem Cytochem 47 1255-1264... 

After i.c.v. injection, the rate of elimination from the CNS compartment is dominated by cerebrospinal fluid dynamics. The CSF, which is secreted by the choroid plexus epithelium across the apical membrane, circulates along the surface and convexities of the brain in a rostral to caudal direction. It is reabsorbed by bulk flow into the peripheral bloodstream at the arachnoid vUh within both cranial and spinal arachnoid spaces [62]. Of note is that the turnover rate of total CSF volume is species dependent and varies between approximately... 

Photoreceptor outer segment layer Retina pigment epithelium layer Bruch s membrane Choroid layer... 

The ciliary body, situated posterior to the iris, performs several functions. It connects the anterior part of the choroid to the circumference of the iris, and contains the ciliary muscles necessary for accommodation. The ciliary body secretes aqueous humor into the posterior together with nutrients to nourish the lens. In the ciliary body, melanin is located only in the outer pigmented epithelium [24]. Aqueous humor is actively secreted and passively filtered by the ciliary body. Although the rate of secretion is about 2 pL/min the same volume is drawn-off via Schlemm s canal from where it is conducted into veins [25]. 

Pitkanen et al. [51] reported the isolated bovine RPE-choroid was up to 20 times more permeable to lipophilic than hydrophilic beta-blockers. Furthermore, the in vitro permeability of bovine RPE-choroid to hydrophilic compounds and macromolecules was 10 to 100 times less compared to sclera, whereas the permeability for lipophilic molecules was in the same range for both tissues. The isolated bovine RPE-choroid also exhibited differential permeation by molecular weight and Stokes radius. The permeation rate of 4, 10, and 20 kDa FITC dextrans was moderate compared to a good permeation rate for the 376 Da carboxyfluor-escein and a poor penetration rate for 40 and 80 kDa FITC-dextrans. The permeability to carboxyfluorescein was 35 times more than to 80 kDa FITC-dextran [51]. In a study on the permeability of the human ciliary epithelium to a horseradish peroxidase, Tonjum and Pedersen [52] reported that ciliary and iridial epithelium contained a system of paracellular zonulae occludentes. Peroxidase was applied on the stromal side of ciliary body and iris specimens obtained from freshly enucleated eyes. The 40 kDa peroxidase was blocked apically in the lateral intercellular spaces of the CNPE whereas in the iris the progression of peroxidase was blocked apically in the lateral intercellular spaces of the IPE. Freddo [53] studied the intercellular junctions in the posterior IPE cells of the rhesus monkey by electron microscopy, freeze-fracture, and horseradish peroxidase. Intravenously injected horseradish... 

The secretion of CSF and ECF is essentially driven by an osmotic gradient created by the Na+/K+-ATPase, expressed in the abluminal membrane of the BBB endothelium and the apical membrane of the choroid plexus epithelium, which produces water movement into brain ECF and produces volume secretion. 

Lippens, S. et al. Caspase 14 is expressed in the epidermis, the choroid plexus, the retinal pigment epithelium and thymic HassaH s bodies. Cell Death Differ. 10, 257, 2003. 

In addition to the work presented here, several alternative viral-vectored approaches have been reported recently. An adeno-associated viral vector (AAV) encoding the soluble VEGF receptor 1, sFlt-1, shows promise for long-term inhibition of two types of ocular neovascularization (Lai et al., 2002). This vector, when injected into the anterior chamber, resulted in expression in both the corneal endothelium and iris pigment epithelium and reduced corneal NV by 36%. Subretinal injection of the same vector reduced choroidal NV subsequent to laser lesions around the optic nerve. These results suggest that a secretable factor expressed in one or more transduced cell populations can be elfective in the control of ocular NV occurring in a disparate cell population. 

Angeletti RH, Novikoff PM, Juwadi SR, Fritschy JM, Meier PJ, Wolkoff AW. The choroids plexus epithelium is the site of the organic anion transport protein in the brain. PNAS 1994 94 283-286. 

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