Zonulae occludentes

The presence of only two dietary carotenoids in the retina, lutein and zeaxanthin, out of about 14 normally present in the plasma indicates their highly specific uptake and retention (Bernstein et al., 2001 Bone and Landrum, 1992 Bone et al., 1988,1997,1993 Davies and Morland, 2004 Khachik et al., 1997, 2002). The retina-blood barrier is formed by the tight zonulae occludentes of the endothelial cells in the inner retina and of the RPE, a monolayer of cells which separates the outer retina from its choroidal blood supply (Strauss, 2005). 

Claude P and Goodenough DA [1973] Fracture faces of zonulae occludentes from tight and leaky epithelia. 1 Cell Biol 58 390-400... 

The blood-brain barrier is markedly different from peripheral capillaries Peripheral capillaries are fenestrated with openings up to 50 nm wide. In contrast, cerebral endothelial cells are closely connected by tight junctions and zonulae occludentes, resulting in extremely high transendothelial resistances of up to 1500-2000 12 cm2 [16] (Figure 17.1). The capillaries are surrounded by a basal membrane enclosing intermittently pericytes, which have been postulated to be involved in host defense. The outer surface of the basement membrane is covered by astrocytic foot processes. Most likely, secretion of soluble growth factors by astrocytes plays an important role in endothelial cell differentiation. 

Luciano, L., Real, E., Rechkemmer, G., and von Engelhard , W., Structure of the zonula occludentes and the permeability of the epithelium to short-chain fatty acids in the proximal and the distal colon of guinea pig, J. Membr. Biol., 82 145-156 (1984). 

In the respiratory tract, cilia-bear-ing epithelial cells are also joined on the luminal side by zonulae occludentes, so that the bronchial space and the inter-stitium are separated by a continuous phospholipid barrier. 

FIGURE 24.2 Cornea. EPITH, epithelium ENDOT, endothelium BM, basement membrane BWN, Bowman s layer ST, stroma DSM, Descemet s membrane ZO, zonulae occludentes. 

The eye globe is a hollow structure filled anteriorly by the aqueous humor and posteriorly by the vitreous. The aqueous humor (Figure 24.1) is continuously formed by the ciliary body. It is secreted into the posterior chamber from which it passes through the pupil into the anterior chamber and is drained at the anterior chamber periphery. Due to the blood aqueous barrier formed by zonulae occludentes of the nonpigmentary ciliary epithelium, macromolecules such as proteins can pass to the aqueous in very small quantities, regardless of their plasma concentrations. In humans, the aqueous humor protein level is around 20 mg/100 mL, less than 0.5% of the normal plasma total protein levels (Table 24.1) [22]. In the human eye, the rate of aqueous formation is around 2.5 pL/min, whereas in the case of rabbit eye it is... 

After permeation through the corneal epithelium, molecules should then permeate through the hydrophilic corneal stroma and the corneal endothelium before reaching the aqueous humor. Thus, while the corneal epithelium favors the penetration of lipophilic compounds, the hydrophilic stroma favors the permeation of hydrophilic compounds. The corneal endothelium monolayer is not sealed by paracellular zonulae occludentes. Consequently this layer has only a minimal effect on the overall permeability of the cornea. Overall, the cornea as a whole is tailored to allow the permeation of moderately lipophilic compounds (close to log P = 2) [40], On the contrary, the reported optimal lipophilicity for compounds penetrating through an isolated epithelial layer of the cornea was around log P=4 [41],... 

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... 

Diffusion and effect. During diffusion from the injection site (i. e., the interstitial space of connective tissue) to the axon of a sensory nerve, the local anesthetic must traverse the perineurium. The multilayered perineurium is formed by connective tissue cells linked by zonulae occludentes (p. 22) and therefore constitutes a closed lipophilic barrier. 

Furuse M et al (2001) Conversion of zonulae occludentes from tight to leaky strand type by introducing claudin-2 into Madin-Darby canine kidney I cells. J Cell Biol 153(2) 263-272... 

Landmann, L., 1980, Zonulae occludentes in the epidermis of the snake Natrix natrix L., Experientia, 36 110. 

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