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GIUSEPPINA RAVIOLA; Effects of Paracentesis on The Blood-Aqueous Barrier: An Electron Microscope Study on Macaca Mulatto, Using Horseradish Peroxidase as a Tracer. Invest. Ophthalmol. Vis. Sci. 1974;13(11):828-858.
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Horseradish peroxidase (MW 40,000; a0 ∼ 2.5 nm.; 0.5 Gm. per kilogram of body weight; HRPO) was injected into the femoral vein of rhesus monkeys (Macaca mulatta) and the diffusion of the tracer was studied in the anterior segment of the eye in control animals and after paracentesis. The following regions of the eye were examined: ciliary body, iris, Schlemm's canal, juxtacanalicular connective tissue, and trabecular meshwork. In the control animals the tracer which escapes from the fenestrated vessels of the ciliary body stroma permeates the intercellular spaces between the pigmented cells and those intervening between the facing apices of the pigmented and nonpigmented cells; it is finally blocked by the tight junctions which connect the lateral surfaces of the nonpigmented cells. In the iris, the vessel wall is impermeable to the tracer. The Schlemm canal, juxtacanalicular connective tissue, and trabecular meshwork are consistently free of HRPO. Following paracentesis, the secondary aqueous humor displays the typical purple color of HRPO in solution. The permeability properties of both the ciliary epithelium and vessels of the iris are unmodified. The lumen of the Schlemm canal is invaded by tracer-containing blood; through newly formed gaps in the endothelial lining of the inner wall of the canal, tracer rapidly diffuses into the juxtacanalicular connective tissue, trabecular meshwork, and anterior chamber. The idea is therefore advanced that the sudden fall of the intraocular pressure upon aqueous humor withdrawal causes a reflux of blood from the episcleral veins into the Schlemm canal and plasma proteins rapidly leak into the anterior chamber. On the basis of the present experiment, the widespread belief that the plasma proteins of the plasmoid aqueous humor originate from the ciliary epithelium or the iris is no longer acceptable. Paracentesis has no effect on the permeability properties of the junctions which represent the morphologic counterpart of the blood-aqueous barrier. It breaks the hydrodynamic equilibrium which ensures the unidirectional flow of the aqueous humor from the posterior chamber to the episcleral veins.
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