A light and electron microscopic study of the permeability of the rabbit iris vessels to horseradish peroxidase in experimental uveitis

A contemporary concept of the blood-aqueous barrier

This review traces the evolution of the concept of the blood-aqueous barrier (BAB) during the past 20 years. The Classical model simply stipulated that the tight junctions of the iris vasculature and ciliary epithelium excluded plasma proteins from the aqueous humor (AH). It failed to reconcile the presence of AH protein levels equal to 1% of that found in plasma. Moreover, models of barrier kinetics assumed that the processes of AH secretion and plasma protein entry were directly linked. Thus, elevations of AH protein levels could only be explained by a pathological breakdown of the BAB. Over the last 20 years it has been shown that the plasma proteins in normal AH by-pass the posterior chamber entirely. Instead, these proteins diffuse from the capillaries of ciliary body stroma, into the iris stroma and then into the anterior chamber. This creates a reservoir of plasma-proteins in the iris stroma that is not derived from the iris vessels. This reservoir is prevented from diffusing posteriorly by tight junctions in the posterior iris epithelium. The one-way valve created by the pupil resting on the anterior lens capsule, combined with the continuous, forward flow of AH through the pupil, prevents protein reflux into the posterior chamber. Importantly, in the new paradigm, secretion of AH and the entry of plasma proteins into AH, are semi-independent events. This opens the possibility that AH protein levels could increase in the absence of breakdown of the BAB. Clinical consequences of this new paradigm of the BAB are discussed.

An experimental uveitis induced by bovine serum albumin. A transmission and scanning electron microscopic study

An acute experimental uveitis was produced by injecting bovine serum albumin (BSA) intravitreally into presensitized albino rabbits. Histopathological changes in the anterior uvea were followed during a period of 2 weeks using transmission and scanning electron microscopy. Both the ciliary body and the iris showed prominent ultrastructural changes. Based on the histopathological characteristics, the inflammatory reaction was divided into the 3 different phases: 1) a rapid and transient polymorphonuclear phase (3 h-day 1; 2) a slower and prolonged mononuclear phase (day 1-day 14) and 3) a phase which is characterized by tissue damage (day 7-). At the beginning of the mononuclear phase, inflammatory cells were seen to leak through the intercellular spaces of adjacent ciliary epithelial cells into the posterior chamber, later to cover large areas of the ciliary processes with fibrin. During the first 2 days, inflammatory changes were also detected in the ciliary body of the contralateral eye. The experiment was controlled using both unsensitized as well as presensitized rabbits which were intravitreally injected with sterile saline.

The ciliary body and the iris in experimental uveitis in rabbits. A scanning electron microscopic study

Acute anterior immunogenic uveitis of varying degree was induced in rabbits by the injection of human serum albumin into the vitreous body. After fixation and drying by the critical point method, the lens, the zonules, the vitreous and, in some specimens, coagulated exudate were removed by careful dissection. The surface of the ciliary body and the posterior surface of the iris were examined by scanning electron microscopy. In moderate uveitis, the ciliary body was covered by a thin layer of leucocytes enmeshed in fibrin strands. Scattered deposits of fibrin and a few leucocytes were also found in the iris. In severe uveitis, the processes of the ciliary body had nearly disappeared because of oedema and exudate, which also contained erythrocytes. The numerous leucocytes had marked excrescences formed by folds of the cell membrane. Fibrin seemed to play a role in the attachment of the leucocytes on the surface of the ciliary body.

Electron microscopic studies on the blood-aqueous barrier of prostaglandin-treated rabbit eyes. II. Iris

The distribution of intravenously injected horseradish peroxidase in the iris of prostaglandin (E1 and E2)-treated rabbit eyes, has been studied with the electron microscope. Peroxidase was demonstrated in the stroma of all parts of the iris. The tracer was found throughout interendothelial clefts of iris vessels, indicating that these vessels had become permeable to peroxidase. The distribution of peroxidase in the iris epithelium indicates that the posteroir epithelial cells are girdled by zonulae occludentes.