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Structural alterations of the rat iris associated with function and growth Lim, Wan Cheng


1. The structural alterations of the adult rat iris associated with its function of dilating and constricting the pupil were examined. The rat eyes were regularly dilated or constricted with a few drops of a mixture of phenylephrine hydrochloride and cyclopentolate, or echothiophate iodide, respectively. The eyes were prepared for examination with the light, transmission and scanning electron microscopes by well-known methods. In pupillary dilation, scanning electron microscopic studies reveal that the posterior epithelial cells are arranged in circumferential ridges which bifurcate, taper down and join with adjacent ridges. On the anterior surface of the iris, blood vessels, generally circumferentially oriented, bulge prominently outwards. In transmission electron micrographs the epithelial cells are high and are discretely separated from each other. The nuclei show indentations of the nuclear envelope. Bundles of intracellular filaments form a 'hammock' around the nucleus. The dilator muscle layer is thick. Dilator hillocks and dilator processes are found all along the boundary zone with the stroma. The nuclei of the dilator muscle cells also show nuclear indentations. With the light microscope, it is clearly shown that the stromal components and the collagen network appear to be arranged in columns perpendicular to the posterior surface of the iris. A low magnification scanning electron micrograph of the rat iris in pupillary constriction shows that most of the posterior surface of the iris is smooth. In extreme pupillary constriction, where the pupil is a mere pinhole, the posterior epithelial cells around the pupil are arranged in radial ridges which peter out peripherally. Bulbous structures are often seen in amongst the radial epithelial ridges. On the anterior surface of the iris the blood vessels do not bulge out as prominently, as they zig-zag from the periphery to the pupillary margin. The morphology of the iridic crypts and pores are well illustrated. In transmission electron micrographs, the posterior epithelial cells in pupillary constriction are low and they form a continuous layer. The nuclei, with smooth nuclear outlines, and the intracellular filaments are disposed parallel to the length of the cells. The dilator muscle cell layer is low. Dilator hillocks and dilator processes are absent. The nuclei of the dilator muscle cells have a smooth outline and lie along the length of the cells. The stromal elements and the connective tissue framework are oriented parallel to the posterior surface of the iris, as seen light microscopically. 2. The overall developmental changes in the structure of the various components of the fetal and post-natal rat iris were observed on toluidine blue stained plastic sections. In the immature iris, the brick-shaped posterior epithelial cells form a continuous layer. By two weeks after birth, the epithelial cells have acquired characteristics of the adult iris. The anterior epithelium develops to give rise to the sphincter and dilator muscles. The stromal elements stream into the iris parallel to the posterior surface of the iris in very close association with the stromal surface of the developing dilator muscle cells. With development, the stromal elements move away from the dilator. A scanning electron microscopic study of the posterior surface of the developing rat iris shows the changes in the surface configurations of the posterior epithelial cells. Initially the posterior surface of the iris is smooth. By a gradual process, the epithelial cells begin to bulge out posteriorly. By two weeks after birth, the epithelial cells are beginning to be arranged in rows. The topography of the pupillary membrane and its relationship with the hyaloid system is shown. Most of the blood vessels of the pupillary membrane appear to come from the iris stroma with perhaps some contribution from the hyaloid system. The thin-walled blood vessels are suspended within a scaffolding of connective tissue fibers. The rat pupillary membrane is still present during the first few days after birth. Changes in the permeability of the iris blood vessels to an intra-vascularly injected solution of HRP were investigated. The iris capillaries of the fetal and early post-natal rats, up to two weeks after birth, are readily permeable to HRP. They then become impermeable to HRP, as in the adult iris.

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