![]() ![]() The peripheral portion of the iris inserts as the pectinate ligaments in the area where Descemet’s membrane ends. The anterior border is formed by fibroblasts and melanocytes. The anterior border of the iris is often discussed as possessing an epi- or endothelial cell layer. In animals with blue eyes the pigment is absent in the stroma, while in the albino it is lacking in both the stroma and the posterior epithelium. The color of the iris depends on the pigmentation of both the stromal melanocytes and the posterior epithelium. The color of the iris can vary with species, age, and sex, and can even differ between eyes or within eyes in the same animal. This central portion of the iris normally rest against the anterior lens surface and in the absence of this support, the iris will tremble. The pupil varies in shape between species and can be circular (dog, rabbit), oval (horse, cow), or vertical (cat). Embryologically, the iris is comprised of components that originate from neural ectoderm, the sphincter and dilator muscles and the 2-layered posterior epithelium, and mesodermal components that form the iris stroma. The iris is the anterior most portion of the vascular tunic and functions as a moveable diaphragm between the anterior and posterior chambers. The vascular tunic is also refered to as the uvea. The vascular tunic is mesodermal in origin and is situated between the outer fibrous tunic and the inner nervous tunic. The vascular tunic is comprised of three distinct regions, (1) the iris, (2) the ciliary body, and (3) the choroid.
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