Ultrastructural changes of the myoepithelium of the dilator pupillae during miosis and mydriasis in the rat iris.

Changes in the three-dimensional structures of the myoepithelium of the dilator pupillae (MDP) during mydriasis and miosis were investigated in the rat by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Following fixation, SEM specimens were treated with sodium hydroxide to expose the muscle surface. Significant morphological differences were noted in the anterior surface of the MDP between mydriasis and miosis. In the mydriatic eye, a highly rugged structure with numerous linear folds was oriented circularly or obliquely together with spherical bulges. These structures, presumably corresponding to the highly contractile portion of the myoepithelial cells, were more prominent near the pupillary margin than near the ciliary margin, indicating that the MDP may contract much more strongly in the pupillary margin. In the miotic eye, the anterior surface of the MDP showed less conspicuous linear folds in the pupillary area, and was almost flat in the ciliary area. Radially oriented ridges were observed only in the pupillary area. These findings suggest that the contraction of the sphincter pupillae in miosis induces a stretching of the MDP toward the pupil and a circular shrinkage of the MDP. Ultrastructural changes of the MDP particularly near the pupillary margin may play an important role in regulation of the pupil diameter as a diaphragm of the eye because morphological changes such as the linear folds, spherical bulges, and ridges were more prominent near the pupillary margin than those near the ciliary margin.