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 th e dilator pupillae (MDP) during mydriasis and miosis were investigated in the rat by scanning electron microscopy (SEM) and transmission ele ctron microscopy (TEM). Following fixation, SEM, specimens were treate d with sodium hydroxide to expose the muscle surface, Significant morp hological differences were noted in the anterior surface of the MDP be tween mydriasis and miosis. In the mydriatic eye, a highly rugged stru cture with numerous linear folds was oriented circularly or obliquely together with spherical bulges. These structures, presumably correspon ding to the highly contractile portion of the myoepithelial cells, wer e more prominent near the pupillary margin than near the ciliary margi n, indicating that the MDP may contract much more strongly in the pupi llary margin, In the miotic eye, the anterior surface of the MDP showe d less conspicuous linear folds in the pupillary area, and was almost hat in the ciliary area, Radially oriented ridges were observed only i n the pupillary area, These findings suggest that the contraction of t he sphincter pupillae in miosis induces a stretching of the MDP toward the pupil and a circular shrinkage of the MDP. Ultrastructural change s of the MDP particularly near the pupillary margin may play an import ant role in regulation of the pupil diameter as a diaphragm of the eye because morphological changes such as the linear folds, spherical bul ges, and ridges were more prominent near the pupillary margin than tho se near the ciliary margin.