Not many years ago the nature and origin of the dilator muscle of the pupil were the subject of much speculation. Improvement of the methods of depigmentation and systematic investigation of embryonic development gradually brought to light its epithelial origin and its fine histologic structure.
As a basis for the further exposition of this subject I shall briefly review the established details of the embryology of the iris. The most anterior portion of the inner layer of the secondary optic vesicle becomes the pigment epithelium of the iris, the cells at the point of reflection being differentiated into the muscle cells of the sphincter iridis and the most anterior portion of the outer epithelial layer being transformed into the cells of the dilator muscle during the seventh month of fetal life. Some of the outer epithelial cells become separated from the continuous layer and preserve their epithelial character throughout life.
KLIEN BA. THE CILIARY MARGIN OF THE DILATOR MUSCLE OF THE PUPIL: WITH REFERENCE TO SOME MELANOMAS OF THE IRIS OF EPITHELIAL ORIGIN. Arch Ophthalmol. 1936;15(6):985–993. doi:10.1001/archopht.1936.00840180029002
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