The development of the sphincter muscle in the iris of the albino rat

Abstract

The iris which first appeared on the 18th day of gestation was a double-layered structure budded off from the anterior rim of the optic cup. On the following day myofilaments were seen in the cells of the anterior epithelium of the iris near the pupil. A group of cells appeared at this point the next day, and became a multi-layered structure by the 21st day. At birth more newly-appeared cells were discovered in the region of the sphincter pupillae. Cells situated on the anterior surface of this newly-appeared cells group were elongated, and contained more myofilaments but fewer ribosomes and melanosomes than cells located more deeply to the anterior epithelium. The number of myofilaments in the individual sphincter muscle cell was increased subsequently. Mitotic cells did not contain myofilaments, and were not found among differentiated sphincter muscle cells. Nerves, blood vessels and melanocytes were found among the sphincter muscle cells as the 6th day after birth. By the 12th day the sphincter muscle was fully developed, and of adult appearance. During the initial differentiation of the epithelium of the iris the basement membrane was present on the free surface of the epithelium, but by the 2nd day after birth it surrounded the well differentiated sphincter muscle cells individually. The transverse diameters of the sectioned sphincter muscle cells decreased steadily from the 12th day after birth, reaching its smallest diameter in the adult. There was only a slight decrease in the diameters of the nuclei of these cells, so that the nucleo-cytoplasmic ratio progressively increased during later stages of development. The number of nuclei encountered in a single section per 100 sphincter muscle cell profiles was also decreased dramatically during development.