The fine structure of iridial constrictor pupillae muscle of Alligator mississippiensis

The iridial constrictor pupillae muscle of Alligator mississippiensis is found to consist of admixed smooth, myoblast‐like, and striated muscle cells. Melanophores, bundles of collagen fibrils, nerve cell processes, and smooth muscle cells, which contain melanin granules, are found scattered in the constrictor pupillae muscle. Protoplasmic bridges are present between myoblast‐like cells and between myoblast‐like and striated muscle cells. Myoblast‐like cells contain agranular reticulum, free ribosomes and randomly scattered, or longitudinally oriented filaments of two sizes (50–100 A and 200–250 A). Free ribosomes are found individually and in clusters of 5 to 10. The agranular reticulum is found as anastomosing tubule‐shaped and cisternal elements. The striated muscle fibers consist of myofibrillae and a poorly organized sarcoplasmic reticulum and T system, such that triads and diads are not precisely ordered, but randomly scattered throughout the muscle fiber. This data is discussed in terms of the differentiation and function of striated muscle. On the basis of the disposition of triads and dyads it is concluded that the striated muscle of the constrictor pupillae muscle of Alligator mississippiensis is a slow‐acting rather than fast‐acting muscle.