Histochemistry of mouse extraocular muscle.

The organization, histochemical and endplate characteristics, and percentage fiber composition are described for mouse extraocular muscle (EOM). Both recti and obliques, but not the retractor bulbi, have two concentric layers, an inner global and superficial orbital. Three histochemical fiber types, coarse, fine and granular, are present in the EOM. The intermediate diameter coarse fibers are focally innervated and occur in both layers in all muscles. The large diameter granular fibers are focally innervated and occur in all EOM in the global layer. The small diameter fine fibers have multiterminal endplates and occur in both layers of the recti. Fine fibers are not seen in the obliques or retractor bulbi. Focal endplates are confined to a broad diagonal band across the middle one third of the muscle, whereas multiterminal endplates are scattered throughout the length of the muscle.