The ultrastructure and innervation of the ear of the gar, Lepisosteus osseus.

The endorgans of the inner ear of the gar were examined using transmission and scanning electron microscopy as well as nerve staining. The ultrastructure of the sensory hair cells and supporting cells of the gar ear are similar to cells in other bony fishes, whereas there are significant differences between the gar and other bony fishes in the orientations patterns of the sensory hair cells on the saccular and lagenar sensory epithelia. The saccular sensory epithelium has two regions, a main region and a secondary region ventral to the main region. The ciliary bundles on the main region are divided into two groups, one oriented dorsally and the other ventrally. Furthermore, as a result of curvature of the saccular sensory epithelium, the dorsal and ventral ciliary bundles on the rostral portion of the epithelium are rotated ninety degrees and are thus oriented on the animal's rostro-caudal axis. Hair cells on the secondary region are generally oriented ventrally. The lagenar epithelium has three groups of sensory hair cells. The groups on the rostral and caudal ends of the macula are oriented dorsally, whereas the middle group is oriented ventrally. Hair cell orientations on the utricular epithelium and macula neglecta are similar to those in other bony fishes. Nerve fiber diameters can be divided into three size classes, 1-8 microns, 9-13 microns, and 14 microns or more, with the smallest size class containing the majority of fibers. The distribution of the various classes of fiber diameters is not the same in nerve branches to each of the end organs. Similarly, the ratio of hair cells to axons differs in each end organ. The highest hair cell to axon ratio is in the utricle (23:1) and the smallest is in the macula neglecta (7:1). The number of sensory hair cells far exceed the number of eighth nerve axons in all sensory epithelia.