RELATIONSHIP OF SACCULAR ULTRASTRUCTURE TO OTOLITH GROWTH IN THE TELEOST OREOCHROMIS-NILOTICUS

The sacculus of Oreochromis niloticus is anatomically separated from the utriculus and semicircular canals. The saccular wall is composed of the sensory epithelium, transitional epithelia, and squamous epithelium. Cellular granules are abundant in the sensory and transitional epithelia but scarce in the squamous epithelium. Over the dorsal side of the dorsal transitional epithelium there exists an oval patch of cells with distinctive microvilli. New finding is a shallow groove which extends from the anterior end of the sensory epithelium approximately halfway down along the ventral perimacular transitional epithelium. Small vesicles, which appear "empty" under transmission electron microscopy (TEM), are aggregated in the posterior region of the groove. These small vesicles are also present in both the sensory and transitional epithelia. A second kind of vesicle is comparatively large and appears filled with stainable contents. These vesicles are restricted to the sensory region. Both kinds of vesicles appear to be involved in apical secretion and possibly provide the otolithic membrane with fibers. The otolithic membrane is composed of a gelatinous layer and subcupular meshwork. The meshwork appears to contribute to the formation of the otolith. The small empty vesicles appear to originate in sensory and transitional epithelial cells and may form the subcupular meshwork. The larger filled vesicles are derived predominantly from sensory cells in the sensory epithelium and appear to contribute to the gelatinous layer of otoliths.