Light and electron microscopic studies on the development of the inner ear and otolith of the ayu Plecoglossus altivelis

The development of the inner ear, especially sensory maculae and otoliths, was studied in the ayu Plecoglossus altivelis, using light and electron microscopy. Light microscopy indicated: 1) The auditory vesicle first appears by 3 days after fertilization, The saccular macula overlain by a round otolith appears and possesses apical hairs by 4 days, while the utricular macula appears by 5 days after fertilization. 2) 9 days after fertilization (hatching day), both the saccular and utricular maculae are well differentiated: composed of hair cells and supporting cells, and overlain by saccular and utricular otoliths, respectively. Electron microscopy revealed: 1) By 5 days after fertilization, sensory hair bundles and bud-like microvilli protrude from the apical surface of the saccular and utricular maculae into the endolymphatic lumen. 2) Secretory materials, such as electron dense cytoplasmic extrusions, vesicles containing spherules, and dense granules, seem to occur from the apical surface of sensory epithelium, probably supporting cells, and to contribute to the formation of the otolith and/or otolith increments. 3) By 9 days after fertilization, the saccular otolith possesses an inner incremental layer around the central nucleus and a thick belt of peripheral incremental layer. Consequently, it was suggested that the hearing mechanism in the ayu may be established earlier compared with other fishes.