Embryonic development of the saccular sensory epithelium in relation to otolith growth in the inner ear of the silver carp, Hypophthalmichthys molitrix (Valenciennes, 1844) (Teleostei: Cyprinidae): light and electron microscopic study

The development of the saccular sensory epithelium in the inner ear of the silver carp, Hypophthalmichthys molitrix, was studied from an early stage (2 days after fertilization), when the otic vesicle or otocyst first formed, to the post-larval stage (7 days after hatching), when the development of the inner ear approximates that of the juvenile stage. Light microscopy revealed: (1) the otic vesicle, the primordia of the saccular otolith and the stato-acoustic ganglion were observed by 2 days after fertilization; (2) the saccular macula overlain by a small round otolith is established at 3 days after fertilization, while the utricular macula with its otolith is first observed at 4 days after fertilization; (3) the saccular macula began to differentiate at 3 days after hatching and became well differentiated at 7 days after hatching. At 7 days after hatching, transmission electron microscopy (TEM) revealed: (1) the apical surface of each hair cell is covered with a ciliary bundle formed of a long kinocilium and a short bundle of stereocilia. The supporting cells are provided with microridges on their apical surface and seemed to be covered with small and large vesicles, suggesting that they have a secretory function beside the supporting one; (2) secretory materials such as multivesicular bodies, electron dense granules, empty vesicles and cytoplasmic extrusions are observed scattering over the saccular sensory epithelium, which probably contribute to the formation of the otolith and/or otolithic membrane. Using scanning electron microscopy (SEM), two types of hair bundles are distinguished: the first type consisted of numerous short stereocilia and a kinocilium; the second type has a small number of short stereocilia with a very long kinocilium. Most of these results are discussed with special regard to the environmental factors affecting the early development of the inner ear in teleost fishes.