Structure and development of the saccular sensory epithelium in relation to otolith growth in the perch Perca fluviatilis (Telostei)

Abstract

The development of the saccular sensory epithelium in relation to otolith growth in the inner ear of the perch Perca fluviatilis was studied by light and transmission electron microscopy. Development is studied from the early embryo (6 days after fertilization) when the otocyst first forms, to the newly hatched larva when the development of the inner ear approximates that of the juvenile. Light microscopy revealed: 1) The auditory vesicle and the otolith primordia or otoconia first appeared 6 days after fertilization. The saccular macula overlain by a small round otolith is established by 7 days after fertilization and the stato-acoustic ganglion which contains the neuronal precursors of the macular epithelium is also seen at this stage. 2) The saccular macula began to differentiate 12 days after fertilization and became well differentiated and distinctive at one day after hatching. It is composed of two types of epithelia: sensory and transitional. The former is differentiated into lightly-stained sensory hair cells and dark-stained supporting cells. Electron microscopy indicated: 1) The apical surface of each hair cell is covered with a ciliary bundle which varies in length in different epithelial regions. Each bundle is formed from a long single kinocilium and a short bundle of stereocilia. The supporting cells are provided with microvilli on their apical surface and seem to be covered with small and large vesicles, suggesting that they have a secretory function beside the supporting one. 2) Secretory materials, such as vesicular dilations, cytoplasmic extrusions, various kinds of vesicles and electron dense granules seemed to be produced by different cells of the saccular sensory epithelium and scattered into the endolymphatic lumen, just over the saccular macula region. These secretory materials probably contribute to the formation of the otolith and/or otolithic membrane. The present findings suggest also that the fish otolithic membrane may function to some degree in the otolith formation, but the function zone appears to be the subcupular meshwork layer and not the gelatinous layer. Most of these results are discussed with special regard to the environmental factors on early development in teleost fishes.


KEY WORDS: inner ear, development, sacculus, otolith, Perca fluviatilis (Teleostei)


Egyptian Journal of Biology Vol.3(2) 2001: 1-12