Calcium gradients in the fish inner ear sensory epithelium and otolithic membrane visualized by energy filtering transmission electron microscopy (EFTEM)

Immunohistochemical and ultrastructural characterization of the inner ear epithelial cells of splitnose rockfish (Sebastes diploproa)

The inner ear of teleost fish regulates the ionic and acid-base chemistry and secretes protein matrix into the endolymph to facilitate otolith biomineralization, which is used to maintain vestibular and auditory functions. The otolith is biomineralized in a concentric ring pattern corresponding to seasonal growth, and this calcium carbonate (CaCO3) polycrystal has become a vital aging and life-history tool for fishery managers, ecologists, and conservation biologists. Moreover, biomineralization patterns are sensitive to environmental variability including climate change, thereby threatening the accuracy and relevance of otolith-reliant toolkits. However, the cellular biology of the inner ear is poorly characterized, which is a hurdle for a mechanistic understanding of the underlying processes. This study provides a systematic characterization of the cell types in the inner ear of splitnose rockfish (Sebastes diploproa). Scanning electron microscopy revealed the apical morphologies of six inner ear cell types. In addition, immunostaining and confocal microscopy characterized the expression and subcellular localization of the proteins Na+-K+-ATPase, carbonic anhydrase, V-type H+-ATPase, Na+-K+-2Cl--cotransporter, otolith matrix protein 1, and otolin-1 in six inner ear cell types bordering the endolymph. This fundamental cytological characterization of the rockfish inner ear epithelium illustrates the intricate physiological processes involved in otolith biomineralization and highlights how greater mechanistic understanding is necessary to predict their multistressor responses to future climate change.

A comparative morphological investigation of otoliths of six parrotfish species (Scaridae) from the Solomon Islands

A comparative morphological investigation of otoliths from six parrotfish species and two genera, Scarus and Hipposcarus, was conducted to identify taxonomically diagnostic species-specific characters. Ontogenetic trends in otolith morphology were evident: Otoliths of small-sized specimens were most like those of large-sized Scarus oviceps and were least similar to large-sized Hipposcarus longiceps.

Morphological characteristics of otoliths for Dussumieria acuta and Dussumieria elopsoides (Pisces: Clupeidae) from the Northern Oman Sea

Otolith shape were investigated to identify two species of genus Dussumieria inhabiting the northern Oman Sea, south of Iran. The main aim of the investigation was to analyze otolith shape differences between these species. The sagittal otoliths of the rainbow sardine Dussumieria acuta Valenciennes, 1847, and the slender rainbow sardine Dussumieria elopsoides Bleeker, 1849, belonging to different length groups were described. The results showed two groups of special characteristics of the sagittal otoliths in D. acuta, and D. elopsoides. The first group is the characteristics that are useful to separate these species from other clupeid species, however these characteristics are closely related to one another in these species of Dussumieria genus. The second group comprises characteristics that vary due to genetically guided mechanisms and biological factors, but that may be useful to define species and are species-specific.

Cadherin-11 controls otolith assembly: evidence for extracellular cadherin activity

Cadherin-11/Cdh11 is expressed through early development and strongly during inner ear development (otic placode and vesicle). Here we show that antisense knockdown of Cdh11 during early zebrafish development interferes with otolith formation. Immunofluorescence labeling showed that Cdh11 expression was concentrated on and within the otolith. Cdh11 was faintly detected at the lateral surface (sites of cell-cell contact) of otic epithelial cells and in the cytoplasm. Strongly labeled Cdh11 containing puncta were detected within the otolymph (the fluid within the otic vesicle) and associated with the otolith surface. BODIPY-ceramine-labeled vesicular structures detected in the otolymph were larger and more numerous in Cdh11 knockdown embryos. We present evidence supporting a working model that vesicular structures containing Cdh11 (perhaps containing biomineralization components) are exported from the otic epithelium into the otolymph, adhere to one another and to the surface of the growing otolith, facilitating otolith growth.

Otolith crystals (in Carapidae): Growth and habit

The biomineralization of otoliths results mainly from the release of soluble Ca(2+), which is in turn precipitated as CaCO(3) crystals. In some Carapidae, sagittae sections have been shown to reveal a three-dimensional asymmetry with a nucleus close to the sulcal side, an unusual position. This study seeks to understand otolith formation in Carapus boraborensis. The unusual shape of the otolith is partly explained by the distribution of the epithelium cells, and particularly the sensory epithelium. Experimental evidence shows for the first time that aragonite growth takes place along the c-axis. These aragonite needles present two different habits. On the sulcal side is found the acicular form resulting from rapid growth during a short period of time. On the anti-sulcal side, the prismatic form seen there is due to a slower growth speed over longer periods. The otolith surface was observed each hour during a period of 24h in fishes reared in similar conditions. This allowed for the first time the direct observation on the otolith surface of the deposition of the two layers (L-zone and D-zone). In C. boraborensis, the organic-rich layer (D-zone) develops during the day, whereas the CaCO(3) layer (L-zone) seems to be deposited during the night.

On the role of the central nervous system in regulating the mineralisation of inner-ear otoliths of fish

Stato- or otoliths are calcified structures in the organ of balance and equilibrium of vertebrates, the inner ear, where they enhance its sensitivity to gravity. The compact otoliths of fish are composed of the calcium carbonate polymorph aragonite and a small fraction of organic molecules. The latter form a protein skeleton which determines the morphology of an otolith as well as its crystal lattice structure. This short review addresses findings according to which the brain obviously plays a prominent role in regulating the mineralisation of fish otoliths and depends on the gravity vector. Overall, otolith mineralisation has thus been identified to be a unique, neuronally guided biomineralisation process. The following is a hypothetical model for regulation of calcification by efferent vestibular neurons: (1) release of calcium at tight junctions in the macular epithelia, (2) macular carbonic anhydrase activity (which in turn is responsible for carbonate deposition), (3) chemical composition of matrix proteins. The rationale and evidence that support this model are discussed.