Preliminary observations on the morphology of the inner ear of the fetal narwhal (Monodon monoceros)

The inner ear of the narwhal (Monodon monoceros) was investigated on the basis of a complete series of microslides of an early fetus. In this well‐preserved specimen, the cochlea is about twice as large as the vestibular apparatus. The latter exhibits a high degree of specialization. Whereas the utriculus and sacculus are of normal size in comparison with those of other mammals, the semicircular canals are miniaturized but not obliterated, as in the case of some adult toothed whales. In comparison with other mammals, there is relatively little perilymphatic space around the vestibular organ, but the inner radius of the lymphatic ducts comes close to that of the adult human. Although there are some hints that the sensitivity of the vestibular system may be high in this species, detailed analysis of the adult narwhal ear is needed to confirm this inference.     

Hair cells in the inner ear of the pirouette and shaker 2 mutant mice

The shaker 2 (sh2) and pirouette (pi) mouse mutants display severe inner ear dysfunction that involves both auditory and vestibular manifestation. Pathology of the stereocilia of hair cells has been found in both mutants. This study was designed to further our knowledge of the pathological characteristics of the inner ear sensory epithelia in both the sh2 and pi strains. Measurements of auditory brainstem responses indicated that both mutants were profoundly deaf. The morphological assays were specifically designed to characterize a pathological actin bundle that is found in both the inner hair cells and the vestibular hair cells in all five vestibular organs in these two mutants. Using light microscope analysis of phalloidin-stained specimens, these actin bundles could first be detected on postnatal day 3. As the cochleae matured, each inner hair cell and type I vestibular hair cell contained a bundle that spans from the region of the cuticular plate to the basal end of the cell, then extends along with cytoplasm and membrane, towards the basement membrane. Abnormal contact with the basement membrane was found in vestibular hair cells. Based on the shape of the cellular extension and the actin bundle that supports it, we propose to name these extensions “cytocauds.” The data suggest that the cytocauds in type I vestibular hair cells and inner hair cells are associated with a failure to differentiate and detach from the basement membrane.     

The surface periplast component of the protistKomma caudata (Cryptophyceae) self-assembles from a secreted high-molecular-mass polypeptide

Summary The cell covering of the cryptomonadKomma caudata (Geitler) Hill is a trilaminar structure consisting of a surface periplast component (SPC) and an inner periplast component (IPC) that sandwich the plasma membrane. In order to investigate the development of the periplast, we have raised monoclonal antibodies against the cell surface ofK. caudata. Immunoblot analyses using one of these antibodies, K1/D.10, showed that it labeled a high-molecular-mass polypeptide. Immunofluorescence and pre- and post-embedding immunogold labeling studies demonstrated that the antibody recognized sites on the cell surface corresponding to the SPC plates and anotherK. caudata cell surface component, the rosulate scales. Labeling was also detected on surface domains devoid of periplast, namely the vestibular/gullet region of the cell. Post-embedding immunocytochemistry revealed that intracellular sites labeled with K1/D.10 included the Golgi apparatus and its associated vesicles. We propose that the subunits of theK. caudata cell covering are antigenically related molecules and that they self-assemble on the cell surface after secretion via the endomembrane system and deployment at the vestibular/gullet region or, in dividing cells, the cytokinetic furrow.     

Distinct contributions from the hindbrain and mesenchyme to inner ear morphogenesis

A mature inner ear is a complex structure consisting of vestibular and auditory components. Microsurgical ablations, rotations, and translocations were performed in ovo to identify the tissues that control inner ear morphogenesis. We show that mesenchyme/ectoderm adjacent to the developing ear specifically governs the shape of vestibular components - the semicircular canals and ampullae - by conferring anteroposterior axial information to these structures. In contrast, removal of individual hindbrain rhombomeres adjacent to the developing ear preferentially affects the growth and morphogenesis of the auditory subdivision, the cochlear duct, or basilar papilla. Removal of rhombomere 5 affects cochlear duct growth, while rhombomere 6 removal affects cochlear growth and morphogenesis. Rotating rhombomeres 5 and 6 along the anteroposterior axis also impacts cochlear duct morphogenesis but has little effect on the vestibular components. Our studies indicate that discrete tissues, acting at a distance, control the morphogenesis of distinct elements of the inner ear. These results provide a basis for identifying factors that are essential to vestibular and auditory development in vertebrates.     

A unique ball-shaped Golgi apparatus in the rat pituitary gonadotrope: its functional implications in relation to the arrangement of the microtubule network

In polarized exocrine cells, the Golgi apparatus is cup-shaped and its convex and concave surfaces are designated as cis and trans faces, functionally confronting the rough endoplasmic reticulum and the cell surface, respectively. To clarify the morphological characteristics of the Golgi apparatus in non-polarized endocrine cells, the investigators immunocytochemically examined its precise architecture in pituitary gonadotropes, especially in relation to the arrangement of the intracellular microtubule network. The Golgi apparatus in the gonadotropes was not cup-shaped but ball-shaped or spherical, and its outer and inner surfaces were the cis and trans faces, respectively. Centrioles were situated at the center of the Golgi apparatus, from which radiating microtubules isotropically extended to the cell periphery through the gaps in the spherical wall of the Golgi stack. The shape of the Golgi apparatus and the arrangement of microtubules demonstrated in the present study could explain the microtubule-dependent movements of tubulovesicular carriers and granules within the gonadotropes. Furthermore, the spherical shape of the Golgi apparatus possibly reflects the highly symmetrical arrangement of microtubule arrays, as well as the poor polarity in the cell surface of pituitary gonadotropes.     

The Candidate Splicing Factor Sfswap Regulates Growth and Patterning of Inner Ear Sensory Organs

The Notch signaling pathway is thought to regulate multiple stages of inner ear development. Mutations in the Notch signaling pathway cause disruptions in the number and arrangement of hair cells and supporting cells in sensory regions of the ear. In this study we identify an insertional mutation in the mouse Sfswap gene, a putative splicing factor, that results in mice with vestibular and cochlear defects that are consistent with disrupted Notch signaling. Homozygous Sfswap mutants display hyperactivity and circling behavior consistent with vestibular defects, and significantly impaired hearing. The cochlea of newborn Sfswap mutant mice shows a significant reduction in outer hair cells and supporting cells and ectopic inner hair cells. This phenotype most closely resembles that seen in hypomorphic alleles of the Notch ligand Jagged1 (Jag1). We show that Jag1; Sfswap compound mutants have inner ear defects that are more severe than expected from simple additive effects of the single mutants, indicating a genetic interaction between Sfswap and Jag1. In addition, expression of genes involved in Notch signaling in the inner ear are reduced in Sfswap mutants. There is increased interest in how splicing affects inner ear development and function. Our work is one of the first studies to suggest that a putative splicing factor has specific effects on Notch signaling pathway members and inner ear development.     

Effects of Mutations at the W Locus (c-kit) on Inner Ear Pigmentation and Function in the Mouse

The W locus encodes a tyrosine kinase receptor, c-kit, which affects survivial of melanoblasts from the neural crest. The primary cochlear defect in Viable Dominant Spotting (W^v/W^v) mutants is a lack of melanocytes within the stria vascularis (SV) associated with an endocochlear potential (EP) close to zero and hearing impairment. In this study, we compare inner ear pigmentation with cochlear potentials in three other W alleles (W^x, W^sh, and W⁴¹) and reveal an unequivocal correlation between presence of strial melanocytes and presence of an EP. Asymmetry was common, and 8.3% of W^sh/W^x, 25% of W^sh/W^sh, 60% of W⁴¹/W^x, and 69.2% of W⁴¹/W⁴¹ ears had a pigmented stria and an EP, while the remainder had no strial melanocytes and no EP. In those mutants that partially escaped the effects of the mutation, strial melanocytes rarely extended the entire length of the stria, but were confined to the middle and/or basal turns of the cochlea. The extent of strial pigmentation was unrelated to the EP value, which was measured from the basal turn only. Compound action potential (CAP) responses recorded from ears with an EP were variable and they showed greatly raised thresholds or were absent in all ears where the EP was close to zero. In controls, melanocytes in the vestibular part of the ear were found in the utricle, crus commune, and ampullae, whereas in many mutants only one or two of these regions were pigmented. There was a broad correlation between pigmentation of the stria and pigmentation of the vestibular region but this was not absolute. All W⁴¹/W^x, W^sh/W^sh, and W⁴¹/W⁴¹ mutants had some pigment on the pinna but, in contrast to controls where melanocytes were found in the epidermis and dermis of the pinna, pigment cells were reduced in number and generally restricted to the dermis. Injection of normal neural crest cells into 9.5-day-old mutant embryos increased the extent of skin pigmentation on the head and coat of adult chimeras and was associated with a small increase in the proportion of pigmented strias.     

Distribution and morphological changes of the Golgi apparatus during Drosophila spermatogenesis

In spermatogenesis, the Golgi apparatus is important for the formation of the acrosome, which is a sperm‐specific organelle essential for fertilization. Comprehensive examinations of the spatiotemporal distribution and morphological characterizations of the Golgi in various cells during spermatogenesis are necessary for functional analyses and mutant screenings in the model eukaryote Drosophila. Here, we examined the distribution and morphology of the Golgi during Drosophila spermatogenesis with immunofluorescence and electron microscopy. In pre‐meiotic germ cells, the Golgi apparatuses were distributed evenly in the cytoplasm. In contrast, they were located exclusively in two regions near the poles during the meiotic metaphase, where they were segregated prior to the chromosomes. In cells in anaphase to telophase, the Golgi were predominantly left behind in the equatorial region between the separating daughter nuclei. After completion of meiosis, the dispersed Golgi were assembled at the apical side of the spermatid nucleus to form the acrosome. Further investigation of the Golgi distribution in β2‐tubulin mutants showed aberrant and uneven distributions of the Golgi among sister cells in the meiotic spermatocytes and in the post‐meiotic spermatids. At the ultrastructural level, the Golgi apparatus in pre‐meiotic spermatocytes comprised a pair of stacks. The two stacks were situated adjacent to each other, as if they had duplicated before entering into meiotic division. These results highlight the dynamic nature of the Golgi during spermatogenesis and provide a framework for analyzing the correlations between the dynamics of the Golgi and its function in sperm development.     

A mutation in CCDC50, a gene encoding an effector of epidermal growth factor-mediated cell signaling, causes progressive hearing loss

We previously mapped a novel autosomal dominant deafness locus, DFNA44, by studying a family with postlingual, progressive, nonsyndromic hearing loss. We report here on the identification of a mutation in CCDC50 as the cause of hearing loss in the family. CCDC50 encodes Ymer, an effector of epidermal growth factor (EGF)-mediated cell signaling that is ubiquitously expressed in different organs and has been suggested to inhibit down-regulation of the EGF receptor. We have examined its expression pattern in mouse inner ear. Western blotting and cell transfection results indicate that Ymer is a soluble, cytoplasmic protein, and immunostaining shows that Ymer is expressed in a complex spatiotemporal pattern during inner ear development. In adult inner ear, the expression of Ymer is restricted to the pillar cells of the cochlea, the stria vascularis, and the vestibular sensory epithelia, where it shows spatial overlap with the microtubule-based cytoskeleton. In dividing cells, Ymer colocalizes with microtubules of the mitotic apparatus. We suggest that DFNA44 hearing loss may result from a time-dependent disorganization of the microtubule-based cytoskeleton in the pillar cells and stria vascularis of the adult auditory system.     

The Asymmetrical Structure of Golgi Apparatus Membranes Revealed by In situ Atomic Force Microscope

The Golgi apparatus has attracted intense attentions due to its fascinating morphology and vital role as the pivot of cellular secretory pathway since its discovery. However, its complex structure at the molecular level remains elusive due to limited approaches. In this study, the structure of Golgi apparatus, including the Golgi stack, cisternal structure, relevant tubules and vesicles, were directly visualized by high-resolution atomic force microscope. We imaged both sides of Golgi apparatus membranes and revealed that the outer leaflet of Golgi membranes is relatively smooth while the inner membrane leaflet is rough and covered by dense proteins. With the treatment of methyl-β-cyclodextrin and Triton X-100, we confirmed the existence of lipid rafts in Golgi apparatus membrane, which are mostly in the size of 20 nm –200 nm and appear irregular in shape. Our results may be of significance to reveal the structure-function relationship of the Golgi complex and pave the way for visualizing the endomembrane system in mammalian cells at the molecular level.     

Transport of rosettes from the golgi apparatus to the plasma membrane in isolated mesophyll cells ofZinnia elegans during differentiation to tracheary elements in suspension culture

Summary Rosettes of six particles have been visualized by freeze-fracture in the protoplasmic fracture (PF) faces of: a) the plasma membrane, b) Golgi cisternae, and c) Golgi-derived vesicles in mesophyll cells ofZinnia elegans that had been induced to differentiate synchronously into tracheary elements in suspension culture. These rosettes have been observed previously in the PF face of the plasma membranes of a variety of cellulose-synthesizing cells and are thought to be important in cellulose synthesis. InZinnia tracheary elements, the rosettes are localized in the membrane over regions of secondary wall thickening and are absent between thickenings. The observation of rosettes in the Golgi cisternae and vesicles suggests that the Golgi apparatus is responsible for the selective transport and exocytosis of rosettes in higher plants, as has been previously indicated in the algaMicrasterias (Giddings et al. 1980). The data presented indicate that the Golgi apparatus has a critical role in the control of cell wall deposition because it is involved not only in the synthesis and export of matrix components but also in the export of an important component of the cellulose synthesizing apparatus. The rosettes are present in the plasma membrane and Golgi vesicles throughout the enlargement of the secondary thickening, suggesting that new rosettes must be continually inserted into the membrane to achieve complete cell wall thickening.Abbreviations EF Golgi vesicles, exoplasmic fracture; the plasma membrane, extracellular fracture - PF protoplasmic fracture     

A mouse model for benign paroxysmal positional vertigo with genetic predisposition for displaced otoconia

Abnormal formation of otoconia, the biominerals of the inner ear, results in balance disorders. The inertial mass of otoconia activates the underlying mechanosensory hair cells in response to change in head position primarily during linear and rotational acceleration. Otoconia associate exclusively with the two gravity receptors, the utricle and saccule. The cristae sensory epithelium is associated with an extracellular gelatinous matrix known as cupula, equivalent to otoconia. During head rotation, the inertia of endolymphatic fluids within the semicircular canals deflects the cupula of the corresponding crista and activates the underlying mechanosensory hair cells. It is believed that detached free‐floating otoconia particles travel ectopically to the semicircular canal and cristae and are the culprit for benign paroxysmal positional vertigo (BPPV). The Slc26a4 mouse mutant harbors a missense mutation in pendrin. This mutation leads to impaired transport activity of pendrin and to defects in otoconia composition and distribution. All Slc26a4 ^loop/loop homozygous mutant mice are profoundly deaf but show inconsistent vestibular deficiency. A panel of behavioral tests was utilized in order to generate a scoring method for vestibular function. A pathological finding of displaced otoconia was identified consistently in the inner ears of mutant mice with severe vestibular dysfunction. In this work, we present a mouse model with a genetic predisposition for ectopic otoconia with a clinical correlation to BPPV. This unique mouse model can serve as a platform for further investigation of BPPV pathophysiology, and for developing novel treatment approaches in a live animal model.     

Hair cell ribbon synapses

Hearing and balance rely on the faithful synaptic coding of mechanical input by the auditory and vestibular hair cells of the inner ear. Mechanical deflection of their stereocilia causes the opening of mechanosensitive channels, resulting in hair cell depolarization, which controls the release of glutamate at ribbon-type synapses. Hair cells have a compact shape with strong polarity. Mechanoelectrical transduction and active membrane turnover associated with stereociliar renewal dominate the apical compartment. Transmitter release occurs at several active zones along the basolateral membrane. The astonishing capability of the hair cell ribbon synapse for temporally precise and reliable sensory coding has been the subject of intense investigation over the past few years. This research has been facilitated by the excellent experimental accessibility of the hair cell. For the same reason, the hair cell serves as an important model for studying presynaptic Ca(2+) signaling and stimulus-secretion coupling. In addition to common principles, hair cell synapses differ in their anatomical and functional properties among species, among the auditory and vestibular organs, and among hair cell positions within the organ. Here, we briefly review synaptic morphology and connectivity and then focus on stimulus-secretion coupling at hair cell synapses.     

Identification and characterization of AtCASP,a plant transmembrane Golgi matrix protein

Golgins are a family of coiled-coil proteins that are associated with the Golgi apparatus. They are necessary for tethering events in membrane fusion and may act as structural support for Golgi cisternae. Here we report on the identification of an Arabidopsis golgin which is a homologue of CASP, a known transmembrane mammalian and yeast golgin. Similar to its homologues, the plant CASP contains a long N-terminal coiled-coil region protruding into the cytosol and a C-terminal transmembrane domain with amino acid residues which are highly conserved across species. Through fluorescent protein tagging experiments, we show that plant CASP localizes at the plant Golgi apparatus and that the C-terminus of this protein is sufficient for its localization, as has been shown for its mammalian counterpart. In addition, we demonstrate that the plant CASP is able to localize at the mammalian Golgi apparatus. However, mutagenesis of a conserved tyrosine in the transmembrane domain revealed that it is necessary for ER export and Golgi localization of the Arabidopsis CASP in mammalian cells, but is not required for its correct localization in plant cells. These data suggest that mammalian and plant cells have different mechanisms for concentrating CASP in the Golgi apparatus.†These authors have contributed equally to the work     

The tubular network of the Golgi apparatus

 Golgi apparatus of both plant and animal cells are characterized by an extensive system of approximately 30 nm diameter peripheral tubules. The total surface area of the tubules and associated fenestrae is thought to be approximately equivalent to that of the flattened portions of cisternae. The tubules may extend for considerable distances from the stacks. The tubules are continuous with the peripheral edges of the stacked cisternae, but the way they interconnect differs across the stack. In plant cells, for example, tubules associated with the near-cis and mid cisternae often begin to anastomose close to the peripheral edges of the stacked cisternae, whereas the tubules of the trans cisternae are less likely to anastomose and are more likely to be directly continuous with the peripheral edges of the stacked cisternae. Additionally, the tubules may blend gradually into fenestrae that surround some of the stack cisternae. Because of the large surface area occupied by tubules and fenestrae, it is reasonable to suppose that these components of the Golgi apparatus play a significant role in Golgi apparatus function. Tubules clearly interconnect closely adjacent stacks of the Golgi apparatus and may represent a communication channel to synchronize stack function within the cell. A feasible hypothesis is that tubules may be a potentially static component of the Golgi apparatus in contrast to the stacked cisternal plates which may turn over continuously. The coated buds associated with tubules may represent the means whereby adjacent Golgi apparatus stacks exchange carbohydrate-processing enzymes or where resident Golgi apparatus proteins are introduced into and out of the stack during membrane flow differentiation. The limited gradation of tubules from cis to medial to trans offers additional possibilities for functional specialization of Golgi apparatus in keeping with the hypothesis that tubules are repositories of resident Golgi apparatus proteins protected from turnover during the flow differentiation of the flattened saccules of the Golgi apparatus stack. Accepted: 3 November 1997     

雌激素及其受体在内耳发育和功能上的作用

雌激素在生殖系统、认知记忆系统、骨骼和神经的发育及其功能维持等多种生理功能中扮演了重要的作用。近年来,在内耳发育及其功能研究过程中,许多学者发现在听力和平衡系统功能上的性别差异可能归根因于不同性别的雌激素水平差异。这些研究表明,雌激素及其受体在内耳发育、听力和平衡系统功能维持上也具有重要作用。该文用一个新的视角聚焦于雌激素及其受体在内耳发育和功能上的研究进展。该综述能为进一步研究雌激素在听力和平衡系统中的作用机制及相关疾病的临床治疗提供参考。     

Development of the inner ear efferent system across vertebrate species

Inner ear efferent neurons are part of a descending centrifugal pathway from the hindbrain known across vertebrates as the octavolateralis efferent system. This centrifugal pathway terminates on either sensory hair cells or eighth nerve ganglion cells. Most studies of efferent development have used either avian or mammalian models. Recent studies suggest that prevailing notions of the development of efferent innervation need to be revised. In birds, efferents reside in a single, diffuse nucleus, but segregate according to vestibular or cochlear projections. In mammals, the auditory and vestibular efferents are completely separate. Cochlear efferents can be divided into at least two distinct, descending medial and lateral pathways. During development, inner ear efferents appear to be a specific motor neuron phenotype, but unlike motor neurons have contralateral projections, innervate sensory targets, and, at least in mammals, also express noncholinergic neurotransmitters. Contrary to prevailing views, newer data suggest that medial efferent neurons mature early, are mostly, if not exclusively, cholinergic, and project transiently to the inner hair cell region of the cochlea before making final synapses on outer hair cells. On the other hand, lateral efferent neurons mature later, are neurochemically heterogeneous, and project mostly, but not exclusively to the inner hair cell region. The early efferent innervation to the ear may serve an important role in the maturation of afferent responses. This review summarizes recent data on the neurogenesis, pathfinding, target selection, innervation, and onset of neurotransmitter expression in cholinergic efferent neurons.