The ultrastructural organization and properties of the mouse tectorial membrane matrix

The polyphenolic compound tannic acid and the cationic stains ruthenium red, Alcian blue and lanthanum chloride have been used to reinvestigate the ultrastructural organization of the tectorial membrane matrix. Tannic acid treatment reveals that the matrix both in between and within the Type A protofibril bundle system has a high degree of structural organization. The basic unit of this matrix is best described as a 'striated sheet'. These striated sheets are formed by alternating 'dark' and 'light' fibrils which run parallel to one another and lie within the plane of each sheet. In sodium based buffers both light and dark fibrils have diameters of approximately 7 nm and the distance between each dark fibril in a sheet varies from 30 to 46 nm. Dark and light fibrils are coupled by periodic, staggered cross-bridges which occur at approximately 12 nm intervals along the fibrils. Fibril diameters in tectorial membranes prepared and fixed in potassium based buffers are from 10-20% greater than they are in tectorial membranes prepared and fixed in sodium based buffers. Fine fibrils can also be resolved in the matrix with the cationic stains lanthanum chloride and ruthenium red, but the organization of these fibrils into a regular matrix structure is most clearly resolved with tannic acid treatment. The striated sheets are largely destroyed by treating the tectorial membranes with neutral trypsin and are insensitive to treatment with bacterial collagenase. In contrast, the Type A protofibril system is trypsin resistant and collagenase sensitive. Treatment of tectorial membranes with salt solutions containing either 5 nM EDTA or 5 mM EGTA and 2 mM MgCl2 results in a complete loss of organized striated sheets and the appearance of randomly dispersed fibrillar material and small particles. Re-addition of Ca2+ ions causes the striated sheets to reform, indicating that the structure can undergo at least one cycle of depolymerization and polymerization in vitro. Reduction of disulphide bonds with beta-mercaptoethanol causes a loss of structural organization similar to that observed after EDTA or EGTA treatment. The results demonstrate that the non-collagenous components of the tectorial form a matrix with a degree of organization that has been previously unrecognised.

The morphology and physiology of hair cells in organotypic cultures of the mouse cochlea

Organotypic explant cultures were prepared from the cochleas of 1 to 3 day post-natal mice and maintained in vitro for up to 5 days. The hair cells retain morphological integrity for the duration of the culture period although they exhibit embryological features such as a kinocilium and additional microvilli on their apical surfaces. The resting membrane potentials of mouse inner hair cells (IHCs) in vitro are similar to those of guinea-pig IHCs in vivo but the membrane potentials of outer hair cells (OHCs) in the mouse cochlea in vitro are less polarized than the resting membrane potentials of OHCs in the basal turn of the guinea-pig cochlea in vivo. The voltage responses of IHCs and OHCs to sinusoidal displacements of their stereocilia are similar to each other in waveform and dynamic range, although the responses of IHCs are larger than those of OHCs. The relationship between transducer conductance and stereocilia displacement in IHCs and OHCs is non-linear and largely accounts for the depolarizing asymmetry of the voltage response. The receptor potentials of IHCs and OHCs reverse close to 0 mV indicating that the transducer conductance is non-selective for cations. The voltage responses of IHCs and OHCs to intracellular current injection rectify when the membrane potentials are more depolarized than about -30 mV. This rectification is most pronounced in OHCs. OHCs also exhibit a time-dependent, voltage-sensitive conductance although they do not behave as electrical resonators.

Development of the electromotor system of Torpedo marmorata: distribution of extracellular matrix and cytoskeletal components during acetylcholine receptor focalization

A combination of direct fluorescence and indirect immunofluorescence microscopy has been used to compare the distribution of the acetylcholine receptor with the distribution of major cytoskeletal and extracellular matrix components during electrocyte differentiation in the electric organs of Torpedo marmorata. Laminin, fibronectin and extracellular matrix proteoglycan are always more extensively distributed around the differentiating cell than the acetylcholine receptor-rich patch that forms on the ventral surface of the cell. The distribution of acetylcholinesterase within the ventral surface of the differentiating electrocyte closely resembles the distribution of the acetylcholine receptor. Areas of apparently high acetylcholine receptor density within the ventrally forming acetylcholine receptor-rich patch are always areas of apparently high extracellular matrix proteoglycan density but are not always areas of high laminin or fibronectin density. Desmin levels appear to increase at the onset of differentiation and desmin initially accumulates in the ventral pole of each myotube as it begins to form an electrocyte. During differentiation F-actin-positive filament bundles are observed that extend from the nuclei down to the ventrally forming acetylcholine receptor-rich patch. Most filament bundles terminate in the acetylcholine receptor-rich region of the cell membrane. Electron-microscopic autoradiography suggests that the filament bundles attach to the membrane at sites where small acetylcholine receptor clusters are found. The results of this study suggest that, out of the four extracellular matrix components studied, only the distribution of acetylcholinesterase (which may be both matrix- and membrane-bound at this stage) closely parallels that of the acetylcholine receptor, and that F-actin filament bundles terminate in a region of the cell that is becoming an area of high acetylcholine receptor density.

Expression of cell adhesion molecules during embryonic induction. III. Development of the otic placode

During embryonic development, the inner ear develops from a placode into a richly differentiated structure with defined borders between neural and non-neural elements. In an effort to define the origin of such differentiation boundaries from the time of appearance of the placode, immunocytochemical methods have been used to map the developmental distributions of the cell adhesion molecules, N-CAM, L-CAM, and Ng-CAM, and the extracellular matrix molecules, cytotactin and fibronectin, in the cochlea of the chicken embryo. As the otic placode was induced by the underlying N-CAM-containing rhombencephalon and mesoderm, the placode expressed both N-CAM and L-CAM. During the period when the otic vesicle differentiated to give rise to the acoustic ganglion and to the differentiated structures of the cochlea, N-CAM increased in the innervated sensory regions while L-CAM increased in the non-sensory areas of the auditory epithelium adjacent to the sensory regions. During subsequent development, the differential expression of N-CAM and L-CAM again formed striking borders within the epithelium between the five morphologically and functionally distinct regions of the cochlea. This pattern of CAM expression is consistent with previous observations suggesting that primary CAMs of different binding specificities are expressed in two different modes to form borders at all sites of embryonic induction and at sites of further cytodifferentiation (K. L. Crossin, C -M. Chuong, and G. M. Edelman, 1985, Proc. Natl. Acad. Sci. USA 82, 6942-6946). Unlike inductive sites involving mesenchyme, however, the placode showed only changes in which an epithelium containing both CAMs loses one or the other or remains unchanged. As differentiation occurred during innervation of the sensory region, the secondary Ng-CAM appeared. Ng-CAM-positive fibers penetrated into the basilar papilla and Ng-CAM and the matrix protein cytotactin appeared within the epithelium in a radial pattern that was consistent with the previously described roles of these molecules in neurite movement. Immunoblot analyses confirmed the identity and biochemical properties of the CAMs and also revealed that N-CAM underwent embryonic to adult conversion during inner ear formation. These studies support the idea that CAMs are expressed in specific modal patterns in the cell collectives participating in inductive events, and strongly suggest that cellular regulation of these patterns is correlated with border formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Polypeptide composition of the mammalian tectorial membrane

The effects of the enzymes collagenase, pepsin, chondroitinase ABC and keratanase on the polypeptide composition of the mammalian tectorial membrane have been analysed using one dimensional SDS-polyacrylamide gel electrophoresis (SDS-PAGE). After reduction at least ten polypeptides can be consistently and clearly recognized in SDS gels with molecular weights relative to globular protein standards of 245, 235, 190, 165, 155, 145, 100, 93, 60-73 and 35-49 kDa. With the exception of the 60-73 and 35-49 kDa bands all these polypeptides are sensitive to digestion with bacterial collagenase. The 235, 165, 155, 145 and 93 kDa bands also resist degradation by cold, acidic pepsin. Amino acid analysis of whole tectorial membranes demonstrates that glycine accounts for nearly 25% of the total amino acid content, that proline, hydroxyproline and hydroxylysine are present and that amine sugars can be detected in fairly high concentrations. Estimates based on hydroxyproline content suggest that collagens account for 25-50% of the total tectorial membrane protein. Immunoblotting techniques demonstrate the presence of polypeptides cross reacting with antisera to Type II collagen, Type IX collagen and Type V collagen. Results from immunohistochemical studies confirm that these polypeptides are present in the tectorial membrane and are not contaminants of the isolation procedure. Collagenase treatment of tectorial membranes reveals the presence of an additional non-collagenous polypeptide with an apparent molecular weight of 173 kDa on 7.5% polyacrylamide gels, and polydisperse high molecular weight material spreading over a broad range at the top of the gels. This high molecular weight material and the 173, 60-73 and 35-49 kDa non-collagenous polypeptides are pepsin sensitive and all bind wheat germ agglutinin (WGA) suggesting that they contain N-acetyl glucosamine. The 173 kDa band also binds soybean agglutinin (SBA) suggesting the presence of N-acetyl galactosamine. In the absence of reducing agent the 173 and 60-73 kDa bands are no longer observed and high molecular weight material forming a broad band at the top of the separating gel is seen. The electrophoretic behaviour of this non-collagenous, glycosylated, disulphide bonded, high molecular weight material is altered by treatment with keratanase but not by chondroitinase ABC. The results of this study indicate the tectorial membrane contains at least three different collagen types and, in addition to these collagenous proteins, several non-collagenous, glycosylated polypeptides that may account for as much as 50% of the total tectorial membrane protein.

Mechanosensitivity of mammalian auditory hair cells in vitro

Intracellular responses recorded in vitro from the cochleas of anaesthetized mammals have shown that the mechanoreceptive inner and outer hair cells are sharply tuned, accounting for many of the properties of the afferent fibres in the auditory nerve. However, in vivo it has not been possible to measure directly the excitatory mechanical input to these cells (the displacement of their mechanosensitive stereocilia) and thus to determine the relationship between the receptor potentials and displacement of their stereocilia. As a means of circumventing this technical difficulty, we have developed an organ culture of the mouse cochlea and here we describe the receptor potentials generated by the hair cells in response to direct displacement of their stereocilia.

Torpedo electromotor system development: biochemical differentiation of Torpedo electrocytes in vitro

The accumulation of 2 postsynaptic proteins--the acetylcholine receptor and acetylcholinesterase, total protein and lactate dehydrogenase levels, and the evolution of the multiple molecular forms of acetylcholinesterase (exhibiting apparent sedimentation coefficients of 17, 13, 11 and 6S) have been examined in aneural cultures of embryonic Torpedo electric organ explanted before, during or after electrocyte differentiation and the onset of synaptogenesis. During electrocyte differentiation in vitro, with explants taken before the 38 mm stage, the relative proportions of the 17, 13 and 11S forms change in vitro as in vivo but the 6S form remains abnormally dominant. In tissue explants taken from 38 to 47 mm stage embryos, the 4 major molecular forms of acetylcholinesterase differentiate in a manner identical to that observed in vivo. In explants taken after the onset of synaptogenesis (55-80 mm stages), the proportions of the acetylcholinesterase forms change as in vivo only during the first week in vitro whilst accumulation is occurring at the normal in vivo rate. The switch to the high acetylcholine receptor and acetylcholinesterase accumulation rate that occurs when synaptogenesis begins in vivo is not observed after any time lag in vitro with tissue explanted before the stage (55 mm) at which synaptogenesis begins. The effects on acetylcholinesterase and acetylcholine receptor accumulation of supplementing the medium with a neural tissue extract are described. The experiments were designed to elucidate the factors and mechanisms that regulate the differentiation and formation of chemical synapses using the electric organ of Torpedo marmorata as a model system. The results demonstrate that the complex changes occurring in the multiple molecular forms of acetylcholinesterase during electrocyte differentiation are not under direct neural control but that the switch to an increased acetylcholinesterase and acetylcholine receptor accumulation rate may be triggered by an external, possible neural factor.

The responses of inner and outer hair cells in the basal turn of the guinea-pig cochlea and in the mouse cochlea grown in vitro

Until recently the responses of the mechanosensitive hair cells of the cochlea have been inferred from their morphology, morphological relationships with other structures in the cochlea, and by indirect electrophysiological measurements. With the advent of techniques for making intracellular recordings from hair cells in the cochleas of anaesthetised mammals it has been possible to measure the responses of hair cells to acoustic stimulation and to assess their roles in sensory transduction in the cochlea. Intracellular recordings of the responses of inner and outer hair cells in the basal turn of the guinea-pig cochlea show that they differ considerably from each other. The receptor potentials of inner hair cells are larger, predominantly depolarizing to low frequency tones and at their best frequencies (16-20 kHz) they generate depolarizing dc receptor potentials. Outer hair cells generate predominantly hyperpolarizing potentials to low frequency tones. They do not produce significant voltage responses at high frequencies except at high intensities when they generate slowly rising depolarizing potentials which are associated with loss of cochlear sensitivity. At low frequencies the receptor potentials of the inner hair cells phase lead those of the outer hair cell. Measurements of their frequency selectivity show that inner and outer hair cells are both sharply tuned. It is proposed that the responses of inner and outer hair cells are consistent with sensory and motor roles respectively in mechanoelectric transduction and that the outer hair cells are the site of an active mechanical process responsible for the frequency selectivity and sensitivity of the cochlea. Intracellular recordings from hair cells in the mouse cochlea maintained in vivo have provided a direct measure of the mechanosensitivity of cochlear hair cells (approximately 30 mV per degree of displacement of their stereociliary bundles) and indirect evidence that the transfer characteristics of the outer hair cells in vivo may be due to their mechanoelectrical interaction with the tectorial membrane. This is because the transfer characteristics of the inner and outer hair cells are similar in vitro in the absence of a tectorial membrane. Considerable importance is attributed to the shape of the transfer characteristics of the inner and outer hair cells. Changes in these characteristics during anoxia and following exposure to intense tones are associated with depolarization of the outer hair cells and loss of cochlear sensitivity and frequency selectivity. Current-voltage studies of hair cells in vivo show the inner and outer hair cells to be electrically nonlinear.(ABSTRACT TRUNCATED AT 400 WORDS)

Torpedo electromotor system development: neuronal cell death and electric organ development in the fourth branchial arch

The fourth branchial arch of Torpedo marmorata has been examined at the light and electron microscopic level during development. Of interest was the determination of the extent of electric organ tissue reported to be present in this arch and its possible relationship to electromotoneuron cell death in the electric lobes. The main electric organ of the torpedo is derived from the hyoid and first three branchial arches and is innervated by four major electromotor nerves. Extensive electromotoneuron cell death occurs in the electric lobes and most notably in the posterior poles. This feature could be due to a tendency for these neurons to innervate the fourth branchial arch where little or no electric tissue is formed. Our findings support this conclusion but are not entirely consistent with the idea that a population mismatch has occurred. This is because cell death precedes the genesis of the target cells. The presence of innervated differentiated electric tissue in this arch is also reported, leading to the conclusion that Torpedo marmorata possesses an accessory electric organ.

Torpedo electromotor system development: developmentally regulated neuronotrophic activities of electric organ tissue

Explant cultures of electric lobe from 45-60 mm stage Torpedo embryos and both ganglionic and dissociated cell cultures prepared from 8-day chick ciliary ganglia have been used to determine whether the electric organs of Torpedo marmorata contain developmentally regulated neuronotrophic activity. Electric lobe explants were evaluated by measuring their neurone density, choline acetyltransferase (CAT0, and low salt, Triton X-100-soluble protein contents. Addition of soluble extracts prepared from the electric organs of late stage embryos (85-105 mm) to standard medium results in the maintenance of nearly theoretical neurone densities in electric lobe explants during a 7-day culture period. Soluble electric organ extracts from early embryonic stages (42-59 mm) do not increase neurone density relative to control cultures but cause an elevation in the CAT content of the explants over control values. On the basis of this analysis it is concluded (1) that late embryonic stage and adult electric organs contain neuronotrophic activity that allows electromotor neurones to survive in vitro and (2) that activity increases rapidly in the electric organs between the 59 nd 72 mm stages of development at a time when rapid increases in postsynaptic membrane markers in the electric organs occur and when peripheral synaptogenesis begins. The activity of late stage embryonic electric organs is heat stable and lost on dialysis. Using ciliary ganglion explants and evaluating both the initial fibre outgrowth and the CAT content after 4 days in vitro, trophic activity is found to be maximal at early embryonic stages (45-55 mm) and to decline thereafter. It is shown that the decline in activity is not due to an increase in toxicity. Using established dissociated ganglionic cell survival assays the specific activity of neuronotrophic factors allowing survival is constant between the 45 and 73 mm stages in the electric organs and then rapidly declines, but activity per electric organ increases rapidly between the 45 and 73 mm stages and then remains at a constant level. The use of poly-dl-ornithine substrates coated with heart-conditioned medium for the cell survival assay results in up to tenfold increase in the trophic titre of the electric organ extracts. The neuronotrophic activity supporting survival of ciliary motorneurones present in embryonic electric organs is heat labile and retained on dialysis. It is concluded that developing electric organs contain at least two neuronotrophic factors that have different properties and are differently regulated. Both factors may contribute during development to bringing naturally occurring electromotor neurone cell death to an end.

Investigations into a bioelectric component of synaptogenesis

Synaptogenesis has been investigated in the electric organ of Torpedo marmorata with the objective of determining whether a bioelectric effect could be demonstrated. Answers to 3 questions were sought. (1) Are currents and/or fields present within the organ? (2) Can they be localized? (3) Are they involved with the synaptogenic process? Voltage measurements across pieces of electric organ revealed the presence of a dorsal positive potential in the low millivolt range. Injection of DC current against this dorsal positive dipole had the effect of reducing the percent of neuritic coverage on the ventral surface as measured by quantitative electron microscopy. These results indicate the presence of a field potential, dorsal positive which, when reversed, causes a retardation in the synaptogenic rate. They are consistent with published reports of neurites growing preferentially towards cathodal sources and implicate that bioelectric forces may be one component of the synaptogenesis process.

The developmental morphology of Torpedo marmorata: the electric lobes

The development of the electric lobes of Torpedo marmorata has been investigated using light and electron microscopical techniques. The lobe Anlagen become visible in the rhombencephalon along the floor of the 4th ventricle at the 10-mm stage. Many of the neuroepithelial cells in the Anlagen differentiate, Becoming postmitotic and axonic by the 24 mm stage. Proliferative zones of neuroepithelial cells disappear from the electric lobes by the 30-mm stage. After their initial, early differentiation the electromotor neurons remain monopolar until the 40-mm stage when dendrite formation begins. The differentiation of the electromotor neuron from a mono- to an immature multi polar form occurs between the 40- and 55-mm stages and involves, in addition to dendrite formation, a change from a pear-shaped to a spherical cell body, a dramatic increase in cytoplasmic volume, a centralization of the nucleus, an enlargement of the nucleolus and its migration away from the nuclear membrane, and differentiation of the axon hillock. The electric lobes are invaded by sinusoids at the 24-mm stage but formation of the capillary network by sprouting cords of endothelial cells begins later at the 40-mm stage. Neuronal cell death (26-74-mm stages) appears to be mainly an autolytic process and the debris is removed by immature glial cells. Afferent fiber growth cones are first recognized in the lobes at the 60-mm stage but synapses are not observed until the 78-mm stage. Myelination begins in the electric lobes concomitantly with the onset of synaptogenesis. A twofold increase in dendrite length occurs over the period when synapses begin to form in the lobes but dendritic maturation is not complete until the neonatal (120-mm) stage. The results are discussed in relation to the development of the electric organs.

The developmental morphology of Torpedo marmorata: electric lobe-electromotoneuron proliferation and cell death

Electromotoneuron proliferation and cell death have been quantitatively studied in the electric lobe of Torpedo marmorata from an embryonic body-length stage of 26-mm to adult animals. These neurons project to the electric organ and form synapses with electrocytes which possess a remarkably large postsynaptic target surface. For this reason cell death would not be predicted to occur if synaptic competition were to be hypothesized as the cause. Isolated observations at the ultrastructural level suggested, however, that cell death was indeed taking place and therefore it seemed appropriate to examine this question in detail. Our findings show first that neuron production appears to be a continuous process throughout the period studied, generating totals of over 70,000 electromotoneurons per lobe by adulthood. Second, two waves of cell death were identified, one occurring early in embryogenesis (stage 30 mm), well before the onset of synaptogenesis, and a second coincident with the onset of synaptogenesis (stages 55--74 mm). It is difficult to reconcile this latter wave with the hypothesis of synaptic competition as the postsynaptic surface at this time of development is largely devoid of synaptic contacts. We conclude that in the electromotor system of Torpedo, synaptic competition is probably not the mechanism of cell death.

Presynaptic plasma membranes and synaptic vesicles of cholinergic nerve endings demonstrated by means of specific antisera

Antisera were raised to cholinergic presynaptic plasma membranes and synaptic vesicles isolated from the electric organ of Torpedo marmorata and tested by immunochemical and immunohistochemical methods. The antisera responded to many antigens not specific to nerve endings, but it was possible to eliminate these antibodies by means of simple absorption procedures with fractions containing the unwanted antigens. After absorption, staining of thin sections of electric organ by immunofluorescence was limited to the region of nerve endings in the tissue. The remaining antibodies responded in the case of the plasma membrane antisera predominantly to a 33,000 molecular-weight polypeptide and a chloroform/methanol-soluble antigen. In cross reactivity studies it was found that this antiserum not only stains cholinergic nerve endings in Torpedo but also those in mammalian tissue. The antigen responsible for the cross reactivity is restricted to the chloroform/methanol-soluble material. The vesicle antiserum labels cholinergic nerve endings in mammalian tissue as well; the relevant antigen in this case is different from the one described above and is likely to be a glycosaminoglycan. The antisera provide valuable markers for cholinergic nerve terminals. In addition, the vesicle antiserum may now be used to study axonal transport and the life cycle of this organelle in the cholinergic neurone.

The developmental morphology of Torpedo marmorata: electric organ--electrogenic phase

The electrogenic developmental phase of the electric organ of Torpedo marmorata begins at 40 mm of embryo length and is characterized by a horizontal flattening of the vertically orientated myotubes. The first sign of this process is a rounding up of the ventral poles of the myotubes and a disassembly of the myofibrils located therein. Occurring concomitantly with this is a migration of the nuclei to the cell center which results in a horizontal plane of nuclei. Filament bundles are then found within the ventral cytoplasm often projecting upwards from the ventral plasma membrane. The filaments of the bundles are dimensionally similar to the myofilaments of muscle and it is suggested that the bundles play a role in cellular transformation. In contrast the dorsal pole of the cell appears to be integrated "passively" with the final cell shape as no morphological correlates of a retraction process have been found. A canalicular system, composed of a complex network of irregular tubules and vacuoles, appears just below the dorsal plasma membrane characterizing this region of the cell. A mononucleated satellite cell population lies in close proximity to the dorsal surface of the differentiating cell and fusion between the two cell types occurs throughout development. Cell shape transformation is complete by 55 mm of embryo length and the intercolumnar nerves begin to invade the interelectrocyte space. The ingrowing neurites preferentially course along the ventral electrocyte surface establishing junctions similar to motor endplates.

The developmental morphology of Torpedo marmorata: electric organ--myogenic phase

The early development of the electric organ of Torpedo marmorata has been examined by light and electron microscopy to the 40-mm stage of embryo growth. The myogenic nature of this tissue is confirmed ultrastructurally by the presence of myoblasts and myotubes both containing myofibrils cross striated with Z,A and I bands. Fusion between these cells is also found taking place. A scheme is presented to explain the development of the overall structural plan of the organ and specifically the formation of the future electrocyte columns. AT 40 mm, a series of morphological transformations signals the onset of a divergent developmental pattern ultimately leading to the establishment of mature electrocyte columns. These features include rounding up of myotubes, dissolution of myofibrils and the appearance of intermediate size filaments (11 nm) and perhaps a non-muscular actin (5.5 nm). This early myogenic phase of development occurs in the absence of any specific nervous contact even though electromotor nerves are always in close proximity.