Sequential alterations of neuronal architecture in nucleus magnocellularis of the developing chicken: An electron microscope study

The development of the auditory nerve endings and their target cells in nucleus magnocellularis was studied by electron microscopy of perfusion-fixed brains from embryonic day 12 to hatching. Embryonic days 12–13: somatic processes extend from the perikaryon. The cytoplasm of the soma and processes contains free ribosomes, mitochondria, lysosomes, rough endoplasmic reticulum, Golgi apparatus and an eccentric, heterochromatic nucleus. Small profiles of auditory nerve fibers containing round, clear vesicles make specialized contacts, including some synapses, on distal somatic processes but rarely on proximal somatic processes or on the soma. The postsynaptic zones contain a flocculent matrix. Days 15–17: somatic processes disappear and occasional attachment plaques are seen between cell bodies. The nucleus appears euchromatic. Cytoplasmic organelles form a dense matrix indicative of intense metabolic activity. Somatic spines are evident. The afferent axons form large, vesiculated profiles located, increasingly, on the cell body and somatic spines, with many points of synaptic contact. Opposite each ending a band of amorphous, flocculent material fills the postsynaptic cytoplasm. Embryonic day 18-hatching: the somatic cytoplasm becomes less dense; stacks of rough endoplasmic reticulum start to condense. Afferent axon terminals mature, especially the synaptic membrane complex and associated densities. The postsynaptic flocculent material diminishes in extent until it is found associated only with somatic spines.The ultrastructural observations on the maturation of nucleus magnocellularis closely corroborate and extend previous results with the Golgi methods. Developing auditory nerve fibers initially synapse on the distal parts of the somatic processes of the immature cells. As the somatic processes disappear or retract, axonal endings move to the soma and develop into large axosomatic end-bulbs. Possibly, the somatic processes as they retract drag the auditory nerve endings to the cell body. The findings also suggest a role of the transiently appearing, flocculent material of the postsynaptic regions in the formation of synapses.     

The development of innervation patterns in the avian cochlea

The sequence of developmental events leading to the innervation of the cochlea and the differentiation of its receptor cells has been studied in chick embryos with Golgi methods. We describe the morphogenesis of cochlear ganglion cell peripheral processes from their appearance in early embryos to the formation of their mature endings on hair cells in the basilar papilla (organ of Corti) of prehatching chicks. In the stage of peripheral fiber outgrowth, embryonic days 3-5, the fibers emerge from the ganglion cell bodies and grow, in a uniform fashion, toward the undifferentiated receptor epithelium of the otocyst. In the stage of the invasion of the otocyst by the peripheral fibers, embryonic days 6-7, some fibers enter the epithelium directly after reaching it, others enter after traveling some distance longitudinally beneath its basal lamina. The invading fibers appear to encounter resistance at the basal lamina, but, once within the epithelium, at embryonic days 8-9, they form a surfeit of branches in columnar zones oriented radially toward the surface. In early synaptogenesis (embryonic days 8-9) hair cells first become apparent. They differentiate from primitive epithelial cells. These cells withdraw their basal processes, which appear to accompany the growing fibers into the superficial epithelium. At embryonic days 11-13, the stage of mid-synaptogenesis, the fibers develop large, bulbous, preterminal and terminal swellings, which are located below the bases of the hair cells; the surplus branches atrophy or withdraw. Efferent axons are first seen in the epithelium at this time. In late synaptogenesis (embryonic days 14-17), the preterminal swellings disappear and the endings transform into mature foot-shapes at the bases of the hair cells. These morphological changes during the development of the peripheral endings are comparable to those of cochlear axons in nucleus magnocellularis (cochlear nucleus). During mid-synaptogenesis, when the ganglion cells develop swellings in the periphery, their central axons ramify extensively. Late in synaptogenesis, while the peripheral swellings disappear, there is a corresponding condensation of the central terminals to form the end-bulbs of Held. Thus, specific connections of the cochlear ganglion cells and their target cells in the ear and brain may result from two sequential developmental phases: (1) loosely organized and overabundant initial growth of branches from the fibers entering their target tissue; (2) reorganization of these fibers with the disappearance or resorption of the surplus branches during the transformation of their endings into mature synaptic arrangements.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neuronal architecture in nucleus magnocellularis of the chicken auditory system with observations on nucleus laminaris: A light and electron microscope study

This report presents the major structural features of neurons and their afferent input in nucleus magnocellularis, the avian homologue of the mammalian anteroventral cochlear nucleus. Results of light-microscope observations, as seen in Golgi, Nissl, and normal fiber preparations, as well as ultrastructural morphology are reported. In addition, cells and axons in nucleus laminaris, the presumed homologue of the mammalian medial superior olivary nucleus, are also described.In Golgi-impregnated material, the mature principal cell in nucleus magnocellularis has an ovoid soma encrusted with somatic spines. A dendrite, when present, emerges from the cell soma, travels for a short distance and breaks into a tuft of stubby terminal branches. Foremost among the afferents to nucleus magnocellularis are auditory nerve axons that terminate in large, axosomatic endings, or endbulbs, covering a large portion of the somatic surface. Other afferents, which also end in relation to the perikaryon, are of undetermined and perhaps multiple origins. The neurons resemble the bushy cells of the mammalian anteroventral cochlear nucleus. Evidence is presented that individual axons from the nucleus magnocellularis bifurcate and send branches to the nucleus laminaris bilaterally, thus placing constraints on the binaural interactions possibly involved in lateralization functions.In electron micrographs, the end-bulbs appear as large, elongate structures which can cover a third of the cell soma. Multiple sites of synaptic specialization occur along these terminals. The synaptic membrane complexes may form directly on the cell body or on the sides or crests of somatic spines. These complexes are characterized by asymmetric membrane densities with a cluster of clear, spherical vesicles on the axonal side. Other small terminal profiles are also present on the somata receiving the end-bulbs. Dendritic profiles are scarce, in agreement with observations in Golgi impregnations.The structural findings indicate that the medial part of the nucleus magnocellularis is homologous to the anterior part of the mammalian anteroventral cochlear nucleus in that the neurons of nucleus magnocellularis are homologous to the bushy cells of the cat. On this basis, the cells in nucleus magnocellularis could faithfully preserve the acoustic response patterns generated in the auditory nerve. This should, in turn, allow a secure relay of bilateral latency differences essential for binaural interactions in the nucleus laminaris.     

Dual populations of efferent and afferent cochlear axons in the chicken

Cell bodies, dendrites, and axons associated with the cochlear nerve were labelled by injections of horeseradish peroxidase into the inner ear of the chicken. Efferent cochlear neurons, labelled by retrograde axonal transport, were located bilaterally in the ventral medulla, lateral to the abducens nerve root and ventromedial to the superior olive. These cells are characterized by radiate or elongate dendritic morphology. The axons of the efferent neurons extend dorsally adjacent to the abducens nerve root. Those from the contralateral side cross beneath the ventricle and exit, with the ipsilateral efferent fibers, through the vestibular nerve. In the basilar papilla thin, varicose efferent fibers form large, claw-shaped endings on short hair cells and small boutons terminaux on tall hair cells. Afferent components of the cochlear nerve were labelled by anterograde transport. Diffusely-filled cochlear afferents bifurcate in the dorsolateral medulla and send branches to the cochlear nuclei, nucleus angularis and nucleus magnocellularis. In nucleus magnocellularis the primary afferents are of two types. Most are thick and smooth and terminate as end-bulbs of Held; other, relatively thin axons have preterminal and terminal enlargements.We conclude that the brain stem and peripheral auditory system of the bird provide an opportune model for hearing research, which can be compared with that of the mammal. This conclusion is based on the following similarities between the avian and mammalian auditory systems. (1) There are two different types of hair cells with different patterns of efferent and afferent innervation. (2) There are crossed and uncrossed efferent cochlear bundles. (3) There are at least two morphologically distinct types of cochlear efferent neurons and at least two different kinds of afferents.     

Dendrodendritic synapses of cells that have axons: The fine structure of the Golgi type II cell in the medial geniculate body of the cat

Summary This study provides a combined analysis with the Golgi method and electron microscopy of the Golgi type II cells of the thalamus in the cat. In the ventral nucleus of the medial geniculate body these cells constitute a large, morphologically homogeneous population of neurons. They are clearly distinguished from the thalamo-cortical neurons by their size, shape, kinds of dendritic appendages, and cytoplasmic structure. The axon of the Golgi type II cell is exceptionally short and forms a small number of lumpy endings in the vicinity of its origin. The dendrites are often longer and much more elaborately branched than the axon. The shafts of these dendrites bear spiculated appendages, while the distal ends of the dendrites form clusters of very large endings. The appendages and terminal clusters participate in the nests of axonal endings formed by the afferent auditory axons and the dendritic branches of thalamo-cortical neurons. These axonal nests are the synaptic nests observed in electron micrographs. Within the synaptic nests the endings of Golgi type II neurons form dendrodendritic synapses on the dendrites of the thalamocortical neurons. The dendritic endings of Golgi type II neurons also receive synapses from the afferent axons. The dendrodendritic synapses may involve the Golgi type II neurons in an inhibitory role in the thalamo-cortical transformation of auditory signals. The dendrodendritic endings of the Golgi type II neurons continue to grow in the adult cat. Possibly these cells are involved in the evolution of cortical functions and in the plastic changes of neural activities that modify behavior.Supported by U. S. Public Health Service Research Grant NS 06115 and GRS Grant 5S01FR05381-08 to Harvard University.     

The mormyrid brainstem--II. The medullary electromotor relay nucleus: an ultrastructural horseradish peroxidase study

The medullary relay nucleus of the mormyrid weakly electric fish Gnathonemus petersii is a stage in the command pathway for the electric organ discharge. It receives input from the presumed command or pacemaker nucleus and projects to the electromotoneurons in the spinal cord. Its fine structure and synaptology were investigated by electron microscopy. The origin of the terminals contacting the cell membrane of the neurons of this nucleus was determined by horseradish peroxidase (HRP) injections into different brain structures, namely into the bulbar command- and mesencephalic command-associated nuclei. Twenty-five to thirty large cells of about 45 micron in diameter constitute the medullary electromotor relay. Each cell has a kidney-shaped, lobulated nucleus, a large myelinated axon with a short initial segment and several long, richly arborizing primary dendrites. Many, if not all, cells are interconnected with large somatosomatic or dendrosomatic, dendrodendritic and dendroaxonic gap junctions. These junctions often occur in serial or triadic arrangements. The relay cells receive large club endings as well as small boutons. The club endings are found mainly on the soma and primary dendrites and are morphologically mixed synapses. The boutons are characterized by synapses which are only chemical and are distributed all over the cell membrane, but with a definitely higher frequency on secondary dendrites and more distal parts of dendritic processes. Horseradish peroxidase injections into the mesencephalic command-associated nucleus reveal a large number of labelled boutons on the secondary dendrites of the relay cells. Injections into the bulbar command-associated nucleus label the same type of boutons as mesencephalic injections, but also label club endings on relay cell soma and primary dendrites. The results support the conclusion made on the basis of previous light microscopical observations that boutons originate from the bulbar command-associated nucleus, whereas the club endings issue from the presumed pacemaker nucleus (nucleus c). The club endings of the bifurcating axons of this nucleus are labelled by retro- and anterograde transport of horseradish peroxidase; the bifurcating axons project simultaneously to the bulbar command-associated nucleus and the medullary relay nucleus.     

The neuronal architecture and topography of the nucleus vestibularis tangentialis in the late chick embryo

The tangential nucleus is a group of secondary vestibular neurons embedded among the fibers of the vestibular nerve root as it first enters the lateral margin of the medulla oblongata. This paper describes the chief morphological features of the neurons and the endings made by afferent axons in rapid Golgi, reduced silver, and Nissl preparations of the tangential nucleus of the chicken. The topography of the tangential nucleus is presented in a series of transverse sections from a reduced silver preparation.The morphological features and topography of the neuronal architecture are well defined in the 18–19 day chick embryo (Hamburger-Hamilton Stages 44–45), in which they provide a useful guide for both embryological investigations and studies of the neuronal architecture in this nucleus. Three types of neurons are defined: the principal cell, comprising 80% of the neurons, the elongate cell, about 20%, and the giant cell, less than 1% of the neuronal population. The principal cell is characterized by an oval body, eccentric nucleus, horizontal dendrites elongated parallel to the incoming vestibular fibers, and short vertical dendrites. Because of their arrangement with respect to the incoming vestibular fibers, the principal cells are in a position to establish connections with select groups of ganglion cells which innervate discrete portions of the vestibular end-organ. This arrangement could preserve the topographical organization of the vestibular ganglion within the brain. Such a pattern is best demonstrated by the connections of the principal cells with the colossal fibers. These fibers form a distinct population consisting of the thickest vestibular nerve axons, which arise from cells in the vestibular ganglion that peripherally innervate small groups of hair cells by means of large calyces in the cristae ampullares of the semicircular canals. Within the tangential nucleus, the colossal fibers form large ‘spoon’ endings en passant. These endings are spoon-shaped enlargements with digitiform appendages that make axosomatic synapses on the principal cells only. Each colossal fiber forms only one spoon ending, which contacts only one cell body; each principal cell body receives only one spoon. This degree of specificity and the morphology of the colossal fiber-principal cell connection provide the basis for a highly specialized, powerful synapse that may subserve important reflex adjustments. The elongate cell is characterized by a smaller, pyramidal body, a central nucleus, and vertical dendrites which are greatly elongated so as to intercept a wide sector of the vestibular nerve root and which consequently can establish connections with many fine fibers innervating widespread regions of the vestibular end-organ. The giant cell is a large, multipolar neuron with very thick, long, branching dendrites and elongated somatic and dendritic appendages. Fine afferent axons of vestibular and nonvestibular origins probably end in relation to all three types of neurons.The neuronal architecture and topography of the chick tangential nucleus offer some unusual opportunities for morphological and experimental studies of neurogcnesis.     

Morphology of migrating trigeminal motor neuroblasts as revealed by horseradish peroxidase retrograde labeling techniques

The trigeminal motor sensory roots were severed in chick embryos on days 2.5 4.5 of incubation and horseradish peroxidase applied to the wound. This procedure retrogradely labels developing trigeminal motor neuroblasts whose axons are at the level of the incision. In day 2.5 embryos, migrating and lateral trigeminal motor neuroblasts were labeled only when the incision was 8 μm from the metencephalon. Migrating cells did not have somatic processes whereas cells of the lateral nucleus had one dendritc-like process extending dorsally. No cells of the medial column, a cluster of premigratory trigeminal motor neuroblasts, were labeled at this age.In the 3-, 3.5-, 4- and 4.5-day embryos, medial column cells, migratory cells and lateral nucleus cells were retrogradely labeled by this procedure. At all these ages, medial column cells tend to have few somatic filopodia migratory cells tend to have increased filopodia as they proceed laterally, and lateral nucleus cells arc characterized either by a single, long dendrite-like process dorsally directed, or a radiation of short, stubby processes. Axons of medial column and migratory cells take a sinuous course across the metencephalon and frequently exhibit small branches and localized swellings. Axons of lateral nucleus cells rarely have branches within the brain stem and usually enter the motor root at an acute angle from their origin at the soma.The central processes of the trigeminal ganglion cells are also labeled with this procedure. In all embryos these fibers were confined to the trigeminal spinal tract at the level of the trigeminal motor nucleus. Caudally, small fibers were observed to exit this tract in the presumed region of the developing trigeminal spinal nucleus.This study demonstrates that axonal outgrowth into the periphery precedes somatic migration and translocation in the trigeminal motor nucleus During their migration, trigeminal motor neuroblasts appear to be in proximity to axons of adjacent migrating cells. Considerable differentiation occurs during this process.     

Morphological changes in the hypothalamic arcuate nucleus and median eminence in the golden hamster during the neonatal period

The purpose of the present investigation was to study the ultrastructure of the arcuate nucleus (ARC) and median eminence of hamsters on days 1–15 of the neonatal period. From days 1–6, the neurons of the ARC had large nuclei and a small amount of cytoplasm which contained polysomes, mitochondria, RER, lysosomes and Golgi complexes. From days 7–15 there was an increase in the amount of cytoplasm as well as more extensive Golgi complexes and RER. Astrocytes were the predominant glial component in both the ARC and median eminence. Astrocytic processes were in juxtaposition to unmyelinated axons, dendrites, and synapses. Axodendritic and axosomatic synapses containing clear vesicles were observed in the neuropil on day 1. There was an increase in the number of dense-core vesicles in the axonal endings beginning on day 4. Concomitantly, there were increasing numbers of clear and dense-core vesicles (64–70 nm) in terminals of the external layer of the median eminence, whereas larger dense-core vesicles (105–140 nm) were distinguishable by day 10 immediately dorsal to the external layer. The capillaries of the median eminence were composed of nonfenestrated endothelium from days 1–9. Fenestrae began to appear about day 10. Ependymal cells lining the third ventricle had pinocytotic vesicles, microvilli, and bleb-like protrusions on their apical surfaces. Ependymal processes were adjacent to nerve processes in the neuropil of the ARC and in the external layer of the median eminence, where they contacted the perivascular space. Two types of supraependymal cells were seen in animals throughout the neonatal period. One resembled a neuron which sent processes along the ependymal surface and often between cells. The second type was similar to a macrophage. The results of this study demonstrate the maturation of the neural elements in the ARC/median eminence area of the neonatal hamster.     

Development of the octopus cell area in the cat ventral cochlear nucleus

The octopus cell area (OCA) of the posteroventral cochlear nucleus was studied electron microscopically in kittens. The adult OCA, a region of morphologically homogenous neurons receiving heterotypic synapses from the cochlea, was used to define the mature state. The OCA reaches cytological maturity at three weeks postnatally, after progression through four stages, defined on the basis of octopus cell cytology (including relative numbers of somatic and dendritic filopodia and spines) and the frequency, ultrastructure and location of previously defined synaptic terminals. Octopus cell size was also studied in rapid Golgi impregnations. The OCA from birth through three postnatal days (stage 1) showed small neurons, few identifiable synaptic types, small, mostly unmyelinated axons, mitotic cells and undifferentiated glia. Between the fourth and seventh postnatal days (stage 2) distinct type 1 and type 2 endings appeared and dendrites thickened, expanded peripherally and developed mature spines. During stage 3 (8–19 days) loss of filopodia, increased somatic spicules, larger somas and clearer differentiation of type 1 and type 2 synapses occurred. After three postnatal weeks (stage 4) the OCA contained morphologically mature octopus cell somas, all three synaptic types ending upon somas and thick basal dendrites, and fascicles of myelinated fibers. Although cytologically mature, the OCA at this stage (about 20–35 days) is substantially smaller than the adult OCA. This smaller size will facilitate further study of OCA synaptic organization.     

Differentiation of granule cells in relation to GABAergic neurons in the rat fascia dentata

Summary Golgi impregnation was used to study the dendritic differentiation of granule cells in the rat fascia dentata. The impregnated granule cells were gold-toned allowing for a fine structural study of the same identified neurons and of the input synapses onto their cell bodies and dendrites. Due to the long postnatal formation of these cells it was possible to describe a sequence of maturational stages coexisting on the same postnatal day (P5). Characteristic features of the dendritic development of granule cells were i) occurrence of varicose swellings along the dendrites, ii) growth cones on dendritic tips, iii) transient formation of basal dendrites, and iv) progressive development of dendritic spines. Incoming synapses on the differentiating granule cells were mainly found on dendritic shafts. Their membrane specializations were symmetric. At least some of these symmetric synapses were GABAergic because immunostaining of Vibratome sections from the same postnatal stage (P5) demonstrated a well-developed GABAergic axon plexus in the fascia dentata (antibodies against glutamate decarboxylase (GAD), the GABA synthesizing enzyme). Electron microscopy of the immunostained axon plexus revealed numerous GABAergic terminals that formed symmetric synaptic contacts, mainly on shafts of differentiating dendrites but also on cell bodies of granule cells. Our results thus indicate that the plexus of inhibitory GABAergic axons is already well developed at a stage when the target neurons, the granule cells, are still being formed.     

Neurogenesis in the nucleus vestibularis tangentialis of the chick embryo in the absence of the primary afferent fibers

The migration, differentiation, and subsequent fate of the principal neurons of the tangential nucleus have been studied in the absence of the colossal fibers of the vestibular nerve in the chick embryo. Normally, the colossal fibers form large axosomatic synapses on the principal cells by means of the highly specific spoon endings. In this study the development of the vestibular nerve was prevented by unilateral otocyst ablations in 2-day old embryos, which were allowed to survive for varying lengths of time up to 13 days of incubation. At each developmental stage, the principal cells of the tangential nucleus on the operated side were compared with those on the unoperated sides and with those found in normal embryos in both rapid Golgi and reduced silver preparations. In the absence of vestibular input, principal cell neuroblasts can migrate and begin to differentiate up to the time when colossal fibers in the tangential nucleus would usually start to form swellings. Thereafter, the partly differentiated principal cells degenerate.Evidently, migration and the initial differentiation of the axon, cell body, and dendrites do not depend on the primary afferent axons, whereas completion of differentiation, continuation of growth, and cell survival do. The role of non-vestibular inputs in the development of the tangential nucleus remains uncertain. Nevertheless, it is clear that formation of many of the morphological features that distinguish principal cells as a distinct neuronal type does not depend on trophic influences or synaptic activity of the vestibular afferent fibers. These fibers, however, apparently do exert trophic effects on the later stages of neuronal development. Some of these effects must precede the formation of structurally defined synapses by the spoon endings.     

A morphological study of neurogenesis in the nucleus vestibularis tangentialis of the chick embryo

This study is a light microscopic investigation of the normal pattern of morphogenesis in the chick tangential nucleus. Rapid Golgi and reduced silver methods have been used to reconstruct the sequences of morphological changes in the developing neurons and colossal-fiber endings of the vestibular nerve in a staged series of chick embryos. The distinctive morphology of the principal and elongate cells and of the colossal fibers and their spoon endings permits the rigorous identification of these structures from very early stages of development. Three stages are discussed: the cell migration stage from 4½ to 6½ days (Hamburger-Hamilton, Stages 25–29), the young neuron stage from 6½ to 8 days (Stages 30–34), and the stage of spoon formation from 8½ to 13 days (Stages 35–39).In the cell migration stage, although the vestibular nerve fibers have already grown into the medulla, no cell bodies appear within the presumptive tangential nucleus in embryos younger than 6 days (Stage 29), nor are the prospective colossal fibers distinguishable by their axonal diameters. The presumptive tangential nucleus contains the external processes of primitive epithelial cells which extend from the matrix zone to the external limiting layer; these cells in 6–7 day (Stage 30) embryos give rise to the migratory neuroblasts which appear in the mantle zone medial to, and also within, the primordial tangential nucleus. The perikarya of the migratory neuroblasts move through the primitive external processes to the presumptive tangential nucleus, while their primitive internal processes withdraw from the matrix zone and reduce to shorter trailing processes.In the young neuron stage the axon of the principal cell emerges from the primitive internal process at a point far medial to the perikaryon; this outgrowth can occur even before the translocation of the nucleus is completed. The colossal fibers are first distinguishable by their greatly enlarged diameters. During the rest of this stage, the post-migratory neuroblasts lose their primitive processes and begin to form dendrites that continue to develop during the next stage.At the beginning of the stage of spoon formation, each colossal fiber forms a characteristic swelling en passant within the tangential nucleus. By 10 days (Stage 36) a sheet-like outgrowth appears just medial to the swelling, which soon disappears as this outgrowth continues to expand over the principal cell soma to form a young spoon ending.It is concluded that the principal cells undergo a specific, time-locked sequence of structural changes in their development, which proceeds in advance of an equally strict sequence of events in the differentiation of the spoon endings that contact these cells. These findings indicate that the ingrowth of the colossal fibers does not initiate cell migration and differentiation in the tangential nucleus, although the fibers and spoon endings could influence subsequent cell development and maintenance. Perhaps the formation of spoon endings depends on migration and differentiation of the principal cells.     

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: horseradish peroxidase labelling of identified cell types

Golgi impregnations of the posterior part of the cat's anteroventral cochlear nucleus have revealed two types of neurons, bushy cells with short bush-like dendrites and stellate cells with long, tapered processes; Nissl stains have revealed globular and multipolar cell bodies with dispersed and clumped ribosomal patterns, respectively. In the present study, we injected horseradish peroxidase into the trapezoid body. Ipsilaterally, retrograde, diffuse labelling of neurons, presumably through damaged fibers, yielded Golgi-like profiles of numerous bushy cells with typical dendrites and with thick axons projecting toward the trapezoid body. Stellate cells were almost never labelled in this way. Anterograde diffuse labelling of thick axons demonstrated calyx endings in the contralateral medial nucleus of the trapezoid body. In the electron-microscope, the perikarya of diffusely-filled bushy neurons were found to have the dispersed ribosomal pattern and the kinds of synaptic endings typical of globular cells, including large profiles of end-bulbs from cochlear nerve axons. After injections restricted to the medial trapezoid nucleus, granularly-labelled cells in the cochlear nucleus were almost completely confined to the contralateral side; Nissl counterstaining showed them to be globular cells in the posterior part of the anteroventral cochlear nucleus. After larger injections, involving surrounding regions of the superior olivary complex, granular labelling occurred throughout the ventral cochlear nucleus on both sides. There is also evidence that stellate cells in Golgi impregnations correspond to multipolar cell bodies in Nissl stains. We conclude that bushy cells typically correspond to globular cells, which receive end-bulbs from the cochlea and send thick axons to the contralateral medial trapezoid nucleus, where they form calyces on principal cells. Principal cells, in turn, are known to project to the lateral superior olive and to one of the nuclei of origin of the crossed olivo-cochlear bundle, which feeds back to the cochlea. In this circuit, correlations between synaptic patterns and particular physiological signal transfer characteristics can be suggested. These could be related to binaural intensity interactions in the lateral superior olive and to a regulatory loop involving the olivo-cochlear bundles.     

The effects of sound overexposure on the spectral response patterns of nucleus magnocellularis in the neonatal chick

Spectral response plots from single cells in the chick nucleus magnocellularis were obtained following a 48 h exposure to a 0.9 kHz pure tone at 120 dB sound pressure level and after a recovery period of 12 days. Immediately after removal from the exposure, a variety of changes in the spectral response patterns of nucleus magnocellularis cells were noted. In particular, at recovery day 0, there was a significant decrease in spontaneous rate; a substantial loss of threshold sensitivity and frequency selectivity; and alterations in both the slope and dynamic range of the rate-intensity function. Interestingly, the maximum discharge rate appeared unaffected by the intense sound. Twelve days after removal from overstimulation, auditory function appeared to be operating at normal levels of efficacy as spontaneous rate, threshold sensitivity, frequency selectivity, and the dynamic range of the rate-intensity function were statistically identical to similar measures in nonexposed control birds. The loss and restoration of auditory function is correlated to structural damage and repair of the avian basilar papilla.     

The growth of cochlear fibers and the formation of their synaptic endings in the avian inner ear: a study with the electron microscope

The developmental sequence of nerve-epithelial cell contacts, leading up to the formation of the mature receptoneuronal synapse, has been studied in the basilar papilla of chick embryos with electron microscopy. The receptor epithelium before innervation, on embryonic days 3-4, consists of a homogeneous population of primitive cells; hair cells and supporting cells cannot be distinguished. During innervation of the epithelium (embryonic days 5-7), the invading peripheral fibers of cochlear ganglion cells penetrate the basal lamina and form nerve-epithelial attachments with the epithelial cell bases. Once within the epithelium some fibers turn and spread in the transverse dimension across the basilar papilla through channels formed between the basal epithelial processes. Subsequently, nerve-epithelial attachments are observed more superficially within the epithelium. Hair cells and supporting cells differentiate during early synaptogenesis (embryonic days 8-9). Receptoneural synapses, possibly derived from the nerve-epithelial attachments formed during the innervation stage, are first seen during this period. They are characterized by symmetrical or asymmetrical membrane densities, separated by a cleft containing a dense material. At many of these junctions synaptic bodies, as well as dense-cored and coated vesicles, gather in the hair cells. During mid-synaptogenesis (embryonic days 11-13) the hair cells proliferate synaptic bodies, many of which are not located at receptoneural junctions. The preterminal portions of the sensory endings form large swellings, containing flocculent material, endoplasmic reticulum and vesicles. Late in synaptogenesis (embryonic days 15-17) the swellings disappear, while synaptic endings are transformed to foot-shaped terminals. In the hair cells, synaptic bodies not associated with junctions disappear. Efferent synapses are first seen during this period. This sequence of ultrastructural changes, which the developing sensory nerve endings and their target cells undergo in parallel, can be correlated with observations of Golgi preparations from a companion study. These correlations suggest that the innervation of the cochlea involves the following developmental processes. Initially the peripheral fibers of the ganglion cells grow directly toward the otocyst in fascicles. Having reached the base of the primitive receptor epithelium, the axonal endings, including some with growth cones, encounter a barrier in the basal lamina. When they enter some of the fibers attach to the basal end-feet of the primitive epithelial cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Immunocytochemical, Golgi and electron microscopic characterization of putative dendrites in the ventral glial lamina of the rat supraoptic nucleus

Processes of magnocellular neurosecretory cells in the rat supraoptic nucleus which project along the pial surface in the ventral glial lamina were investigated using immunocytochemistry, Golgi stains and electron microscopy. Immunocytochemical studies revealed that although both oxytocin- and vasopressin-containing processes were evident in the ventral glial lamina, vasopressin-containing processes predominated. Ventral processes were thicker and of a different morphology than dorsal axon-like processes which joined the hypothalamo-neurohypophysial tract and exhibited large varicosities along their length or at their apparent termination. Golgi stains revealed that classically defined dendrites of supraoptic neurons projected primarily ventrally and often invaded the ventral glial lamina. No axons were traced to the lamina. Ultrastructurally, processes within the ventral glial lamina characterized as dendrites could be stained immunocytochemically for neurophysin and were post-synaptic to a variety of presynaptic elements. The results suggest that many dendrites from magnocellular neurosecretory cells in the supraoptic nucleus project to the ventral glial lamina and form a restricted, receptive plexus. The previously demonstrated coexistence of catecholamine-containing varicosities and other axon types with these processes in the lamina indicates an important role for supraoptic dendrites in integrating a wide variety of information relevant to neurohypophysial hormone release.     

Projections from auditory cortex to cochlear nucleus: A comparative analysis of rat and mouse

Mammalian hearing is a complex special sense that involves detection, localization, and identification of the auditory stimulus. The cerebral cortex may subserve higher auditory processes by providing direct modulatory cortical projections to the auditory brainstem. To support the hypothesis that corticofugal projections are a conserved feature in the mammalian brain, this article reviews features of the rat corticofugal pathway and presents new data supporting the presence of similar projections in the mouse. The mouse auditory cortex was localized with electrophysiological recording and neuronal tracers were injected into AI. The cochlear nucleus was dissected and examined for terminal fibers by light and electron microscopy. Bouton endings were found bilaterally forming synapses with dendrites of granule cells of the cochlear nucleus. This report provides evidence for direct auditory cortex projections to the cochlear nucleus in the mouse. The distribution of projections to the granule cell domain and the synapses onto granule cell dendrites are consistent with what has been reported for rats and guinea pigs. These findings suggest a general plan for corticofugal modulation of ascending auditory information in mammals. Corticobulbar inputs to the auditory brainstem likely provided a survival advantage by improving sound detection and identification, thus allowing the development of complex social behaviors and the navigation of varied environments.     

The fine structure of nerve endings in the nucleus of the trapezoid body and the ventral cochlear nucleus

Large, calyciform axonal endings, as well as typical terminal boutons, have been previously described in the ventral cochlear nucleus and the nucleus of the trapezoid body by light microscopists. In the present study, these endings were examined with the electron microscope in chinchillas, rats, and a cat after perfusion fixation with osmium tetroxide. The calyces were found to consist of elongated processes arising from myelinated axons and making multiple synaptic contacts with perikarya and dendrites. This finding suggests that an important property of calyces is the large amount of synaptic activity that they can bring to bear on a single post-synaptic structure. Adjacent to the calyciform endings were variable numbers of boutons that made synaptic contacts with the same perikarya and dendrites. The majority of boutons contained smaller synaptic vesicles than those present in the calyces, implying both anatomical and functional differences between these two types of ending. It is suggested that many of these boutons in the ventral cochlear nucleus are endings of centrifugal, inhibitory fibers described by previous workers.     

Synaptology of the command (pacemaker) nucleus in the brain of the weakly electric fish, Sternarchus (Apteronotus) albifrons

The high frequency electric emission of the weakly electric fish Sternarchus (Apteronotus) albifrons depends on the pacemaker activity of a specific brainstem nucleus located in the ventral part of the rhombencephalic reticular formation. The general morphology and fine structure of this nucleus has been investigated, with particular reference to its synaptic connections.Three neuronal components could be distinguished in the nucleus; namely large cells of 80–100 μm diameter, small cells of 30–50 μm diameter and bundles of thin, myelinated fibres. These elements are embedded in a network of thick myelinated fibres. The large cells have a few small and short dendrites whereas the small neurons have long branching dendrites. Large and small neurons possess thick myelinated axons, but only those of the latter show branching patterns and send collaterals which have intranuclear courses only. Two types of synaptic terminals have been found on both neurons: large club endings exclusively with gap junctions and small bouton-like terminals with polarized chemical synapses. Serial semi-thin and ultra-thin sections revealed that the large club endings belong to the pacemaker cells, whereas the small terminals are found in the thin myelinated axons of extranuclear origin.The findings indicate that the small neurons are connected 1) to each other and 2) to the large neurons, by way of their large myelinated axons. Both, small (pacemaker) as well as large (relay), neurons receive chemical synapses from myelinated fine fibers probably originating from higher encephalic centers. Thus, electric organ discharge rhythm can be modulated at the level of pacemaker as well as of the relay cells. No somatosomatic, dendrodendritic or dendrosomatic connections were found between large, small or large and small cells.