D.K. Morest

Sequential alterations of neuronal architecture in nucleus magnocellularis of the developing chicken: A golgi study

Nucleus magnocellularis in the chicken consists predominantly of a population of medium-sized cells which receive large, axosomatic endings from the auditory nerve. The morphological development of these cells and their auditory input were studied with the Golgi methods. At 7 1/2-9 days of incubation (embryonic days 7 1/2-9, staged according to the Hamburger-Hamilton series), cells in nucleus magnocellularis have several long, branched dendrites, which often end in bulbous swellings. By embryonic day 10, efferent axons have already grown out from the cells and characteristic terminal plexuses of these axons are seen in nucleus laminaris bilaterally. The dendrites of cells in nucleus magnocellularis have been replaced by a multitude of long somatic processes, giving the cell body a shaggy appearance. This arrangement is maintained up to embryonic day 15, when a remarkable second transformation occurs. The cells lose their somatic processes and present bald, round profiles. Around embryonic days 17-18 a primitive-looking process with a tip like a growth cone emerges from the cell body and somatic spines are evident. By days 19-20, one or two thin, frail dendritic processes can be seen. Correlated with this dramatic series of changes in the cells is a fixed sequence of transformations of the incoming axons. Around embryonic day 10, primary sensory axons in nucleus magnocellularis end in swellings resembling growth cones. Between days 11 and 13, following the explosive growth of somatic processes there is a corresponding expansion and ramification of the auditory nerve endings. On embryonic day 14, there is a condensation of the terminal axon branches, which now form a compact, highly branched plexus. Between days 16 and 17, the plexus coalesces into a calycine structure, now approaching its final form, the end-bulb of Held, which is achieved by embryonic days 19-20. The transformation of the plexus to the calycine form occurs around the same that the cell loses its somatic processes. The parallel sequence in the morphogenetic stages of the assembly of the end-bulbs and their target cells evinces a correlation, if not a causal relationship between the sensory axons and the developing neurons. The arrangement of the somatic processes and axonal branches during the early, multipolar stage would provide an opportunity for optimum interactions between the synaptogenetic processes of the afferent axons and the target cells. The later morphological transformations could orchestrate the specific, cell-to-cell interactions which accompany, or even depend on the activity of the definitive end-bulb synapse.

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 cats 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 bushy cells in the anteroventral cochlear nucleus of the cat. A study with the electron microscope

Abstract The bushy cells in the anterior division of the anteroventral cochlear nucleus of the cat were studied with the electron microscope. In the anterior part of the anterior division, profiles of bushy cells and their processes are easily identified, since few cells of other types are found in this region. In the posterior and posterodorsal parts of the anterior division, the bushy cells are intermingled with stellate and small cells but can be identified on the basis of light-microscopic descriptions and comparisons with the results from the anterior part. Bushy cells are large, spherical cells with a centrally located nucleus enveloped by sheets of rough endoplasmic reticulum. Thin proximal dendrites jut abruptly from the cell body and contain a relatively pale cytoplasm. The distal dendrites contain few organelles other than numerous, very large mitochondria. The cell soma and proximal dendrites, as well as the axon hillock, receive numerous synaptic terminals, but the distal dendritic processes are contacted by relatively few endings. At least four types of terminals form synaptic contacts with the bushy cells. Very large terminals, containing large, spherical synaptic vesicles and forming multiple asymmetrical contacts, correspond to the end-bulbs of Held from the cochlea. These terminals disappear after cochlear ablations, but the other three types remain. The most numerous of these is a large terminal that contains flattened synaptic vesicles and forms long, nearly symmetrical contacts with the soma and dendrites of bushy cells. The second type of non-cochlear terminal is smaller and contains small, pleiomorphic synaptic vesicles that are not flattened. The third type occurs mainly on bushy cell dendrites, contains small, spherical synaptic vesicles, and forms moderately asymmetrical contacts. The bushy cells probably correspond to the primarylike units described in electrophysiological studies of the anterior division. Primarylike units respond to activity in auditory nerve fibers in a one-to-one manner, a finding compatible with the observation that much of the surface of the soma and dendrites of the bushy cells is contacted by auditory nerve terminals (end-bulbs of Held). Neither the origins nor the functions of the several types of non-cochlear inputs to the bushy cells are known. Further analysis of these inputs and of the other neuronal types in the anterior division, when correlated with physiological and biochemical data from the same cell types, could clarify the functional significance of the observed patterns of synaptic organization.

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 end-bulbs, 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.

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: Electron microscopy

We have studied the posterior division of the anteroventral cochlear nucleus, where the cochlear nerve root enters the brain, in the cat. In Nissl preparations, this region contains two types of neuronal cell bodies: globular and multipolar. The two types can be identified in the electron-microscope by comparing Nissl substance and rough endoplasmic reticulum. Globular cell bodies receive many synaptic terminals, which cover 85% of the surface. In contrast, multipolar cell bodies are almost entirely wrapped by thin glial sheets--synaptic terminals contact less than 15% of the surface and tend to cluster at the bases of dendrites. Synaptic terminals are of three kinds, types 1, 2, and 3, which contain large round, small round-to-oval, and small flattened synaptic vesicles, respectively. Terminals of all three kinds synapse on both types of cell bodies. However, only globular cell bodies receive the largest type 1 terminals, which correspond to end-bulbs, seen in Golgi impregnations to arise from cochlear nerve axons. Cochlear ablation leads to degeneration of type 1, but not type 2 or 3 terminals. We conclude that neurons with globular cell bodies receive heavy somatic input from the cochlear nerve, as well as from other sources. Neurons with multipolar cell bodies receive very little input to their perikarya--giving their dendrites a more important role in determining their response properties. We suggest a morphological basis for correlating individual kinds of neurons with certain electrophysiological response types.

Organization of the neurons in the anterior division of the anteroventral cochlear nucleus of the cat. Light-microscopic observations

Abstract The anterior division of the anteroventral cochlear nucleus of the cat was studied in the light microscope. Criteria were developed to distinguish neurons in the Nissl-stained anteroventral cochlear nucleus which could then be correlated with those types found in Golgi preparations. Based on the patterns of distribution of the neuronal types, their size and shape, and the number of primary dendrites, the bushy cells (Golgi) are shown to correspond to the spherical cells (Nissl), whereas the stellate and small cells (Golgi) correspond to the ovoid cells (Nissl). The present results provide a background for a detailed study of the synaptic organization of the different cell types of the anterior division with the electron microscope and electrophysiological methods.

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: Golgi and Nissl methods

This report characterizes the cells and fibers in one part of the cochlear nucleus, the posterior division of the anteroventral cochlear nucleus. This includes the region where the cochlear nerve root enters the brain and begins to form endings. Nissl stains reveal the somata of globular cells with dispersed Nissl substance and those of multipolar cells with coarse, clumped Nissl bodies. Both parts of the posterior division contain cells with each Nissl pattern, but in different relative numbers and locations. Golgi impregnations demonstrate two types of neurons: bushy cells, with short bush-like dendrites, and stellate and elongate cells, with long tapered dendrites. Several varieties of bushy cells, differing in the morphology of the cell body and in the size and extent of the dendritic field, can be distinguished. Comparison of the distributions of these cell types, as well as cellular morphology, suggest that the globular cells recognized in Nissl stains correspond to bushy neurons, while the multipolar cells correspond to stellate and elongate neurons. Golgi impregnations reveal large end-bulbs and smaller boutons from cochlear nerve fibers, as well as boutons from other, unidentified sources, ending in this region. The particular arrangements of the dendritic fields of the different cell types and the axonal endings associated with them indicate that these neurons must have different physiological properties, since they define different domains with respect to the cochlear and non-cochlear inputs.

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

Abstract 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.

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

Abstract 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.