Growth, differentiation and ribonucleic acid content of the fetal rat spinal ganglion cells in culture

Connectome gradient dysfunction in major depression and its association with gene expression profiles and treatment outcomes

Patients with major depressive disorder (MDD) exhibit concurrent deficits in both sensory and higher-order cognitive processing. Connectome studies have suggested a principal primary-to-transmodal gradient in functional brain networks, supporting the spectrum from sensation to cognition. However, whether this gradient structure is disrupted in patients with MDD and how this disruption associates with gene expression profiles and treatment outcome remain unknown. Using a large cohort of resting-state fMRI data from 2227 participants (1148 MDD patients and 1079 healthy controls) recruited at nine sites, we investigated MDD-related alterations in the principal connectome gradient. We further used Neurosynth, postmortem gene expression, and an 8-week antidepressant treatment (20 MDD patients) data to assess the meta-analytic cognitive functions, transcriptional profiles, and treatment outcomes related to MDD gradient alterations, respectively. Relative to the controls, MDD patients exhibited global topographic alterations in the principal primary-to-transmodal gradient, including reduced explanation ratio, gradient range, and gradient variation (Cohen's d = 0.16-0.21), and focal alterations mainly in the primary and transmodal systems (d = 0.18-0.25). These gradient alterations were significantly correlated with meta-analytic terms involving sensory processing and higher-order cognition. The transcriptional profiles explained 53.9% variance of the altered gradient pattern, with the most correlated genes enriched in transsynaptic signaling and calcium ion binding. The baseline gradient maps of patients significantly predicted symptomatic improvement after treatment. These results highlight the connectome gradient dysfunction in MDD and its linkage with gene expression profiles and clinical management, providing insight into the neurobiological underpinnings and potential biomarkers for treatment evaluation in this disorder.

The astroglial cell that guides nerve fibers from growth cone to synapse in organotypic cultures of the fetal mouse spinal cord

We present electron microscopic and autoradiographic studies done using organotypic cultures of spinal cord explants excised from 15 days of gestation mouse embryos. Nerve fibers growing from the spinal cord explant carry at their tips immature mitotic astrocytic cells that lead their growth cones. These glial cells divide only during the active phase of neuronal growth, and correspond ultrastructurally to radial glia. They provide a specific cellular substrate for neuronal growth. Some growth cones form axoglial synapses with smooth membranes of immature glial cells. In contrast, maturing glial cells sprout cytoplasmic processes that tightly wrap individual growth cones and effectively arrest their growth. Next, the processes gather nerve endings into islets and nerve fibers into bundles. After internalizing nerve endings, the glial processes withdraw, bringing the endings into contact with each other. The direct neuronal appositions lead to the transformation of growth cones into presynaptic endings, signaled by their collection of presynaptic vesicles. Clustering of the vesicles at presynaptic axoglial or axodendritic membranes indicates the onset of synaptogenesis-completed by differentiation of spinous and compound synapses. Concomitant with the progress of synaptogenesis, astrocytic investment within the neuropil progressively diminishes. The differentiating astrocytic processes show secretory and tethering activity toward nerve fibers and their endings. Our observations demonstrate that astroglial cells-depending on their developmental stage-first promote and then arrest neuronal growth, and induce synaptogenesis. Thus, at any time, the growing nerve fibers are not only supported but also controlled by the astroglial cells.

Influence of neurotrophins on the synaptogenesis of inner hair cells in the deaf Bronx waltzer (bv) mouse organ of Corti in culture

The Bronx waltzer (bv) deaf mouse is characterized by massive degeneration of the primary auditory receptors, the inner hair cells, which occurs during the time of expected afferent synaptogenesis. The process is associated with degeneration and protracted division of the normally postmitotic afferent spiral ganglion neurons. To investigate the potential role of neurotrophins in the afferent synaptogenesis of inner hair cells, we exposed bv newborn cochleas in organotypic culture to brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and nerve growth factor (NGF), and also to gamma aminobutyric acid (GABA), for up to 8 days. The study was done using light and electron microscopy. Only about 20% of the inner hair cells survived in culture, regardless of the treatment, similar to the number in the intact mutant in our colony. Depending on the exogenous treatment, this population consisted of either innervated ultrastructurally normal cells or denervated dedifferentiated cells wrapped-in lieu of nerve endings-by the supporting inner phalangeal and border cells. In the control and GABA cultures, inner hair cells were mostly denervated. BDNF and NT-3 alone or combined increased synaptogenesis and hair cell survival only during the first 3 days (by about 10%); however, the cells became denervated by 8 postnatal (PN). Only NGF induced stable innervation and differentiation of neurosensory relationships, including supernumerary innervation characteristic of the intact bv. Denervation among the remaining 20% of inner hair cells induced a reactive wrapping by inner phalangeal and border cells which evidently extended inner hair cell survival. Immunocytochemical studies of these reactive supporting cells were done in the intact (8 PN) mutant cochlea. The supporting cells that provide sustenance to the denervated inner hair cells displayed strong BDNF (and possibly NT-3) immunoreactivity. Subsequently, we revealed the presence of all three neurotrophins in the inner hair cell region of the developing (1-8 PN) cochlea of the normal ICR mouse. The inner hair cells expressed all three neurotrophins; BDNF prevailed in the inner phalangeal cells, NT-3 in the pillar cells and inner phalangeal cells, and NGF in the pillar cells.

IN CONCLUSION

initially, the 80% loss of inner hair cells is apparently caused by their failed afferent synaptogenesis. Exogenous neurotrophins influence synaptogenesis in the bv in culture, but NGF alone is successful in promoting stable neurosensory relationships. The presence of neurotrophins in supporting cells in the normal and degenerating cochlea indicates their role in the sustenance of inner hair cells.

Expression of trk and neurotrophin mRNA in dorsal root and sympathetic ganglia of the quail during development

The nerve growth factor (NGF) family of neurotrophins exerts effects by binding to products of the trk family of proto-oncogenes. We examined the expression of both trk and neurotrophin mRNA during the entire range of development of quail dorsal root ganglia (DRG) and sympathetic ganglia (SG) using in situ hybridization and reverse transcriptase-polymerase chain reaction (RT-PCR). TrkC mRNA was present in neurons or their precursors from the time of formation of DRG (stage 18, embryonic day 2.5 [E2.5]) and throughout development. The number of labeled cells changed, however, from a majority to a minority at later developmental stages. Expression of trkA mRNA was not detected in DRG until stage 30 (E6) by in situ hybridization, although results with RT-PCR were positive at stage 23 (E3.5). Labeling was always detected on a majority of neurons or their precursors. SG exhibited low levels of trkC mRNA during the later stages of development, whereas trkA mRNA was present from stage 34 onward in most neurons. We have also shown that NGF, neurotrophin-3 (NT-3), and brain-derived neurotrophic factor (BDNF) mRNA were present at all stages examined (stages 23 through 45 for DRG, stages 35-36 and 45 for SG). In DRG, NGF mRNA expression was limited to support cells, whereas NT-3 and BDNF mRNA were detected in both neurons and support cells. These results suggest that neurotrophins could serve a local function in developing ganglia, which can be correlated with the presence of their respective receptors.

Differential expressions of the topoisomerase II alpha and II beta mRNAs in developing rat brain

Distributions of the topoisomerase II alpha and II beta mRNAs were examined in the developing rat brain, by in situ hybridization with isoform-specific oligonucleotide probes. Intense signals for the topoisomerase II alpha mRNA were detected in the ventricular zone of each brain region at embryonic day 13-15 (E13-E15), and in the external granular layer of the cerebellum at postnatal day 7-14 (P7-P14). Thereafter, the signals rapidly decreased in levels and eventually disappeared from respective regions. Administration of bromodeoxyuridine (BrdU) into embryos at E13 showed that the topoisomerase II alpha mRNA was expressed in the BrdU-incorporated region and its ventricular side of the neural wall, suggesting that transcription of this isoform occurs in neurons from S-phase through M-phase. On the other hand, the topoisomerase II beta mRNA was distributed throughout the brain from E13 through P21, irrespective of the ventricular and mantle zones. Signal levels of the topoisomerase II beta mRNA were much stronger during early developmental stages than at mature stages in various brain regions. The characteristic and differential spatio-temporal expressions suggest that the topoisomerase II alpha is involved in the proliferation, while the topoisomerase II beta is closely related to differentiation and maturation of neurons.

Presynaptic fibres of spiral neurons and reciprocal synapses in the organ of Corti in culture

Isolated segments of the newborn mouse organ of Corti were explanted together with the spiral ganglion components. Within the innervation provided by the spiral neurons, we observed presynaptic vesiculated nerve endings that form reciprocal ribbon-afferent/efferent synapses with inner hair cells. These intracochlear presynaptic fibres are characteristically located between adjoining inner hair cells, on the modiolar side, low and close to the supporting cells. The presynaptic fibres display different modes of synaptic connectivity, forming repetitive reciprocal synapses on single inner hair cells or on adjoining hair cells, or connecting adjoining inner hair cells through simultaneous efferent synapses. Many presynaptic fibres exhibit a distinctive ultrastructure: defined clusters of synaptic vesicles, dense core vesicles, coated vesicles, and mitochondria. These organelles occur focally at the synaptic sites; beyond the efferent synaptic specializations, the endings appear quite nondescript and afferent-like. We believe that the reciprocal synapses, although observed in cultures of the organ of Corti, represent real intracochlear synaptic arrangements providing a feedback mechanism between the primary sensory receptors and a special class of spiral ganglion cells that have yet to be recognized in the organ in situ.

Networks formed by dorsal root ganglion neurites within spinal cord explants: a computer-aided analysis of HRP intracellularly labeled neurons

Dorsal root ganglion (DRG) neurons from rat embryos were explanted either alone or with the attached spinal cord (SC). Neuritic processes were mapped out histologically using an intracellular iontophoretic HRP method. Computer reconstructions and morphometric parameters allowed a comparative quantitative analysis of the DRG neurons and of their neuritic processes in both models. The first model, strongly dependent on nerve growth factor (NGF) for its survival, developed large multidirectional processes. The second model showed a bipolar distribution of its neuritic processes, the central one entering predominantly the dorsolateral part of the cord explant and ramifying both homo- and heterolaterally. The quantitative data revealed a significant decrease in the overall size of the neuritic networks of the second model (with attached SC). A discriminant analysis permitted the recognition of these two populations of DRG neurons. The role of the spinal cord explant, and more precisely the target cells of the dorsal horn, was considered to be a prominent factor in the development of the DRG neuritic networks.

Freeze-fracture and immunomorphological analysis of spiral ganglion cells

Freeze-fracture analysis of adult spiral ganglion cells of CBA/CBA mice revealed two types of membrane specializations. Most cells (type I) had a smooth surface and were surrounded by Schwann cells. Type II spiral ganglion cells showed numerous membrane specializations with well-delineated indentations similar to those previously found on hair cells adjacent to afferent and efferent nerve endings. Immunomorphological analysis (using well-defined monoclonal antibodies directed against different subclasses of intermediate filament proteins) revealed a unique co-expression of neurofilaments, vimentin and cytokeratins in spiral ganglion cells of 8- to 22-week human fetuses.

Perinatal growth of spiral ganglion cells in the kitten

Spiral ganglion cell growth and myelination in the kitten have been studied by means of light and electron microscopy, in one fetus and in kittens of various ages. The growth of spiral ganglion cell bodies and nuclei as studied through their cross-section modifications show two different periods of increase: a period of rapid growth before birth, followed by a slower one after birth up to the first postnatal month. After this stage, the diameter decreases slightly in the adult. Myelination of the cell body begins before birth for the basal part of the cochlea. Myelination is fast during the first postnatal month, then slows down during the following months. Comparison of myelination with the growth of the cell body and its nucleus shows that the postnatal growth of the cell body is due to the increase of the cytoplasmic component of the cell and also to the myelin sheath.

Schwann cells induce morphological transformation of sensory neurones in vitro

Cell-cell interactions are thought to play a crucial part in determining the developmental fate of vertebrate cells and regulating their subsequent differentiation. In the peripheral nervous system, for example, signals from neuronal axons determine whether or not some Schwann cells wrap their plasma membrane concentricially around the axon to form a myelin sheath. Moreover, there is some evidence that the interactions between Schwann cells and neurones are not all one way: for example, Schwann cells are thought to provide signals for neuronal sprouting and regeneration. However, there are no clear examples in which Schwann cells have been shown to influence the normal development of neurones. Here I have used purified populations of embryonic sensory neurones and Schwann cells to demonstrate that Schwann cells have a dramatic influence on the development of these neurones. In the presence of Schwann cells, but not other cell types, the sensory neurones undergo a morphological transformation from an immature bipolar form to a mature pseudo-unipolar form. This provides a striking example of the importance of glial cells for neuronal development.

A comparative ultrastructural study of the trigeminal ganglion in the rat and chicken

The neurons of the trigeminal ganglia of the rat and chicken were characterized by means of light microscopic, electron microscopic, and histochemical methods. Light microscopy disclosed four types of neurons, based on the characteristics of Nissl granules: (1) large neurons with diffusely distributed and very fine granules, (2) neurons containing coarse and sparsely distributed Nissl granules, (3) neurons containing dense Nissl granules of varying size, and (4) small neurons with granules concentrated peripherally. Electron microscopy allowed further definition of these four types of neurons by the length and arrangement of flattened cisterns of granular endoplasmic reticulum (gER) and the number of neurofilaments. Type 1 cells were largest, with a mean nuclear area of 139.8 +/- 28.3 micron2. Type 4 cells were smallest, with a mean nuclear area of 74.6 +/- 20.9 micron2. The mean nuclear areas of type 2 and 3 cells were intermediate to those of the type 1 and 4 cells. Type 3 and 4 neurons lacked neurofilaments. Four forms of Golgi apparatus were found: (1) large bent grains forming a network throughout the soma, (2) dispersed fine granular deposits, (3) fine or small granules, and (4) coarse bent deposits arranged confluently in the perinuclear zone. In some rat neurons, the concentration of acid phosphatase reaction products suggested a high enzymatic activity, whereas the chicken ganglion cells showed no such concentration. These findings are discussed and compared with the classifications of previous studies.

Growth in culture of the peripheral axons of the spiral neurons in response to displacement of the receptors

Observations were made on the growth of the peripheral axons of the spiral neurons in injured explants of the cochlea of the mouse. Opening of the cochlear duct during explantation usually results in a displacement of the hair cells into the outgrowth zone if the basilar membrane of the excised organ of Corti faces the substrate. Spiral neurons respond with vigorous growth to the stress created by the displacement of their receptors. Two different growth reactions occur in succession: 1) growth by elongation of the synaptically engaged fibre 2) free growth. The response of the neuron to an incipient displacement of the hair cells is an elongation of the fibre while its contacts with the receptors are, at least partly, preserved. Most or all fibres that are connected with the displaced cells elongate. The growth is well organized, limited to the region of the displaced hair cells, and restricted in length by the position of the receptors. It is inferred that tension on the synaptically engaged fibre may be a stimulus for its growth and that the growth ceases when the tension is relieved. Continuous stress eventually leads to a rupture of the fibre. The break usually occurs near or at the terminals, leaving the terminals attached to the hair cells. The proximal end of the fibre--now free--starts to grow. The growth is independent of the receptors, unrestricted in length, uncontrolled in amount, and continues for at least three weeks after explantation. The path of the fibre is tortuous, and collaterals are often emitted. Free growth proceeds at a markedly faster rate than growth by elongation. Maintenance of the synapse and growth of the fibre seem to be mutually dependent events. A peripheral axon of the spiral neuron can sustain in culture a normal innervation pattern, without any obvious signs of abnormal growth, if its connections with the hair cells are undisturbed; it elongates in an organized manner, if these connections come under stress; it responds with an uninhibited growth if the connections are broken.

Organotypic development of the organ of Corti in culture

The preservation and development of the innervation pattern in the organ of Corti have been studied in culture up to 27 days in vitro. The explants were obtained from the newborn mouse. Segments of the cochlear duct dissected together with the appropriate sectors of the spiral ganglion may retain their structural organization for about two weeks. Maturation of some nonneuronal elements which occurs during that time is followed by a subsequent regression of the organ. Only a fraction of the explanted neurons survive. However, the surviving neurons, if connected with the hair cell region, maintain a complex peripheral innervation pattern that contains all the major fibre components which characterize the normal pattern in a young mouse. The peripheral innervation pattern in culture seems largely composed of preserved fibres, that is, of fibres which at the time of explantation have already ramified within the organ of Corti. Nonetheless, there is evidence for growth or maturation, in culture, of at least some peripheral processes of the spiral neurons. Thus, only in older cultures is the innervation of the apical tip established. Likewise, it is only in older explants that the inner spiral bundle becomes prominent. Spiral neurons survive in culture in several modes. Most frequently, the central process is altogether absent and the neuron is effectively a unipolar cell which maintains only the peripheral process. A distinct minority of neurons is bipolar possessing both the peripheral process and a central axon which grows freely, though no central target is present. A neuron may survive also as a unipolar or, rarely, as a bipolar cell with no processes entering the organ of Corti. The observations imply that (1) most or all major fibre systems in the organ of Corti carry components of spiral neuron origin; (2) a small population of spiral neurons innervating a short segment of the organ contributes importantly not only to the radial but also to the spiral innervation of the segment.