A freeze-fracture study of synaptogenesis in the distal retina of larvalXenopus

Summary Synapse formation between photoreceptor, bipolar and horizontal cells of the larvalXenopus retina was studied by the freeze-fracture technique. Photoreceptors and horizontal cells were joined by ribbon synapses; photoreceptor and bipolar cells by basal junctions. Gap junctions were found between photoreceptors and between horizontal cells. Horizontal cell dendrites invaginated receptor bases before the plasma membrane of either cell showed zones of intramembrane (IMP) particle accumulation. Subsequently the receptor cell began to form a synaptic ridge where P-face IMPs aggregated at a protrusion of the surface membrane. The length of the ridge and the density of its IMPs increased between larval stages 40 and 56. Cross-fractured views of receptor cytoplasm at different larval stages showed that synaptic ribbons and synaptic vesicles developed in conjunction with the ridge. Plasmalemmal deformations suggesting sites of vesicle fusion or uptake were noted adjacent to the apex of the ridge. Horizontal cell dendritic membrane first accumulated P-face IMPs at several small regions; subsequently the IMPs became aligned over a broad membrane area. Both rod- and cone-related horizontal cell dendrites also manifested a loose patch of E-face IMPs which subsequently was transformed into a linear array. Basal junctions were characterized by a P-face IMP aggregate in the photoreceptor membrane and an E-face IMP aggregate in the bipolar cell membrane. Basal junctions appeared suddenly in a mature configuration at larval stage 42.     

Neuronal and synaptic organization of the outer plexiform layer of the pigeon retina

The organization of the outer plexi-form layer (OPL) of the pigeon retina is described by electron microscopy and Golgi impregnation. Six types of photoreceptor, four types of horizontal cell, eight types of bipolar cell, and an interplexiform cell type were found by Golgi impregnation. The OPL was tri-stratified due to the endings of the photoreceptors at three different levels. This stratification was reflected in the laminar arrangement of the dendrites of the horizontal and bipolar cells. Electron microscopy showed that the synaptic endings of the photoreceptors made ribbon synapses, both triads and dyads, and basal junctions with the process of second-order neurons. Horizontal cells formed conventional chemical synapses, while horizontal cell axon terminals were extensively linked by gap junctions.     

Synapses between cones and diffuse bipolar cells of a primate retina

Summary The photoreceptor synapses of three representative cells of the six types of diffuse bipolar cell of the rhesus macaque monkey's retina are described at 3.5–4.0 mm eccentricity. Bipolar cell DB3 was found to be postsynaptic to 11 cones at 155 basal synapses; about 70% of these were triad associated. Bipolar cell DB4 was postsynaptic to eight cones at 52 ribbon synapses; in addition it was found also to make an average of two or three basal (non-ribbon) synapses per cone (total 23). The DB5 bipolar cell type had 57 invaginating synapses with seven cones. It too had basal synapses, but only two with each of three cones. The diffuse invaginating bipolar cell described by Mariani (1981) is identified as a member of the DB5 category. Dendrites of cone bipolar cell types which have axons ending in the a-layer of the inner plexiform layer make only basal synapses with the cone pedicle. Those so far investigated are the flat midget bipolar cell and the DB2 and DB5 flat diffuse bipolar cells. All bipolar cells whose axons terminate in the b-layer of the inner plexiform layer are postsynaptic at the ribbon synapses of the cone pedicles. They now appear to fall into two groups. Those whose dendrites are exclusively postsynaptic at the ribbons; these are the blue cone and invaginating midget bipolar cells. And the diffuse bipolar cell DB4, that has both ribbon and basal synapses in a ratio of about 2.31. It is uncertain into which category cell DB5 should be placed; its basal synapses are so few the cell could be anomalous. It now seems that at least one primate bipolar cell type may be like those of other vertebrates in having, as defined ultrastructurally, two different kinds of synaptic connection with its cones. The results are discussed in the context of a brief review of the photoreceptor synapses of other mammalian bipolar cells.     

Pikachurin, a dystroglycan ligand, is essential for photoreceptor ribbon synapse formation

Exquisitely precise synapse formation is crucial for the mammalian CNS to function correctly. Retinal photoreceptors transfer information to bipolar and horizontal cells at a specialized synapse, the ribbon synapse. We identified pikachurin, an extracellular matrix-like retinal protein, and observed that it localized to the synaptic cleft in the photoreceptor ribbon synapse. Pikachurin null-mutant mice showed improper apposition of the bipolar cell dendritic tips to the photoreceptor ribbon synapses, resulting in alterations in synaptic signal transmission and visual function. Pikachurin colocalized with both dystrophin and dystroglycan at the ribbon synapses. Furthermore, we observed direct biochemical interactions between pikachurin and dystroglycan. Together, our results identify pikachurin as a dystroglycan-interacting protein and demonstrate that it has an essential role in the precise interactions between the photoreceptor ribbon synapse and the bipolar dendrites. This may also advance our understanding of the molecular mechanisms underlying the retinal electrophysiological abnormalities observed in muscular dystrophy patients.     

Synaptic organization of the inner plexiform layer in the retina of the tiger salamander

Summary The synaptic morphology and organization of the inner plexiform layer of the salamander retina was examined in serial sections with the electron microscope. Processes were traced, whenever possible, from their cells of origin, and synapses were classified as ribbon or regular (non-ribbon, conventional). Amacrine processes always make regular synapses, while bipolar processes make both ribbon and regular synapses. For this reason, new and decisive criteria based on the differential morphology of synaptic vesicles and junctional membranes were sought to distinguish between the amacrine and bipolar processes in single sections. Amacrine processes contain a relatively uniform population of small round vesicles and they make regular symmetrical synapses. Bipolar processes contain vesicles that are generally larger and more pleomorphic than those of the amacrine processes, and they make ribbon synapses (monads, dyads and triads) and regular synapses of the asymmetrical type. Amacrine processes synapse upon other amacrine processes, bipolar axons, ganglion cell dendrites, and the perikarya of these three types of cells. Amacrine cells also give rise to somatodendritic synapses. Bipolar processes synapse upon amacrine processes, ganglion cell dendrites and other bipolar processes, but they have not been seen to synapse upon the somata of any cell. Both amacrine and bipolar processes engage in serial synapses, and these two groups often make reciprocal synapses with each other. Gap junctions have been found between two bipolar processes, between two amacrine processes, and less frequently, between a bipolar and an amacrine process.     

Histology of the ferret retina

The retina of the adult ferret, Mustelo furo, was studied with light and transmission electron microscopy to provide an anatomical basis for use of the ferret as a model for retinal research. The pigment epithelium is a simple cuboidal layer of cells characterized by a zone of basal folds, apical microvilli, and pigment granules at various stages of maturation. The distinction between rod and cone photoreceptor cells is based on their location, morphology, heterochromatin pattern and the electron density of their inner segments. The round, light-staining cone cell nuclei occupy the layer of perikarya along the apical border of the outer nuclear layer. The remainder of the outer nuclear layer consists of oblong, deeply-stained rod cell nuclei. Ribbon type synaptic complexes involving photoreceptor cell axons, horizontal cell processes, and bipolar cell dendrites characterize the outer plexiform layer. The inner nuclear layer is comprised of horizontal, bipolar, and amacrine cell perikarya as well as the perikarya of the Müller cells. The light-staining horizontal cell nuclei are prominent along the apical border of the inner nuclear layer. The light-staining amacrine cell nuclei form a more or less continuous layer along the basal border of the inner nuclear layer. Both conventional and ribbon-type synapses characterize the inner plexiform layer. The ganglion cells form a single cell layer. The optic fiber layer contains bundles of axons surrounded by Müller cell processes. Small blood vessels and capillaries are present in the basal portion of the retina throughout the region extending from the internal limiting membrane to the outer plexiform layer. The adult one-year-old retina is compared with the retina at the time of eye opening.     

Synaptic organization of the outer plexiform layer of the turtle retina: an electron microscope study of serial sections

Summary Neural connections in the outer plexiform layer of thePseudemys turtle retina have been studied by electron microscopy of serial ultrathin sections. While the distinguishing features of the photoreceptors have been described elsewhere, in this paper we describe the patterns of connectivity between identified second order neurons and identified photoreceptors or amongst second order neurons themselves. Basal telodendria emitted from double cone pedicles interconnect the two members of the double cone. Three morphologically different types of junction are made between bipolar cells and cone pedicles. H1 horizontal cells can be distinguished from H2 horizontal cells and synapses occur between them. Axon terminals of H1 cells are presynaptic to H1 cell bodies. Photoreceptors, H1 cell bodies and H1 axon terminals engage in electrical junctions while chemical synapses occur from both types of horizontal cell to bipolar cells. On rare occasions, bipolar cell dendrites were seen to be presynaptic to other bipolar cell dendrites. The significance of some of these contacts for the electrophysiological findings on the OPL of the turtle retina is discussed.     

The cone synapses of cone bipolar cells of primate retina

One each of bipolar cell types DB2 and DB4, together with a flat and an invaginating midget bipolar cell, were taken from a Golgi-stained rhesus macaque retina; then serially sectioned for EM examination of their synapses with cone pedicles. The cone input to the dendrites of the DB2 cell was exclusively at basal junctions; it had a characteristic distribution. Fifty per cent of the basal synapses were with cone pedicle membrane immediately adjacent to the dendrite of a bipolar cell invaginating to end opposite the ribbon of a cone triad (this, therefore, is called triad-associated). The remainder were one or more synapses distant from the triad-associated position (and, therefore, non-triad associated). The DB4 cell had both basal (predominantly in the triad-associated position) and ribbon-related synaptic input. But the basal to invaginating ratio differed from that of our previously published cell; 56% basal, 43% invaginating, as compared with 31% basal and 69% invaginating. Like foveal IMB cells the synapses of the mid-peripheral invaginating midget bipolar cell were exclusively invaginating; but were about 25% more numerous. The flat midget bipolar cell made exclusively basal synapses. These were 2.5 times more numerous than those of foveal flat midget bipolar cells, and 3.5 times the number of invaginating midget bipolar synapses at equivalent eccentricity. The synapses between cones and diffuse and midget bipolar cells are characteristic for each particular bipolar cell type, but the details depend on a cells distance from the fovea (eccentricity). A rather constant number of cone pedicle synaptic ribbons 38.6±2.5 (n±:60) was found across mid-peripheral macaque and vervet monkey retinae. The smaller mean number for vervet monkey, 27.4±3.5 (n ±:23), suggests there can also be generic differences in synaptic detail at cone bipolar cell synapses.     

Gap junctions in the outer plexiform layer of the chick retina: thin section and freeze-fracture studies

Summary Previous studies have established that gap junctions between presumptive retinal neurons of the chick retina disappear during the course of embryogenesis. The present study examines the 2–3-week-old chick retina to determine if gap junctions are present in the outer plexiform layer of the more mature animal as would be in accordance with evidence from morphological and physiological studies on a variety of other vertebrates. Thin section and freeze-fracture techniques are used in a complementary manner to demonstrate that gap junctions are present between horizontal cell processes in the distal regions of the outer plexiform layer. These junctions appear to be between axon terminals and between spines that project from axon terminals to rods and double cones. Gap junctions are also observed between photoreceptors. They are seen on the synaptic terminals of all classes of cones and are located between the cone synaptic terminals and cone basal processes. Gap junctions are also seen between unidentified photoreceptor basal processes within the neuropil of both distal and proximal parts of the outer plexiform layer. Gap junctions are also present between cone synaptic terminals and deeply invaginated, vesicle-containing processes the origin of which remains to be determined.     

Identification and localization of an immunoreactiveAMPA-type glutamate receptor subunit (GluR4) with respect to identified photoreceptor synapses in the outer plexiform layer of goldfish retina

L-glutamate, the main excitatory synaptic transmitter in the retina, is released from photoreceptors and evokes responses in second-order retinal neurons (horizontal, bipolar cells) which utilize both ionotropic and metabotropic types of glutamate receptors. In the present study, to elucidate the functional roles of glutamate receptors in synaptic transmission, we have identified a specific ionotropic receptor subunit (GluR4) and determined its localization with respect to photoreceptor cells in the outer plexiform layer of the goldfish retina by light and pre-embedding electron-microscopical immunocytochemistry. We screened antisera to mammalian AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate)-preferring ionotropic glutamate receptors (GluR 1–4) of goldfish retina by light- and electron-microscopical immunocytochemistry. Only immunoreactive (IR) GluR4 was found in discrete clusters in the outer plexiform layer. The cones contacted in this manner were identified as long-wavelength (red) and intermediate-wavelength (green) cones, which were strongly immunoreactive to monoclonal antibody FRet 43 and antisera to goldfish red and green-cone opsins; and short-wavelength (blue) cones, which were weakly immunoreactive to FRet 43 but strongly immunoreactive with antiserum to blue-cone opsin. Immunoblots of goldfish retinal homogenate with anti-GluR4 revealed a single protein at Mr=110 kDa. Preadsorption of GluR4 antiserum with either the immunizing rat peptide, or its goldfish homolog, reduced or abolished staining in retinal sections and blots. Therefore, we have detected and localized genuine goldfish GluR4 in the outer plexiform layer of the goldfish retina. We characterized contacts between photoreceptor cells and GluR4-IR second-order neurons in the electron microscope. IR-GluR4 was localized to invaginating central dendrites of triads in ribbon synapses of red cones, semi-invaginating dendrites in other cones and rods, and dendrites making wide-cleft basal junctions in rods and cones; the GluR4-IR structures are best identified as dendrites of OFF-bipolar cells. The results of our studies indicate that in goldfish retina GluR4-expressing neurons are postsynaptic to all types of photoreceptors and that transmission from photoreceptors to OFF-bipolars is mediated at least in part by AMPA-sensitive receptors containing GluR4 subunits.     

Ectopic synaptogenesis during retinal degeneration in the royal college of surgeons rat

Rod photoreceptor-specific mutations cause ectopic synapses to form between cone photoreceptor terminals and rod bipolar cell dendrites in degenerating retinas of rhodopsin transgenic (P347L) pigs and retinal degeneration mice. Since the mutations occur in rod photoreceptor-specific genes in these two models, it is not known if ectopic synaptogenesis occurs specifically due to some rod photoreceptor cell-autonomous properties of a mutation or as a general consequence of photoreceptor degeneration. In the Royal College of Surgeons (RCS) rat, a mutation in the receptor tyrosine kinase gene, Mertk, causes failure of the retinal pigment epithelial (RPE) cells to phagocytose shed photoreceptor outer segments; subsequently, both rod and cone photoreceptors die. The non-phagocytic phenotype of the RCS rat is RPE cell-autonomous and the photoreceptors degenerate secondarily. Here we show that in 35-day-old RCS rats, where a majority of rod and cone photoreceptors remained, rod bipolar cell dendrites had abnormal (flat-contact type) synaptic contacts with rod and cone terminals.Demonstration of ectopic synapses in the RCS rat suggested that ectopic synaptogenesis could occur as a result of photoreceptor degeneration, even when the rods and cones were developmentally normal. This further supported the hypothesis that ectopic synaptogenesis may be a common step in the disease progression of different forms of retinal degeneration that include photoreceptor death as a feature, such as retinitis pigmentosa.     

Spatial relationships of connexin36, connexin57 and zonula occludens-1 in the outer plexiform layer of mouse retina

Horizontal cells form gap junctions with each other in mammalian retina, and lacZ reporter analyses have recently indicated that these cells express the Cx57 gene, which codes for the corresponding gap junctional protein. Using anti-connexin57 antibodies, we detected connexin57 protein in immunoblots of mouse retina, and found punctate immunolabeling of this connexin co-distributed with calbindin-positive horizontal cells in the retinal outer plexiform layer. Double immunofluorescence labeling was conducted to determine the spatial relationships of connexin36, connexin57, the gap junction-associated protein zonula occludens-1 and the photoreceptor ribbon synapse-associated protein bassoon in the outer plexiform layer. Connexin36 was substantially co-localized with zonula occludens-1 in the outer plexiform layer, and both of these proteins were frequently located in close spatial proximity to bassoon-positive ribbon synapses. Connexin57 was often found adjacent to, but not overlapping with, connexin36-positive and zonula occludens-1-positive puncta, and was also located adjacent to bassoon-positive ribbon synapses at rod spherules, and intermingled with such synapses at cone pedicles. These results suggest zonula occludens-1 interaction with connexin36 but not with Cx57 in the outer plexiform layer, and an absence of connexin57/connexin36 heterotypic gap junctional coupling in mouse retina. Further, an arrangement of synaptic contacts within rod spherules is suggested whereby gap junctions between horizontal cell terminals containing connexin57 occur in very close proximity to ribbon synapses formed by rod photoreceptors, as well as in close proximity to Cx36-containing gap junctions between rods and cones.     

Age-dependent remodelling of retinal circuitry

We have investigated morphological changes in second-order neurons of the mouse retina during aging by using immunohistochemistry and electron microscopy. We observed sprouting of rod bipolar cells dendrites and horizontal cells arborizations: neuronal processes of both neuronal types showed irregular extensions beyond the outer plexiform layer, toward the outer limiting membrane, as well as into the outer nuclear layer (ONL). These processes were first observed in animals of 12 months of age and increased in numbers steadily until 24 months, which represent the last age examined. The ectopic processes are decorated by puncta immunoreactive for pre-synaptic markers typical of photoreceptor terminals juxtaposed to post-synaptic neurotransmitter receptors, demonstrating the presence of the entire molecular machinery of functional synapses. Electron microscopy confirmed that ectopic processes receive synapses from photoreceptor terminals. We conclude that during the second year of life retinal rod bipolar and horizontal cells undergo sprouting and form ectopic synapses in the ONL.     

Cone synapses of a flat diffuse cone bipolar cell in the primate retina

Summary A Golgi-stained flat diffuse cone bipolar cell from a vervet monkey's retina (Cercopithecus aethiops), contacting six cones, was serially sectioned for electron microscopy (EM) to determine the types of synapses it made with the cone pedicles. All the synapses were basal (flat) contacts. Their distribution and ultrastructural type were similar at each pedicle. Approximately half the synapses were definable as triad-associated and the rest were elsewhere on the cone pedicle base. Their ultrastructure is the same regardless of those positions. About 25 synapses were made with each cone. Thus this type (DB2 of Boycott & Wässle, 1991) of flat diffuse cone bipolar cell is in contact with six cones through about 150 synapses. At the eccentricity studied each cone pedicle probably makes 90–100 basal synapses with between three and four DB2 bipolar cells. This is between two and three times the number that are made with all the types of invaginating bipolar cells. A brief review of cone photoreceptor synapses with bipolar cells shows that, for those so far examined in the primate retina, the dichotomy into two types of bipolar cell invaginating (ribbon-related), with axons ending in the b-layer of the inner plexiform layer (IPL) (hence presumptive On-bipolars) and flat (basal synapses), with axons ending in the a-layer of the inner plexiform layer (hence presumptive Off-bipolars) is the rule. But other vertebrate retinae, including that of the cat, also have bipolar cells which vary from this pattern.     

Regressive and reactive changes in the connectivity patterns of rod and cone pathways of P23H transgenic rat retina

We have used the P23H line 1 homozygous albino rat to study how progressive photoreceptor degeneration affects rod and cone relay pathways. We examined P23H retinas at different stages of degeneration by confocal microscopy of immunostained sections and electroretinogram (ERG) recordings. By 21 days of age in the P23H rat retina, there is already substantial loss of rods and reduction in rod bipolar dendrites along with reduction of metabotropic glutamate receptor 6 (mGluR6) and rod-associated bassoon staining. The cone pathway is relatively unaffected. By 150 days, when rods are absent from much of the retina, some rod bipolars remain and dendrites of rod and cone bipolar cells form synaptic complexes associated with cones and horizontal cell processes. These complexes include foci of mGluR6 and bassoon staining; they develop further by 270 days of age. Over the course of degeneration, beginning at 21 days, bipolar axon terminals atrophy and the inner retina undergoes further changes including a reduced and disorganized AII amacrine cell population and thinning of the inner plexiform layer. Electroretinogram (ERG) results at 23 days show reductions in a-wave amplitude, in rod and cone-associated b-waves (using a double flash paradigm) and in the amplitude of oscillatory potentials (OPs). By 38 days, rod scotopic a-wave responses and OPs are lost. B-wave amplitudes decline until 150 days, at which point they are purely cone-driven and remain stable up to 250 days. The results show that during the course of photoreceptor loss in the P23H rat, there are progressive degenerative changes, particularly in the rod relay pathway, and these are reflected in the changing ERG response patterns. Later reactive changes involving condensation of cone terminals and neurotransmitter receptors associated with rod and cone bipolar dendrites and with horizontal cell processes suggest that at this stage, there are likely to be complex changes in the relay of sensory information through the retina.     

Neuro-epitheliomuscular cell and neuro-neuronal gap junctions inHydra

Summary Gap junctions have been described ultrastructurally between neurons and epitheliomuscular cells and between neurons and their processes in the hypostome peduncle and basal disc ofHydra. All gap junctions examined inHydra exhibit two apposed plasma membranes having a 2–4 nm gap continuous with the extracellular space. The gap junctions are variable in length from 0.1–1.6 m and appear linear or V-shaped in section. Neuronal gap junctions inHydra occur infrequently as compared to chemical synapses. Electron microscopy of serial sections has demonstrated the presence of adjacent electrical and chemical synapses (neuromuscular junctions) formed by the same neuron. In addition multiple gap junctions were present between two neurons. This is the first ultrastructural demonstration of electrical synapses in the nervous system ofHydra. Such synapses occur in neurons previously characterized as sensory-motor-interneurons on the basis of their chemical synapses; these neurons appear to represent a type of stem cell characterized by having both electrical and chemical synapses.     

The absence of the clathrin-dependent endocytosis in rod bipolar cells of the FVB/N mouse retina

The high rate of exocytosis at the ribbon synapses is balanced by following compensatory endocytosis. Unlike conventional synaptic terminals where clathrin-mediated endocytosis (CME) is a predominant mechanism for membrane retrieval, CME is thought to be only a minor mechanism of endocytosis at the retinal ribbon synapses, but CME is present there and it works. We examined the clathrin expression in the FVB/N rd1 mouse, which is an animal model of retinitis pigmentosa. The broadly distributed pattern of clathrin immunoreactivity in the inner plexiform layer was similar in both the control and FVB/N mouse retinas, but the immunoreactive punta within the rod bipolar axon terminals located in the proximal IPL were decreased in number and reduced in size at postnatal days 14 and they came to disappear at postnatal days 21. This preferential decrease of the clathrin expression at ribbon synapses in the rod bipolar cell axon terminals of the FVB/N mouse retina demonstrates another plastic change after photoreceptor degeneration and this suggests that clathrin may be important for normal synaptic function at the rod bipolar ribbon synapses in the mammalian retina.     

Ultrastructural analysis of the glutamatergic system in the outer plexiform layer of zebrafish retina

l-Glutamate, the photoreceptor neurotransmitter, depolarizes horizontal cells and OFF-bipolar cells by ionotropic receptors and hyperpolarizes ON-bipolar cells by metabotropic receptors. Despite extensive light microscopy on the distribution of glutamate receptors in zebrafish retina, there are little ultrastructural data. Given the importance of zebrafish in studies on the genetic manipulation of retinal development and function, precise data on the synaptic neurochemical organization of the zebrafish retina is needed. Immunohistochemical techniques were used to determine the ultrastructural localization of glutamate receptor subunits GluR2, GluR4, NMDA2B (NR2B) and mGluR1α in zebrafish outer plexiform layer (OPL). These antibodies were chosen because of an apparent conservation of localization of GluR2, GluR4 and mGluR1α in the vertebrate OPL, while there is some support for NMDA receptors in the OPL. GluR2-immunoreactivity (IR) was in all horizontal cell dendrites that invaginated cone pedicles and rod spherules. Three arrangements of dendrites contained GluR-IR in rod spherules: classical-type with GluR2-IR on lateral horizontal cell dendrites, a butterfly-shaped horizontal cell dendrite, and a goblet-shaped dendrite, likely of bipolar cell origin. GluR4-IR was restricted to dendrites of OFF-bipolar cells that innervated rod and cone terminals. NR2B-IR was restricted to a subtype of cone ON-bipolar cell. mGluR1α-IR was restricted to ON mixed rod/cone (Mb) bipolar cells whose dendrites innervated rod and cone synaptic terminals. The presence of mGluR1α on Mb bipolar cell dendrites is consistent with a role in retrograde endocannabinoid suppression. The subunit composition of glutamate receptors should affect the kinetics and pharmacology of these cells to glutamate receptor activation.     

The development of the dendritic organization of primary and secondary motoneurons in the spinal cord of Xenopus laevis. An HRP study

Summary During embryonic and larval development of the clawed toad, Xenopus laevis, two different populations of motoneurons appear in the spinal cord. In this study the development of primary motoneurons which innervate the axial musculature (used during embryonic locomotion) and of secondary motoneurons which innervate the extremity musculature (used for locomotion during metamorphosis and thereafter) was analyzed with horseradish peroxidase (HRP) as a neuronal marker. After application of HRP to the axial musculature (rostral five postotic myotomes) the first labeled primary motoneurons were found at stage 24/25. During development gradually more labeled neurons were observed. These primary motoneurons send their dendrites into the marginal zone (white matter). At first only dorsal and lateral dendrites develop (stages 25–33), followed by ventral dendrites (stage 37/38). Up till stage 48 the developing dendrites extend throughout the marginal zone. Hereafter the marginal zone increases particularly at the dorsolateral edge, a development which is not followed by the dendrites of the primary motoneurons. The dendrites of mature primary motoneurons (stages 58–62) occupy the ventral and ventrolateral parts of the marginal zone.At stage 48, shortly after the hindlimb bud arises (stage 46, early metamorphosis), the first neurons related to this developing extremity could be labeled in the ventrolateral part of the lumbar spinal cord. At first these secondary motoneurons bear only a few dorsal dendrites of which only the tips reache out in the adjacent white matter. Already at stage 50 these dorsal dendrites have invaded the whole dorsolateral part of the marginal zone. Also the first ventral dendrites were observed at this stage. Later, at stage 53/54 also some ventral dendrites have reached the white matter together with a few lateral dendrites. At these early metamorphic stages already some primary afferent fibers were found making contact with the dorsomedial dendrites. At stage 58 for the first time recurrent axon collaterals were found, which extend into the ventromedial part of the marginal zone. The development of motoneurons in the spinal cord seems to be characterized by two phases: (1) establishment of contacts between motoneurons and target muscles, and (2) subsequent formation of connections of these motoneurons with other nerve cells within the central nervous system. The dendrites of primary motoneurons follow the development of the marginal zone, while dendrites of secondary motoneurons develop into an already well developed marginal zone. Generally, the dendrites of mature motoneurons of the axial musculature were observed in the ventromedial and ventrolateral parts of the marginal zone. The dendrites of themotoneurons which innervate the musculature of the hindlimbs were observed predominantly in the dorsolateral and ventromedial parts of the marginal zone.     

Structure and function of a complex sensory synapse

Vision is the most important of the senses for humans, and the retina is the first stage in the processing of light signals in the visual system. In the retina, highly specialized light-sensing neurons, the rod and cone photoreceptors, convert light into neural signals. These signals are extensively processed and filtered in the subsequent retinal network before transmitted to the higher visual centres in the brain, where the perception of viewed objects and scenes is finally constructed. A key feature of signal processing in the mammalian retina is parallel processing. Visual information is segregated in parallel pathways already at the rod and cone photoreceptor terminals, which provide multiple output synapses for the faithful encoding and transfer of the visual signals to the post-receptoral retinal network. This review aims at highlighting the current knowledge about the structural and functional pre- and post-synaptic specializations of rod and cone photoreceptor ribbon synapses, which belong to the most complex chemical synapses in the central nervous system.