Stratum Fibrosum Et Griseum Superficiale Research Articles

Neuron mapping in the Molly fish optic tectum: An emphasis on the adult neurogenesis process.

Teleost fish exhibit the most pronounced and widespread adult neurogenesis. Recently, functional development and the fate of newborn neurons have been reported in the optic tectum (OT) of fish. To determine the role of neurogenesis in the OT, this study used histological, immunohistochemical, and electron microscopic investigations on 18 adult Molly fish specimens (Poecilia sphenops). The OT of the Molly fish was a bilateral lobed structure located in the dorsal part of the mesencephalon. It exhibited a laminated structure made up of alternating fiber and cellular layers, which were organized into six main layers. The stratum opticum (SO) was supplied by optic nerve fibers, in which the neuropil was the main component. Radial bipolar neurons that possessed bundles of microtubules were observed in the stratum fibrosum et griseum superficiale (SFGS). Furthermore, oligodendrocytes with their processes wrapped around the nerve fibers could be observed. The stratum album centrale (SAC) consisted mainly of the axons of the stratum griseum centrale (SGC) and the large tectal, pyriform, and horizontal neurons. The neuronal cells of the SO and large tectal cells of the SAC expressed autophagy-related protein-5 (APG5). Interleukin-1β (IL-1β) was expressed in both neurons and glia cells of SGC. Additionally, inducible nitric oxide synthase (iNOS) was expressed in the neuropil of the SAC synaptic layer and granule cells of the stratum periventriculare (SPV). Also, transforming growth factor beta (TGF-β), SRY-box transcription factor 9 (SOX9), and myostatin were clearly expressed in the proliferative neurons. In all strata, S100 protein and Oligodendrocyte Lineage Transcription Factor 2 (Olig2) were expressed by microglia, oligodendrocytes, and astrocytes. In conclusion, it was possible to identify different varieties of neurons in the optic tectum, each with a distinct role. The existence of astrocytes, proliferative neurons, and stem cells highlights the regenerative capacity of OT. RESEARCH HIGHLIGHTS: The OT of the Molly fish exhibited a laminated structure made up of alternating fiber and cellular layers, which were organized into six main layers. Radial bipolar neurons that possessed bundles of microtubules were observed in the stratum fibrosum et griseum superficiale (SFGS). The stratum album central (SAC) consisted mainly of the axons of the stratum griseum centrale (SGC) and the large tectal, pyriform, and horizontal neurons. Inducible nitric oxide synthase (iNOS) was expressed in the neuropil of the SAC synaptic layer and granule cells of the stratum periventricular (SPV). Also, transforming growth factor beta (TGF-β), SRY-box transcription factor 9 (SOX9), and myostatin were clearly expressed in the proliferative neurons. The existence of astrocytes, proliferative neurons, and stem cells highlights the regenerative capacity of OT.

Two different areas of the nucleus glomerulosus in the South American pufferfish, Colomesus asellus.

The nucleus glomerulosus (NG) in paracanthopterygian and acanthopterygian teleost fishes receives afferents from neurons of the nucleus corticalis (NC), whose dendrites extend to the layers, stratum fibrosum et griseum superficiale (SFGS) and stratum griseum centrale (SGC), of the tectum opticum. A re-examination in this study revealed, by means of tracer experiments using biotinylated dextran amine, a separation among both tectal layers, portions of the NC, and target areas in a laminated type of the NG in the South American pufferfish, Colomesus asellus. Neurons of the lateral part of the NC send their dendrites to the SFGS and project to an area located dorsolaterally and centrally in the NG. In contrast, dendrites from neurons of the medial part of the NC run to the SGC, and projections from these neurons terminate in the NG in an area extending from dorsomedial to ventrolateral in the outer portion. Therefore, these two areas in the NG receive input from different sources. The NG in the visual system of tetraodontids may be involved in higher cognitive functions requiring much energy, becoming apparent by its very high level of cytochrome c oxidase activity.

Role of En2 in the tectal laminar formation of chick embryos.

The chick optic tectum consists of 16 laminae. Here, we report contribution of En2 to laminar formation in chick optic tecta. En2 is specifically expressed in laminae g-j of stratum griseum et fibrosum superficiale (SGFS). Misexpression of En2 resulted in disappearance of En2-expressing cells from the superficial layers (laminae a-f of SGFS), where endogenous En2 is not expressed. Misexpression of En2 before postmitotic cells had left the ventricular layer indicated that En2-misexpressing cells stopped at the laminae of endogenous En2 expression and that they did not migrate into the superficial layers. Induction of En2 misexpression using a tetracycline-inducible system after the postmitotic cells had reached superficial layers also resulted in disappearance of En2-expressing cells from the superficial layers. Time-lapse analysis showed that En2-misexpressing cells migrated back from the superficial layers towards the middle layers, where En2 is strongly expressed endogenously. Our results suggest a potential role of En2 in regulating cell migration and positioning in the tectal laminar formation.

Histological Structure of The Optic Tectum in The Fresh Water Turtle in The Clemmys caspica caspica (Gmelin, 1774)

A histological study was conducted to examine the structure of optic tectum in Clemmys caspica caspica. The results showed that the optic tectum consist of seven strata and the third stratum consists of three substrata, as follows: The stratum zonula (SZ), the stratum opticum (SO), the stratum griseum et fibrosum superficiale (SGFS) which consist of three substrata are a, b, c, the stratum griseum centrale (SGC), the stratum album centrale (SAC), the stratum griseum periventriculare (SGP), and the stratum album periventriculare (SAP). The stratum zonula (SZ) was the thinnest stratum in the optic tectum and its main thickness was (17-19) μm, while the stratum griseum et fibrosum superficiale (SGFS) was the thickest and its main thickness was (234-243) μm.The stratum opticum (SO), which represents the second stratum called the optic fibers tract, it was found that the optic fibers enter the optic tectum through this stratum while these fibers goout from optic tectum through the (SGP) which is the sixth stratum.The stratum griseum et fibrosum superficiale (SGFS) which represents the third stratum contain a large number of stallete and piriform multipolar neurons and fusiform neurons.

Identification of retinotectal projection pathway in the deep tectal laminae in the chick

The optic tectum is a visual center of nonmammalian vertebrates that receives retinal fibers in a retinotopic manner. It has been accepted that retinal fibers project to some superficial laminae of the tectum, but do not go through lamina g of stratum griseum et fibrosum superficiale (SGFS). By a novel fiber-tracing method, we found a novel pathway of retinal fibers that run through deep laminae of the tectum. The retinal fibers that would run through the newly identified pathway first run caudally along the medial edge after invading the tectum, turn laterally, and extend toward the lateral side through the deep pathway. The deep pathway runs through stratum album centrale and stratum fibrosum periventriculare. The fibers that run through the deep pathway do not enter the stratum opticum, where the conventional retinal fibers run. As development proceeds, these fibers decrease and disappear by the adult stage. By the new method, we found that some of the conventional retinal fibers transiently run through lamina g of SGFS and invade laminae h/i. In conclusion, we found distinct but transient retinal fiber pathway in the deep tectal laminae, which have not been thought to be retinorecipient.

Characterization of Genetically Targeted Neuron Types in the Zebrafish Optic Tectum

The optically transparent larval zebrafish is ideally suited for in vivo analyses of neural circuitry controlling visually guided behaviors. However, there is a lack of information regarding specific cell types in the major retinorecipient brain region of the fish, the optic tectum. Here we report the characterization of three previously unidentified tectal cell types that are specifically labeled by dlx5/6 enhancer elements. In vivo laser-scanning microscopy in conjunction with ex vivo array tomography revealed that these neurons differ in their morphologies, synaptic connectivity, and neurotransmitter phenotypes. The first type is an excitatory bistratified periventricular interneuron that forms a dendritic arbor in the retinorecipient stratum fibrosum et griseum superficiale (SFGS) and an axonal arbor in the stratum griseum centrale (SGC). The second type, a GABAergic non-stratified periventricular interneuron, extends a bushy arbor containing both dendrites and axons into the SGC and the deepest sublayers of the SFGS. The third type is a GABAergic periventricular projection neuron that extends a dendritic arbor into the SGC and a long axon to the torus semicircularis, medulla oblongata, and anterior hindbrain. Interestingly, the same axons form en passant synapses within the deepest neuropil layer of the tectum, the stratum album centrale. This approach revealed several novel aspects of tectal circuitry, including: (1) a glutamatergic mode of transmission from the superficial, retinorecipient neuropil layers to the deeper, output layers, (2) the presence of interneurons with mixed dendrite/axon arbors likely involved in local processing, and (3) a heretofore unknown GABAergic tectofugal projection to midbrain and hindbrain. These observations establish a framework for studying the morphological and functional differentiation of neural circuits in the zebrafish visual system.

Periventricular efferent neurons in the optic tectum of rainbow trout

The efferent connections and axonal and dendritic morphologies of periventricular neurons were examined in the optic tectum of rainbow trout to classify periventricular efferent neurons in salmonids. Among the target nuclei of tectal efferents, tracer injections to the following four structures labeled periventricular neurons: the area pretectalis pars dorsalis (APd), nucleus pretectalis superficialis pars magnocellularis (PSm), nucleus ventrolateralis of torus semicircularis (TS), and nucleus isthmi (NI). Two types of periventricular neurons were labeled by injections to the APd. One of them had an apical dendrite ramifying at the stratum fibrosum et griseum superficiale (SFGS), with an axon that bifurcated into two branches at the stratum griseum centrale (SGC), and the other had an apical dendrite ramifying at the SGC. Two types of periventricular neurons were labeled after injections to the TS. One of them had an apical dendrite ramifying at the boundary between the stratum opticum (SO) and the SFGS, and the other had dendritic branches restricted to the stratum album centrale or stratum periventriculare. Injections to the PSm and NI labeled periventricular neurons of the same type with an apical dendrite ramifying at the SO and a characteristic axon that split into superficial and deep branches projecting to the PSm and NI, respectively. This cell type also possessed axonal branches that terminated within the tectum. These results indicate that periventricular efferent neurons can be classified into at least five types that possess type-specific axonal and dendritic morphologies. We also describe other tectal neurons labeled by the present injections.

The distribution of expression of doublecortin (DCX) mRNA and protein in the zebra finch brain

Using in situ hybridization, we measured the distribution of expression of doublecortin (DCX), a microtubule-associated protein, in zebra finch adult and nestling (P9–11) brains. In adult brain, DCX mRNA was detected mainly in the mesopallium (M), medial striatum (MSt), septum, Area X, diencephalon, telencephalic subventricular zone (SVZ), and Purkinje cells in the cerebellum. The expression at posthatch day 9 (P9) was heavy in almost the entire telencephalon and showed heavier expression in SVZ and song regions such as the high vocal center (HVC) and the robust nucleus of arcopallium (RA). Outside of the telencephalon at P9, we found distinct label in nucleus ovoidalis (OV), nucleus spiriformis lateralis (SpL), and nucleus subpretectalis (SP) in the midbrain, almost the entire diencephalon including nucleus dorsomedialis posterior thalami (DMP), stratum griseum et fibrosum superficiale (SGF) in optic tectum, and Purkinje cells in cerebellum. Most of the heavily labeled areas by in situ hybridization overlapped with immunohistochemical staining for DCX, indicating that DCX mRNA is probably translated into protein in those regions. No sex difference was found in DCX expression at P9 or in the adult except that Area X was labeled only in the adult male. The intensity of expression in the adult was significantly lower than that at P9, which suggests a particular role for DCX in early song bird brain development. If DCX is predominantly expressed in migrating neurons, as suggested from studies in mammals, the present results offer no evidence for a sex difference in neuronal migration.

Afferent and Efferent Connections of the Nucleus Geniculatus Lateralis Ventralis Demonstrated by WGA‐HRP in the Chick

Fibre connections of the chick nucleus geniculatus lateralis ventralis (GLv) were investigated using the axonal tracing method with wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). After an injection of WGA-HRP into the GLv, many labelled neurons were observed in layer i of the stratum griseum et fibrosum superficiale (SGFS) in the ipsilateral tectum opticum (TO) and in the nucleus lentiformis mesencephali (LM). In the TO-GLv projection, cells of origin were located in the deeper part of layer i of the TO and were topographically distributed along the direction from the rostrodorsal part to the caudoventral part of the TO relating to a rostrocaudal axis of the GLv. In the LM-GLv connection, the dorsal and ventral parts of the LM connected reciprocally with the rostral and caudal halves of the GLv, respectively. In contrast, in the GLv efferent connection, labelled axon terminals spread widely in the ipsilateral area pretectalis without any clear topographical arrangement.

The role of Pax7 in determining the cytoarchitecture of the superior colliculus.

Pax genes are a family of transcriptional regulators vital for embryonic development. One member of the family, Pax7, functions early in neural development to establish dorsal polarity of the neural tube, and continuous refinement of its expression affords regional identity to brain nuclei, in particular the superior colliculus. Pax7 expression within the superior colliculus is eventually restricted to the stratum griseum et fibrosum superficiale (SGFS), the retinorecipient layer to which the optic nerve projects. The key role of Pax7 in specification of the superior colliculus has been highlighted by misexpression studies which result in ectopic formation of superior collicular tissue with characteristic laminae innervated by retinal ganglion cell axons. Here we review the role of Pax7 in formation of the superior colliculus and discuss the possibility that Pax7 may also assist in refinement of correct topographic mapping.

Fiber connections of the torus longitudinalis and optic tectum in holocentrid teleosts.

Fiber connections of the torus longitudinalis (TL) and target(s) of toral recipient tectal neurons (pyramidal cells) in the optic tectum were examined by tract-tracing methods in holocentrids. Injections into the stratum marginale (SM) labeled neurons in the stratum opticum and stratum fibrosum et griseum superficiale (SFGS). They had superficial spiny dendrites, with a fan-shaped branching pattern in SM and a thick basal dendrite that gave rise to bushy horizontal branches at the boundary between the SFGS and the stratum griseum centrale (SGC), where an axon and a thin dendrite arose. The axon terminated in a middle cellular layer of the SGC, and the thin dendrite ramified slightly deeper to this cellular layer. The SM injections also labeled cells in the ipsilateral TL. Injections into either the lateral or the medial part of TL labeled terminals in the ipsilateral SM and neurons in the bilateral nucleus paracommissuralis (NPC) and nucleus subvalvularis and ipsilateral nucleus subeminentialis. Only medial TL injections labeled cells in the ipsilateral SGC. These neurons had a basal dendrite that branched in the middle cellular layer of SGC, suggesting that they receive inputs from the pyramidal cells and project back to the TL to form a closed circuit. Only lateral TL injections labeled terminals in the corpus cerebelli. A visual telencephalic portion projects to the NPC and sublayers of SGC, where dendrites of the pyramidal cells and SGC neurons ramify. The present results therefore suggest that the TL and SM are components of an intricate circuitry that exerts telencephalic descending visual influence on the optic tectum and corpus cerebelli.

Tectothalamic visual projections in turtles: their cells of origin revealed by tracing methods.

In two species of turtle (Emys orbicularis and Testudo horsfieldi), retrograde and anterograde tracer techniques were used to study projections from the optic tectum to the nucleus rotundus (Rot) and to the dorsal lateral geniculate nucleus (GLd). The ipsilateral Rot received the most massive tectal projections, stemming from numerous neurons located in the stratum griseum centrale (SGC). These neurons varied in size and shape, many of them having a wide zone of dendritic arborization within both the (SGC) and the stratum griseum et fibrosum superficiale (SGFS). Projections from the tectum to the GLd were ipsilateral, were extremely scarce, and arose from a small number of neurons of various shapes situated in the SGFS; these cells were, as a rule, smaller than those projecting to the Rot. For the most part, these neurons were radially oriented, with rather restricted dendritic arborizations in the most superficial sublayers of the SGFS; smaller numbers of projection neurons were horizontally oriented, with long dendrites branching throughout the layer. Some neurons located in the stratum griseum periventriculare (SGP) projected to both the Rot and the GLd. Most of these neurons had dendritic arborizations within the retinorecipient zone of the SGFS. We were unable to rule out the possibility that some cells projecting to the GLd were situated in the SGC. Both the GLd and the main body of the Rot did not contain neurons projecting to the optic tectum. Thalamic neurons projecting to the tectum were observed in the ventral lateral geniculate nucleus, the intergeniculate leaflet and the interstitial nuclei of the tectothalamic tract, and the nucleus of the decussatio supraoptica ventralis. The question of whether variation in the laminar organization of the tectorotundal and tectogeniculate projection neurons in reptiles, birds, and mammals may be related to different degrees of differentiation of the tectal layers is discussed.

Fiber Connections of the Nucleus isthmi in the Carp (Cyprinus carpio) and Tilapia (Oreochromis niloticus)

The nucleus pretectalis (NP) is a prominent nucleus in the percomorph pretectum and has been shown to project to the nucleus isthmi in the filefish by an HRP tract-tracing method [Ito et al., 1981], but a homologous nucleus to the NP is apparently lacking in ostariophysans. The present study examined fiber connections of the nucleus isthmi in an ostariophysan teleost, the carp (Cyprinidae, Cyprinus carpio), to identify a nucleus homologous to the percomorph nucleus pretectalis. Identical studies in a percomorph tilapia (Cichlidae, Oreochromis niloticus) were also performed. Injections of biotinylated dextran amine (BDA) or biocytin to the carp nucleus isthmi labeled cells in the ipsilateral optic tectum and nucleus ruber of Goldstein [1905]. Labeled tectal neurons were located in the stratum periventriculare (SPV) and the stratum fibrosum et griseum superficiale (SFGS). The somata in the SPV were pyriform and those in the SFGS were fusiform. No labeled cells were found in the pretectum. Labeled terminals were seen in the ipsilateral nucleus pretectalis superficialis pars parvocellularis (PSp), optic tectum, and bilateral nucleus ruber. Terminals in the nucleus ruber appear to come from tectal neurons in the SFGS labeled by isthmic injections. Thus the nucleus isthmi has reciprocal fiber connections with the ipsilateral optic tectum, receives projections from the ipsilateral nucleus ruber, and projects to the ipsilateral PSp. The nucleus pretectalis homologue is apparently absent in the carp. Studies in tilapia showed that the nucleus isthmi receives bilateral projections from the NP and optic tectum. In addition, the present study revealed a previously unknown afferent from the nucleus ruber to the percomorph nucleus isthmi. The tilapia nucleus isthmi projects to the same targets as in the carp. Isthmic projection neurons in the tilapia optic tectum were located in the SPV and pyriform with a similar shape to the carp SPV neurons that project to the nucleus isthmi. No labeled cells were found in the SFGS of tilapia optic tectum. The fusiform neurons in the SFGS of the carp optic tectum possess various hodological similarities with the NP and may correspond to the NP neurons of percomorphs.

Immunocytochemical localization of the GABAA/benzodiazepine receptor beta2/beta3 subunits in the optic tectum of the salmon.

The optic tectum of the salmon is a primary visual center with direct input from the retina via the optic tract. The structure is homologous with the superior colliculus of the mammalian brain. We have studied the distribution of immunoreactivity against the GABAA/benzodiazepine receptor beta2/beta3 subunits with a monoclonal antibody (BD-17) in the optic tectum of the salmon brain. A weak immunoreactivity is found in the rostral stratum marginale (SM), strong labelling of the neuropil is shown in a thin band in stratum opticum (SO), two bands in stratum fibrosum et griseum superficiale (SFGS) and two bands in stratum griseum centrale (SGC). Immunoreactive perikarya with neurites that extend radially through the stratum album centrale (SAC) are located in the stratum periventriculare. BD-17 immunoreactivity is to a great extent located in tectal layers that receive direct retinal input, i.e. the SO, SFGS and SGC. These layers are known to receive input also from other visual centers, such as the pretectum (SO, SFGS), the nucleus isthmi (SO, SFGS, SGC), as well as non-visual regions as the telencephalon (SGC). High levels of 2-[125I]-iodomelatonin binding sites have previously been demonstrated in all layers of the salmon optic tectum except the SM and SPV. Thus it appears likely that GABA and/or benzodiazepines and melatonin play a role in visual processing in the optic tectum of teleost fish.

Tangential neuronal migration in the avian tectum: cell type identification and mapping of regional differences with quail/chick homotopic transplants

This paper is a sequel to a previous report, using quail/chick chimeras with partial tectal transplants, in which a tangential invasion of host (chick) tectal territories by cells originating in the quail graft was demonstrated. The cells displaying this secondary tangential migration appeared restricted to two strata (stratum griseum centrale (SGC) and stratum griseum et fibrosum superficiale (SGFS)). Here we describe the morphology of the tangentially displaced neurons, as well as their overall distribution in the host tectal lobe, by means of an antibody that specifically recognizes quail cells, staining them in a Golgi-like manner. Neurons that migrated into the SGC are identified as multipolar projection neurons, typical of this stratum. The majority of cells that migrated into the SGFS correspond to horizontal neurons, as was also corroborated by observations in Golgi-impregnated material. These horizontal cells are concentrated in laminae b, d and f, where their processes form well delimited axonal plexuses. In confirmation of previous results, SGC neurons have a limited range of migration, whereas SGFS cells translocate across much longer distances. In reconstructions of appropiate cases, a remarkable polarity was noted. Significant invasion of chick tectum by quail cells mostly occurred in the rostral half of the host tectum. The long-range migration of superficial horizontal cells frequently reached, but did not cross, the rostral tectal boundary. Conversely, tangential migration in the caudal half of the host tectum was scarce and coincided with a typical arrangement of quail-derived radial columns interdigited with chick-derived columns. These findings are discussed in relation to existing data on immature neuronal populations, molecular marker distribution and polarity of the avian optic tectum.

Interspecific Allometry of the Teleost Visual System; a New Approach

The capabilities of spatial vision of 31 species of fishes were estimated using a mathematical model that is based upon the allometry of the eye and the retinorecipient tectal layers, the Stratum opticum (SO) and the Stratum fibrosum et griseum superficiale (SFGS). The four parameters of the model are the focal length of the eye, the linear size of SO + SFGS, the thickness of SO + SFGS and the density of the optic projection within these layers. The visual index is defined as the product of these parameters, which show a high mutual positive correlation. The volume of SO + SFGS is, irrespective of fish size and species, linearly proportional to the retinal surface area. To establish intraspecific allometric relations, the allometric function y = kxd(1-alogx) was used, with d being a natural number dependent on the dimensions of x and y. The visual index correlates with published data on psychophysically determined minimum resolvable angles and it appears to be interrelated with behavioural and environmental characteristics. High indices are characteristic of pelagic, diurnal species living in clear waters and are mostly found among predatory and especially piscivorous species. Species with low indices live in turbid waters and/or are often benthic. Water depth and social behaviour appear to be unrelated to the visual index. The visibility is the characteristic which is strongest related to the visual index.

Distribution, laminar location, and morphology of tectal neurons projecting to the isthmo‐optic nucleus and the nucleus isthmi, pars parvocellularis in the pigeon (Columba livia) and chick (Gallus domesticus): A retrograde labelling study

Retrograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L), fluorogold, fast blue, rhodamine labelled microspheres, and horseradish peroxidase (HRP) was employed to study the distribution, laminar location within the optic tectum, and morphology of tectal cells projecting upon the isthmo-optic nucleus (ION) and the nucleus isthmi, pars parvocellularis (Ipc), in the pigeon and chick. Following injections into the ION, all retrograde markers labelled tecto-ION neurons and their dendrites in the ipsilateral tectum. The cells were located within a relatively narrow band at the border between layers 9 and 10 of the stratum griseum et fibrosum superficiale (SGFS). Retrogradely labelled neuronal somata were different in both dendritic branching and shape; however, tecto-ION neurons generally possessed non-spiny radially oriented and multi-branched dendrites. The apical processes extended into the retino-recipient layers (2-7) of the SGFS and basal dendrites extended into layers 12-14 of the SGFS. Positive neuronal somata were observed throughout the rostro-caudal extent of the optic tectum. The average distance between adjacent tecto-ION neurons varied from one region to another. Specifically, retrogradely labelled cells were more numerous in the caudal, lateral, and ventral tectum, and less numerous at rostro-dorsal levels. Approximately 12,000 tecto-ION neurons were labelled within the ipsilateral optic tectum following either PHA-L or fluorescent dye injections. While the regional distribution of tecto-Ipc neurons was not examined, the morphology indicated that the cells had a single radially oriented dendritic process. Therefore, the apical dendrites are more restricted than those of tecto-ION cells. Moreover, the dendrites were spiny and arborized within layers 3, 5, and 9 of the ipsilateral optic tectum. The axon of tecto-Ipc cells arise from the apical process as a shepherd's crook and descend into the deep layers of the optic tectum. These results indicate that 1) tecto-ION and tecto-Ipc neurons are possibly monosynaptically activated by retinal input, 2) tecto-ION neurons are heterogeneous in morphology, and 3) there is a differential distribution of the tecto-ION neurons throughout the rostro-caudal extent of the optic tectum, suggesting a greater representation of the caudo-ventral portion of the optic tectum within the ION. The discussion primarily concerns the organization of the retino-tecto-ION-retinal circuit in light of the distribution and morphology of tecto-ION neurons within the optic tectum.

Immunohistochemical localization of choline acetyltransferase in the chicken mesencephalon.

Choline acetyltransferase, a specific marker for cholinergic neurons, has been immunohistochemically localized in the mesencephalon and in the caudal diencephalon of the chicken. A complete series of transverse sections through the mesencephalon is presented. In the diencephalon, cholinergic fibers were found in the stria medullaris, the fasciculus retroflexus, and the ventral portion of the supraoptic decussation. The nucleus triangularis and the nucleus geniculatus lateralis, pars ventralis also contained cholinergic fibers. Small cholinergic cell bodies were found in the medial habenula. In the pretectum, cholinergic fibers innervated the nucleus lentiformis mesencephali and the tectal gray. The nucleus spiriformis lateralis also contained cholinergic fibers, while most of the cell bodies in the nucleus spiriformis medialis were cholinergic. In the mesencephalon, labelled fibers were found in the nucleus intercollicularis and in all layers of the optic tectum except the stratum opticum. The highest density of tectal cholinergic fibers was in the stratum griseum et fibrosum superficiale (SGFS), layer f. Radial cells located in SGFS, layer i were also cholinergic. In the isthmic nuclei, cholinergic fibers were found in the pars magnocellularis, while the pars parvicellularis and the nucleus semilunaris contained labelled cells. The oculomotor, Edinger-Westphal, trochlear, and trigeminal motor nuclei all had cholinergic cell bodies. Cholinergic axons were present in the oculomotor and trochlear nerves. In the tegmentum, cell bodies were labelled in the nucleus mesencephalicus profundus, pars ventralis, while the nucleus interpeduncularis had dense cholinergic innervation. Our localization of cholinergic cell bodies and fibers has been compared with earlier autoradiographic and anatomical studies to help define cholinergic systems in the avian brain. For example, the results indicate that the chicken may have a cholinergic habenulointerpeduncular system similar to that reported in the rat. Establishing the cholinergic systems within the avian midbrain is important for designing future neurophysiological and pharmacological studies of cholinergic transmission in this region.

Spatial arrangement of radial glia and ingrowing retinal axons in the chick optic tectum during development

Neuroanatomical tracing of retinal axons and axonal terminals with the fluorescent dye, Dil, was combined with immunohistochemical characterization of radial glial cells in the developing chick retinotectal system. Emphasis was placed on the mode of the tectal innervation by individual retinal axons and on the distribution and fate of the tectal radial glial cells and their spatial relation to retinal axons. It was obvious from fluorescent images obtained from anterogradely filled axons that these axons deserted the superficial stratum opticum (SO) to penetrate the stratum griseum et fibrosum superficiale (SGFS) by making right-angled turns within the SO. Frequently, axons which had invaded the SGFS were bifurcated and had a superficial branch which remained within the SO. Terminal axonal arborization occurred at various depths within the SGFS. Characterization of the tectal glial cells and their radial fibers by means of the anti-filament antibody, R5, and post-mortem staining with the fluorescent dye, Dil, revealed the following. (a) At least from day E8 to P1, tectal glial fibers traversed all tectal layers from the periventricular location of their somata to the superficial interface between SO and pia mater. In this interface they enlarged and formed characteristic endfeet. (b) Glial endfeet covered the whole tectal surface. They showed at early ages anterior-posterior differences having a higher density in the posterior tectum. These differences disappeared at embryonic day E13. (c) After innervation, glial endfeet of the anterior tectal third were arranged in rows parallel to the retinal fibers within the SO. This arrangement was not observed in eyeless embryos. (d) Radial glial fibers could be stained with R5 from day E8 to late embryonic stages throughout their entire length. (e) At the first posthatching days, only the segments of the radial glial fibers restricted to the thickness of the SO were R5-positive, although the fibers still traversed throughout the depth of the tectum. The results are discussed in context to the genesis of the retinotectal projection.

Locally correlated activity in the goldfish tectum in the absence of optic innervation

Despite a substantial literature on the role of correlated presynaptic activity in the patterning of nerve connections during synaptogenesis in the central nervous system, few studies have focused on postsynaptic activity during this process. To address the possibility that the target exhibits correlated activity independently of presynaptic activity, extracellular activity was recorded from the main optic innervation layer stratum fibrosum et griseum superficiale (SFGS) in goldfish in which the optic nerve was crushed or the eye removed. At about 2 weeks after denervation, multiunit recordings revealed phasic temporally correlated discharge between different tectal units. Auto-correlation analysis of these trains showed a broad peak 75–100 ms wide confirming temporal correlation. Using cross-correlation analysis of two simultaneous recordings at different distances across tectum, this correlation was shown to be local. Strong positive correlations were detected over about 200 μm and decrease with greater distances disappearing beyond about 400 μm. These correlograms showed a broad symmetrical peak about 75–100 ms wide. This pattern of activity persisted from the day following nerve crush into the period of activity dependent reinnervation at 1 month. When the eye was removed, the pattern could be demonstrated for up to 3 months of denervation indicating the circuitry responsible for the correlated activity was quite stable in the absence of optic innervation. We conclude that tectal elements are capable of locally correlated discharge independently of optic innervation. We propose that locally correlated discharge represents cooperative groups of tectal cells and that these groups, rather than single cells, are the target of the activity dependent synaptic rearrangement such as ocular dominance columns which occurs during synaptogenesis.