Developmental changes in the spatial pattern of mesencephalic trigeminal nucleus (Mes5) neuron populations in the developing chick optic tectum

The developing mesencephalic trigeminal nucleus (nucleus of the fifth cranial nerve; Mes5) is composed of four neuron populations: 1) the medial group, located at the tectal commissure; 2) the lateral group distributed along the optic tectum hemispheres; 3) a group outside the neural tube; and 4) a population located at the posterior commissure. The present work aims to elucidate the site of appearance, temporal evolution, and spatial distribution of the four Mes5 populations during development. According to detailed qualitative observations Mes5 neurons appear as a primitive unique population along a thin dorsal medial band of the mesencephalon. According to quantitative analyses (changes in cell density along defined reference axes performed as a function of time and space), the definitive spatial pattern of Mes5 neurons results from a process of differential cell movements along the tangential plane of the tectal hemispheres. Radial migration does not have a relevant developmental role. Segregation of medial and lateral group populations depends on the intensity of the lateral displacements. The mesenchymal population appears as an outsider subset of neurons that migrate from the cephalic third of the neural tube dorsal midregion to the mesenchymal compartment. This process, together with the intensive lateral displacements that the insider subset undergoes, contributes to the disappearance of this transient population. We cannot find evidence indicating that neural crest-derived precursors enter the neural tube and differentiate into Mes5 neurons. Our results can be better interpreted in terms of the notion that a dorsal neural tube progenitor cell population behaves as precursor of both migrating peripheral descendants (neural crest) and intrinsic neurons (Mes5).     

Synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi in Rana pipiens

The nucleus isthmi is reciprocally connected to the ipsilateral optic tectum. Ablation of the nucleus isthmi compromises visually guided behavior that is mediated by the tectum. In this paper, horseradish peroxidase (HRP) histochemistry and electron microscopy were used to explore the synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi. After localized injections of HRP into the optic tectum, there are retrogradely labeled isthmotectal neurons and orthogradely labeled fibers and terminals in the ipsilateral nucleus isthmi. These terminals contain round. Clear vesicles of medium diameter (40–52 nm). These terminals make synaptic contact with dendrites of nucleus isthmi cells. Almost half of these postsynaptic dendrites are retrogradely labeled, indicating that there are monosynaptic tectoisthmotectal connections. Localized HRP injection into the nucleus isthmi labels terminals primarily in tectal layers B, E, F, and 8. The terminals contain medium-sized clear vesicles and they form synaptic contacts with tectal dendrites. There are no instances of labeled isthmotectal terminals contacting labeled dendrites. Retrogradely labeled tectoisthmal neurons are contacted by unlabeled terminals containing medium-sized and small clear vesicles. Fifty-four percent of the labeled fibers connecting the nucleus isthmi and ipsilateral tectum are myelinated fibers (average diameter approximately 0.6 μm). The remainder are unmyelinated fibers (average diameter approximately 0.4 μm). © 1994 Wiley-Liss, Inc.     

Oculomotor and sensory mesencephalic trigeminal neurons in lungfishes: phylogenetic implications.

The location and number of neurons in the brainstem with projections to the eye muscles were investigated by means of fluorescent tracers in the African lungfish Protopterus dolloi. The oculomotor nucleus (M III) projects bilaterally with a ratio of 3:1 (70 ipsilateral, 20 contralateral neurons). Three subdivisions of this nucleus can be differentiated: one projects exclusively ipsilaterally, another projects exclusively contralaterally, and a third component projects bilaterally. The trochlear nucleus (M IV) is located caudally, distinct from M III, and projects predominantly to contralateral eye muscles with a ratio of 6:1 (18:3 neurons). The abducens nucleus (M VI) contains about 30 neurons with ipsilateral projections only. There is no evidence for an accessory abducens nucleus in Protopterus. Intraocular injections of tracers do not reveal any retinopetal projections in Protopterus. The mesencephalic trigeminal nucleus (Mes V) of Protopterus and Neoceratodus contains about 540-590 neurons on each side. In juvenile Protopterus, up to 75 Mes V neurons are located caudally in a ventral projection of the tectum above the velum medullare anterius. Fifty-five Mes V neurons (10% of the total number) have processes that exit the brain with the trochlear nerve. The relatively large number of Mes V neurons in lungfishes correlates with the well developed jaw musculature. The present study provides the first conclusive evidence for the location of oculomotor subdivisions in the brain of a lepidosirenid lungfish. The organization of the oculomotor nucleus is consistent with the observation that lungfishes possess the pattern of eye-muscle innervation seen in elasmobranchs and supports the unconventional view that lungfishes may be the sistergroup of elasmobranchs.     

Reevaluation of projections from the mesencephalic trigeminal nucleus to the medulla and spinal cord: New projections. A combined retrograde and anterograde horseradish peroxidase study

Microinjection of horseradish peroxidase (HRP) into the medullary parvocellular reticular formation (NPvc) resulted in retrograde labeling of neurons throughout the mesencephalic trigeminal nucleus (Mes V). Labeled cells were large and ovoid and were distributed primarily in the expanded pontine part of the nucleus. However, none of the small neurons in Mes V were labeled. Injections of HRP made into adjacent brainstem structures including the nucleus gigantocellularis, ventrolateral reticular formation, vestibular complex, and the spinal trigeminal nucleus failed to label neurons in Mes V. Injections made into the medullary raphe and into regions reported to receive inputs from Mes V–spinal cord, nucleus tractus solitarius, hypoglossal nucleus, and facial nucleus–were also not followed by transport to Mes V. Anterograde axonal transport of HRP from the region of reticular formation innervated by Mes V also labeled axons projecting to Mes V and to visceral and somatic sensorimotor nuclei in the lower brainstem. Recent reports of afferents from the amygdala to Mes V suggest that reflexes involving the mesencephlic trigeminal nucleus might be modulated by signals from limbic and autonomic as well as somatic centers in the brain.     

The afferent connections of the tectum mesencephali in two chondrichthyans,the shark Scyliorhinus canicula and the ray Raja clavata

The afferent connection of the tectum mesencephali were studied in the spotted dogfish Scyliorhinus canicula and the thornback ray Raja clavata by means of the horseradish peroxidase (HRP) technique. Following unilateral injections in the tectum, labeled neurons could be identified in all main divisions of the brain and in the cervical spinal cord. Telencephalic neurons which project to the tectum mesencephali were observed in the caudal part of the pallium. Diencephalic projections to the tectum originate from the thalamus dorsalis pars medialis, the thalamus ventralis pars lateralis, the nucleus medius infundibuli, and the pretectal area. In Scyliorhinus labeled neurons could also be found in the corpus geniculatum laterale. Mesencephalic cells of origin of tectal afferent pathways were identified in the stratum cellulare externum of the contralateral tectum, in the nucleus tegmentalis lateralis, in the ventrolateral tegmentum, and in the nucleus ruber. Rhombencephalic cells projecting to the tectum could be identified in the nucleus cerebelli (only in Scyliorhinus), the nucleus vestibularis superior, the reticular formation, the nucleus funiculi lateralis, the nucleus tractus descendens nervi trigemini, and the nucleus dorsalis and intermedius areae octavolateralis. In addition a number of small-and medium-sized cells of the reticular formation were found labeled. Diffusely scattered labeled cells could be observed in the dorsal part of the cervical spinal cord. It is concluded that the tectal afferent connections in the chondrichthyans studied in general resemble those of other vertebrates, but that some striking differences exist. In particular, tectal afferents originating from the nucleus medius infundibuli, the nucleus cerebelli, and the nucleus dorsalis and intermedius areae octavolateralis have not been reported in other vertebrates.     

Localization of neurons afferent to the optic tectum in longnose gars

Afferent pathways to the optic tectum in the longnose gar were determined by unilateral tectal injections of HRP. Retrogradely labeled cells were observed in the ipsilateral caudal portion of the rostral entopeduncular nucleus and bilaterally in the rostral half of the lateral zone of area dorsalis of the telencephalon. The following diencephalic cell groups were also labeled following tectal injections: the ipsilateral anterior, ventrolateral, and ventromedial thalamic nuclei, the periventricular pretectal nucleus, and the central pretectal nucleus (bilaterally); the ventromedial thalamic and central pretectal nuclei revealed the largest number of labeled cells. At midbrain levels, retrogradely labeled cells were seen in the ipsilateral torus longitudinalis, nucleus isthmi, and accessory optic nucleus; cells were labeled bilaterally in the torus semicircularis and a rostral tegmental nucleus. Only a few cells were labeled in the contralateral optic tectum, suggesting that few of the fibers of the intertectal commissure are actually commissural to the tectum. At hindbrain levels, retrogradely labeled cells were seen bilaterally in the locus coeruleus, the superior, medial, and inferior reticular formations, the eurydendroid cells of the cerebellum, and the nucleus of the descending trigeminal tract; the contralateral dorsal funicular nucleus also exhibited labeling. Clearly, the tectum in gars receives a substantial number of nonvisual afferents from all major brain areas, most of which have been reported in other vertebrates. The functional significance of these afferent sources and their probable homologues in other vertebrate groups are discussed.     

Neuronal death during development in the isthmo-optic nucleus of the chick: sustaining role of afferents from the tectum

Neurons have been counted in the isthmo-optic nucleus following lesions of the optic tectum, its main source of afferents. Late lesions, made at 10.8-12.2 days of incubation, were employed as they cause the fewest non-specific side effects. The lesions spared the isthmo-optic tract, and although they caused many retinal ganglion cells to die, the degeneration did not spread to the inner nuclear layer, which contains the target cells of the isthmo-optic fibers. Hence the effects on the isthmo-optic nucleus were due to its being deprived of afferents. Even in unoperated embryos, 60% of the isthmo-optic neurons are known to die between embryonic days 12 and 17. The tectal lesions greatly increased the cell loss ipsilaterally; this was due to cell death, since other explanations such as migration away or differential cellular shrinkage have been ruled out. The fact that additional neuronal death occurred mainly during the latter half of the period of natural cell death implies that the tectal afferents are important for the survival of the isthmo-optic neurons during this latter half, but not before.     

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.     

Cytoarchitecture,fiber connections,and ultrastructure of the nucleus pretectalis superficialis pars magnocellularis (PSm) in carp

The cytoarchitecture, fiber connections, and ultrastructure of the nucleus pretectalis superficialis pars magnocellularis (PSm) were studied in cypriniform teleosts (Cyprinus carpio). The PSm is an oval nucleus in the pretectum. Medium-sized cells and synaptic glomeruli are the main components of the nucleus. A lesser number of small cells are also present. Most of the medium-sized cells form one or two cell layers on the periphery of the nucleus, and some cells are scattered among synaptic glomeruli in the nucleus. Cell bodies in the peripheral cell layer are pyriform and sprout a thick dendrite directed inward. The dendrite gives off fine dendritic branches, which are postsynaptic elements in synaptic glomeruli. The PSm projects to the ipsilateral corpus mamillare (CM) and sends collaterals to the ipsilateral nucleus lateralis valvulae (NLV). Axons of the PSm neurons have terminals with many varicosities in the CM, and collaterals in the NLV have cup-shaped terminals around the cell bodies of the NLV neurons. Following horseradish peroxidase (HRP) injections into the PSm, HRP-labeled cells are found ipsilaterally in the optic tectum, the nucleus tractus rotundus of Schnitzlein, and the nucleus ruber of Goldstein. The tecto-PSm projections are topographically organized. The rostral optic tectum projects mainly to the rostral portion of the PSm, and the caudal tectum projects to the caudal portion of the PSm. The ventral tectum sends fibers mainly to the ventral part of the PSm. The dorsomedial tectum projects to the medial part of the PSm, and the dorsolateral tectum projects to the lateral part of the PSm. Tectal projection neurons to the PSm are of only one type. The tectal cell body is pyriform and is situated in the superficial part of the ipsilateral stratum periventriculare (SPV). The tectal neurons have a long perpendicular dendrite, which branches out in the stratum opticum (SO). An axon emerges from the branching site in the SO. Judging from the dendritic branching pattern of the tectal projection neurons, we concluded that the PSm receives visual information from the optic tectum. © 1993 Wiley-Liss, Inc.     

Spinal projections from the mesencephalon in the toad

Mesencephalic cell groups projecting to the spinal cord have been identified by means of the retrograde axonal transport of the enzyme horseradish peroxidase (HRP). The injections were made either in the cervical or lumbar enlargements of the toad spinal cord. Following injections in the cervical cord, labeled cells are located in the isthmus region, in the ipsilateral laminated nucleus posteroventralis tegmenti mesencephali (Potter). At more rostral levels the labeled cells are in the nucleus of the fasciculus longitudinalis medialis, in the nucleus interstitialis of the fasciculus longitudinalis medialis, in the contralateral red nucleus, in lamina six of the contralateral optic tectum, bilaterally in the nucleus of the posterior commissure and in the mesencephalic nucleus in the Vth nerve. Injections in the lumbar cord label neurons of the nucleus posteroventralis tegmenti mesencephali (Potter) and nucleus interstitialis of the fasciculus longitudinalis medialis. Nuclei that had not been previously identified in anurans but which were labeled after HRP spinal injections (i.e., the nucleus interstitialis of the fasciculus longitudinalis medialis, the nucleus of the posterior commissure and the red nucleus) have been delimited in normal material in Nissl-stained transverse sections. The spinal pathways from the mesencephalon can be classified into four projections: reticulospinal, rubrospinal, tectospinal and trigeminospinal. A comparison of these descending fiber systems with homologous pathways in other vertebrate species has been made.     

Changes in the topographically organized connections between the nucleus isthmi and the optic tectum after partial tectal ablation in adult goldfish

The projection of the nucleus isthmi to the ipsilateral optic tectum was examined in normal goldfish. This was compared to the projection in animals in which the entire visual field had been induced to compress onto a rostral half tectum by caudal tectal ablation. The isthmo-tectal projection was examined by making localized injections of horseradish peroxidase into the optic tecta and observing the patterns of labeled cells within the nucleus isthmi. The teleost nucleus isthmi consists of a cell sparse medulla covered by a cellular cortex, which is thick on the rostral, medial, and dorsal surfaces of the nucleus. Almost all isthmic cells projecting to the tectum were located in the area of thick cortex. In normal fish, rostral tectal injections labeled cells in the rostroventral portion of the thick cortex; injections midway in the rostrocaudal tectal axis labeled more caudodorsally located cells, and caudal tectal injections labeled cells a little further caudally in extreme dorsal cortex. The rostroventral to caudodorsal isthmic axis was therefore seen to project rostrocaudally along the tectum. This topography contrasts somewhat with the situation seen in amphibia where the rostrocaudal tectal axis receives projections from the rostrocaudal isthmic axis. In fish with half-tectal ablations, injections near the caudal edge of the half tectum (at a site that had originally been midtectal) labeled cells that had previously projected to caudal tectum. Rostral tectal injections in fish with compression of the visual field gave a normal pattern of labeled isthmic cells. The results indicate that a topographically ordered isthmo-tectal projection exists in goldfish that may be induced to compress onto a half tectum.     

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.     

The neuronal architecture of the inferior colliculus in the cat: Defining the functional anatomy of the auditory midbrain

This study defines anatomical subdivisions in Golgi-impregnated material from the inferior colliculus of the cat. The findings demonstrate that the inferior colliculus consists of a mosaic of morphologically distinct parts of neuropil. Each part is also characterized by a unique set of neuronal types. Each part of the inferior colliculus can be defined as tectal or tegmental on the basis of the fundamental pattern of dendritic branching. The main subdivisions of the auditory tectum are the central nucleus, the cortex, and the paracentral nuclei. The central nucleus is distinguished by its laminated neuropil composed of neurons with disc-shaped dendritic fields oriented in parallel arrays with the lemniscal axons. In contrast, the cortex is identified by its broad layers of loosely woven neuropil, which are orthogonal to those in the central nucleus and lack neurons with disc-shaped dendritic fields. The paracentral nuclei, so called because of their scattered arrangement around the central nucleus, are the commissural, dorsomedial, rostral pole, lateral, and ventrolateral nuclei. The main subdivisions of the auditory tegmentum are the pericollicular areas, the nucleus of the brachium of the inferior colliculus, and the sagulum. The pericollicular areas are intercollicular or subcollicular and separate the tectal division from the superior colliculus, central gray, and remaining portions of the tegmentum. The afferent projections to each tectal and tegmental subdivision, as observed in silver-degeneration experiments, distinguish the parcellations based on the Golgi findings. Subdivisions containing tectal cell types receive afferents predominantly from the auditory pathways, in contrast to subdivisions with tegmental cell types, which receive inputs from a wide variety of sources. This suggests a correlation between neuronal types and the nature of their inputs. This analysis of the subdivisions of the inferior colliculus differs from previous studies, especially those relying on Nissl stains. It is likely that subdivisions distinguished by the pattern of the neuropil differ functionally, since the structural components identified in the Golgi-impregnated material are essential parts of the synaptic organization of the auditory midbrain. Future physiological studies should benefit from approaches in which the cell types serve as the focus for the analysis.     

Morphology and location of tectal projection neurons in frogs: A study with hrp and cobalt-filling

Tectal projection neurons were labeled by retrograde transport of horseradish peroxidase (HRP) or cobaltic-lysine. The tracer substances were delivered iontophoretically or by pressure injection or diffusion into various regions of the brain or spinal cord. Histochemical procedures allowed identification of labeled cells projecting to the injected regions. Many neurons were filled with cobaltic-lysine, resulting in a Golgi-like staining. After cobalt injections in the diencephalon most of the labeled cells, identified as small piriform neurons, were located in layer 8 of the tectum. Two types of small piriform neurons were distinguished. Type 1 neurons have flat dendritic arborizations confined to lamina D, while the dendrites of type 2 cells may span all of the superficial tectal strata. In smaller numbers large piriform, pyramidal, and ganglionic cells of the periventricular tectal layers were labeled after diencephalic injections. Rhombencephalic cobalt and HRP injections labeled cells whose axons form the tectobulbospinal tract. The neurons most frequently labeled were large ganglionic cells. Ipsilaterally, the majority of their somata were located in layer 7, and their dendrites arborized mainly in lamina F. Con-tralaterally, labeled ganglionic cell somata occupied the top of layer 6, and most of their dendritic end-branches entered lamina B. The possible functional significance of this anatomical arrangement is discussed. After tectal cobalt injections the topography of the tectoisthmic projection and the terminals of tectal efferent fibers in the diencephalon and brainstem were observed. It is concluded that the organization of frog tec-tofugal pathways is very similar to that of mammals.     

Afferent connections of the optic tectum in lampreys: an experimental study

Tectal afferents were studied in adult lampreys of three species (Ichthyomyzon unicuspis, Lampetra fluviatilis, and Petromyzon marinus) following unilateral BDA injections into the optic tectum (OT). In the secondary prosencephalon, neurons projecting to the OT were observed in the pallium, the subhipoccampal lobe, the striatum, the preoptic area and the hypothalamus. Following tectal injections, backfilled diencephalic cells were found bilaterally in: prethalamic eminence, ventral geniculate nucleus, periventricular prethalamic nucleus, periventricular pretectal nucleus, precommissural nucleus, magnocellular and parvocellular nuclei of the posterior commissure and pretectal nucleus; and ipsilaterally in: nucleus of Bellonci, periventricular thalamic nucleus, nucleus of the tuberculum posterior, and the subpretectal tegmentum, as well as in the pineal organ. At midbrain levels, retrogradely labeled cells were seen in the ipsilateral torus semicircularis, the contralateral OT, and bilaterally in the mesencephalic reticular formation and inside the limits of the retinopetal nuclei. In the hindbrain, tectal projecting cells were also bilaterally labeled in the dorsal and lateral isthmic nuclei, the octavolateral area, the sensory nucleus of the descending trigeminal tract, the dorsal column nucleus and the reticular formation. The rostral spinal cord also exhibited a few labeled cells. These results demonstrate a complex pattern of connections in the lamprey OT, most of which have been reported in other vertebrates. Hence, the lamprey OT receives a large number of nonvisual afferents from all major brain areas, and so is involved in information processing from different somatic sensory modalities.     

Afferents of the lamprey optic tectum with special reference to the GABA input: combined tracing and immunohistochemical study

The optic tectum in the lamprey midbrain, homologue of the superior colliculus in mammals, is important for eye movement control and orienting responses. There is, however, only limited information regarding the afferent input to the optic tectum except for that from the eyes. The objective of this study was to define specifically the gamma-aminobutyric acid (GABA)-ergic projections to the optic tectum in the river lamprey (Lampetra fluviatilis) and also to describe the tectal afferent input in general. The origin of afferents to the optic tectum was studied by using the neuronal tracer neurobiotin. Injection of neurobiotin into the optic tectum resulted in retrograde labelling of cell groups in all major subdivisions of the brain. The main areas shown to project to the optic tectum were the following: the caudoventral part of the medial pallium, the area of the ventral thalamus and dorsal thalamus, the nucleus of the posterior commissure, the torus semicircularis, the mesencephalic M5 nucleus of Schober, the mesencephalic reticular area, the ishtmic area, and the octavolateral nuclei. GABAergic projections to the optic tectum were identified by combining neurobiotin tracing and GABA immunohistochemistry. On the basis of these double-labelling experiments, it was shown that the optic tectum receives a GABAergic input from the caudoventral part of the medial pallium, the dorsal and ventral thalamus, the nucleus of M5, and the torus semicircularis. The afferent input to the optic tectum in the lamprey brain is similar to that described for other vertebrate species, which is of particular interest considering its position in phylogeny.     

Tectal connections in Python reticulatus

The origins of the axons terminating in the mesencephalic tectum in Python reticulatus were examined by unilateral tectal injections of horseradish peroxidase. Kutrogradely labeled cells were observed bilaterally throughout the spinal cord in all subdivisions of the trigeminal system, with the exception of nucleus principalis, which showed labeled cells only on the ipsilateral side. Labeling of the reticular formation occurred bilaterally in nucleus reticular is interiormagnocellularis, nucleus reticularis lateralis, nucleus reticularis medius and the mesencephalic reticular formation. The tectum also receives bilateral projections from the dorsal tegmentaJ field, the nucleus of the lateral lemniscus and nucleus isthmi, and ipsilateral projections from nucleus profundus mesencephali. A few labeled cells were found ipsilaterally in the locus coeruleus and in nuclei vestibulares ventrolateralis and centromedialis. In the diencephalon labeled cells were observed ipsilaterally in nucleus ventrolateralis thalami, nucleus ventromedialis thalami, nucleus suprapeduncularis, and in the dorsal and ventral lateral geniculate nuclei. Bilateral labeling was observed in nucleus periventricularis hypo-thalami. Furthermore, labeling was ipsilaterally present in the ventral telen-cephalic areas. The tectum in Python reticulatus receives a wide variety of afferent connections which confirm the role of the tectum as an integration center of visual and exteroceptive information.     

Frog tectal efferent axons fail to regenerate within the CNS but grow within peripheral nerve implants

Tectal efferent axons, located adjacent to the optic tract, fail to regenerate past diencephalic lesions in Rana pipiens even though optic axons regenerate after the same injury (M. J. Lyon and D. J. Stelzner, J. Comp. Neurol. 255: 511-525). We tested the possibility that tectal efferent axons can regenerate within peripheral nerve implants. A 6- to 8-mm segment of autologous sciatic nerve was implanted into the anterolateral (N = 23) or centrolateral (N = 22) portion of the dorsal surface of the tectum. Frogs survived for 6 (N = 16) or 12 weeks (N = 29) before the free end of the nerve was recut and HRP applied. A control group had the nerve crushed prior to the HRP application. Neurons within the tectum, near and medial to the implant site, were retrogradely labeled from the nerve graft in most experimental operates but no neurons were labeled in controls. In addition, neurons were also labeled in nuclei which projected to the tectum in a number of cases. Three times as many neurons were labeled in 12-week operates (42 +/- 46) as in 6-week operates (15 +/- 12). The morphology and location of labeled neurons in the tectum was similar to tectal efferent neurons except that the somal area of neurons labeled from the graft was significantly larger (41%) than normal tectal efferent neurons. The basic finding is similar to experiments using the same paradigm in the mammalian central nervous system (CNS). One difference is the minimal glial reaction at the graft insertion site.(ABSTRACT TRUNCATED AT 250 WORDS)     

Topographic projections between the nucleus isthmi and the optic tectum in a teleost,Navodon Modestus

Following horseradish peroxidase injections into the optic tectum of a teleost,Navodon modestus, reciprocal and topographic projections between the nucleus isthmi and the ipsilateral optic tectum were determined. The isthmo-tectal fibers diverge to the optic tectum while maintaining the spatial arrangements of the isthmic cells from which the fibers originate. The tecto-isthmic projections also keep the spatial arrangements in the optic tectum. The tectal fibers converge near the nucleus isthmi and terminate in the non-cellular portion of the nucleus. The reciprocal topography is apparent in the combined results of 9 experiments with one tectal injection in each region. No labeled cells and fibers were found in the contralateral nucleus isthmi.