The nucleus isthmi as an intertectal relay for the ipsilateral oculotectal projection in the frog, Rana pipiens

Reconstruction of serial sections through the nucleus isthmi in the frog Rana pipiens shows that the cortex of the nucleus consists of a sheet of cells which is folded back on itself to form two more or less parallel faces. Cyto-architectural features allow three regions to be distinguished: a 7“rim region,” which previous work had suggested may be involved in the physiological pathway from one eye to the ipsilateral tectal lobe, and two additional regions which are here termed the anterior and posterior “nonrim cortex,” re-spectively. The cytoarchitectural features which distinguish the rim region from the other two regions are largely absent in the tadpole. Analysis of retrograde transport of horseradish peroxidase (HRP) following widespread injections into one tectal lobe indicates that the anterior nonrim cortex projects ipsilaterally while the rim cortex and posterior nonrim cortex both project contralaterally. The differing projections of the three regions are paralleled by the pattern of retrograde degeneration observed after unilateral tectal lesions. We have studied the topographic organization of the connections between the nucleus isthmi and the two tectal lobes by making small injections of HRP at tectal loci with known visual field input. Patterns of retrograde cellular labelling show that the anterior nonrim cortex projects topographically to the entirety of the tectal lobe on the same side of the brain. The rim region projects topographically to the binocularly activated part of the opposite tectal lobe; the posterior nonrim cortex projects to the monocularly activated part. There is a discontinuity in the mapping from the nucleus to the opposite tectal lobe which corresponds to the border between binocular and monocular tectum. Patterns of anterograde labelling indicate that the afferent projection from the ipsilateral tectal lobe is topographic and organized so that afferents from a given tectal locus contact cells in the rostral part of the nucleus which project back to that locus. Afferents from tectal re-gions representing binocular visual field in addition continue across the nucleus to terminate within the rim region. The organization of the afferent and efferent projections of this region is such as to link tectal loci which relate to the same direction in visual space. Our findings provide new evidence that the nucleus isthmi is involved in the pathway from one eye to the ipsilateral tectal lobe. They also show how the nucleus is organized to distribute information from a given locus in one tectal lobe to cells which project to the visually corresponding but in general anatomically different pairs of loci in the two tectal lobes. Finally, our findings confirm the existence of a projection from the nucleus isthmi to the opposite monocular tectum and suggest that this may be part of an intertectal circuit linking anatomically corresponding rather than visually corresponding pairs of tectal loci.     

Restoration of vision in genetically eyeless axolotls (Ambystoma mexicanum).

Eyes grafted into genetically eyeless axolotls at embryonic stages 26 or 27 (early tailbud stage) are capable of establishing retinotectal connections and restoring near normal vision. Normal vestibulo-ocular reflexes are also present in most of the eyeless mutants having grafted eyes. The animals are capable of accurately localizing objects in visual space and demonstrate following movements in an optokinetic drum. Evoked potentials can be recorded from the surfaces of the tectal lobes of eyeless mutants having a right eye graft which do not differ significantly from those recorded from a normal animal, except that recordings can still be obtained from the ipsilateral tectal lobe in the former following section of the intertectal fibers. This indication of direct retinotectal connections to the ipsilateral tectum was confirmed by histological examination which also showed that the optic fibers entering the diencephalon high on the lateral wall are initially directed toward the normal optic tract position before proceeding to be tectum.     

Recovery of the ipsilateral oculotectal projection following nerve crush in the frog: evidence that retinal afferents make synapses at abnormal tectal locations

The ipsilateral oculotectal projection in the frog is a topographic mapping of the binocular part of the visual field of one eye on the ipsilateral tectal lobe. The underlying neuronal circuitry consists of the topographic, crossed retinotectal projection and an intertectal pathway which relays information from a given point in one tectal lobe to the visually corresponding point in the other. During optic nerve regeneration, there is a period when the terminals of retinotectal afferents are found at abnormal locations in the opposite tectal lobe. Whether they form functional synapses at this time is not known. If so, one would expect to observe correlated abnormalities in the ipsilateral oculotectal projection. To determine whether such abnormalities exist, we have made parallel electrophysiological studies of the recovery of the retinotectal and ipsilateral oculotectal projections following crush of one optic nerve. The earliest stage of recovery was characterized by a lack of significant topographic order in the retinotectal projection and by the absence of a physiologically observable ipsilateral projection. Within a short time, the retinotectal projection became topographically organized and a similarly organized ipsilateral projection appeared. While topographic, the retinotectal projection at intermediate times was abnormal in that the multiunit receptive fields recorded at individual tectal loci were greatly enlarged. Multiunit receptive fields were similarly enlarged in the ipsilateral projection. In addition, some ipsilateral fields included areas of visual space not normally represented in the projection. The abnormalities in both projections subsequently disappeared over the same time course. Throughout recovery there was a high correlation between multiunit receptive field sizes in the contralateral tectal lobe and those at visually corresponding points in the ipsilateral tectal lobe. Enlarged multiunit receptive fields in the contralateral tectal lobe could not be accounted for in terms of optical or retinal abnormalities since single unit receptive field sizes were normal. Nor could they be accounted for in terms of changes in recording characteristics since simultaneously recorded fields activated by the undisturbed eye were normally sized. We conclude that the enlarged fields in the contralateral tectal lobe indicate the presence at individual tectal loci of afferents from wider than normal retinal regions. Similar considerations ruled out optical, retinal, and recording abnormalities as the explanation for the enlarged multiunit receptive fields in the ipsilateral tectal lobe.(ABSTRACT TRUNCATED AT 400 WORDS)     

Afferent and efferent connections of the optic tectum in the carp (Cyprinus carpio L.)

The afferent and efferent connections of the tectum opticum in the carp (Cyprinus carpio L.) were studied with the HRP method. Following iontophoretic peroxidase injections in several parts of the tectum anterograde transport of the enzyme revealed tectal projections to the lateral geniculate nucleus, dorsal tegmentum, pretectal nuclei, nucleus rotundus, torus longitudinalis, torus semicircularis, nucleus isthmi, contralateral tectum and to the mesencephalic and bulbar reticular formations.Tectal afferents were demonstrated by retrograde HRP transport in the area dorsalis pars centralis of the telencephalon, torus longitudinalis, torus semicircularis, nucleus isthmi, nucleus profundus mesencephali, several pretectal nuclei, dorsomedial and dorsolateral thalamic nuclei, nucleus of the posterior commissure, mesencephalic and bulbar reticular nuclei and nucleus ruber. Visuo-cerebellar circuitry was investigated by means of peroxidase injections in the various parts of the cerebellum. These experiments revealed indirect retino- and tecto-cerebellar pathways via the pretectal nuclei and the nucleus isthmi.     

Retinotectal map formation in dually innervated tecta: A regeneration study in Xenopus with one compound eye following bilateral optic nerve section

Retinotectal map formation was studied during regeneration in young adult Xenopus. Right compound double-temporal eyes (TT) were formed in tailbud stage embryos by the fusion of two temporal halves of the eye blastema in the same orbit. In other animals right compound double-nasal eyes (NN) were prepared. In both combinations the left eye was kept intact. After metamorphosis the right and left optic nerves were sectioned to induce optic fiber regeneration from each eye to both tecta. The patterns of retinotectal projections from the compound and normal eyes were studied from 37 to 364 days after optic nerve section, using electrophysiological recording of the visuotectal projections and ³H-proline autoradiographic assay from one of the two eyes. The left normal eyes projected in a retinotopic fashion, across the entire extent of the right and left dually innervated tecta. In contrast, the right compound eye projections were confined to the rostrolateral or to the caudomedial part of the right and left tecta in TT and NN animals, respectively. These tectal areas corresponded to the termination of temporal and nasal hemiretinal fibers of the normal eye. Discontinuous, interdigitating projection patterns from the right and left eyes were found in parts of the tecta where the compound and the normal eye projections overlapped. These results indicate that the normal optic fiber projections caused the originally expanded compound eye projections to be restricted to the corresponding part of the dually innervated tecta. It is suggested that the orderliness and the extent of the retinotectal map are established by the competition and interaction of optic fibers based on stable positional programming of the retinal ganglion cells.     

Visual and electrosensory circuits of the diencephalon in mormyrids: an evolutionary perspective

Mormyrids are one of two groups of teleost fishes known to have evolved electroreception, and the concomitant neuroanatomical changes have confounded the interpretation of many of their brain areas in a comparative context, e.g., the diencephalon, where different sensory systems are processed and relayed. Recently, cerebellar and retinal connections of the diencephalon in mormyrids were reported. The present study reports on the telencephalic and tectal connections, specifically in Gnathonemus petersii, as these data are critical for an accurate interpretation of diencephalic nuclei in teleosts. Injections of horseradish peroxidase into the telencephalon retrogradely labeled neurons ipsilaterally in various thalamic, preglomerular, and tuberal nuclei, the nucleus of the locus coeruleus (also contralaterally), the superior raphe, and portions of the nucleus lateralis valvulae. Telencephalic injections anterogradely labeled the dorsal preglomerular and the dorsal tegmental nuclei bilaterally. Injections into the optic tectum retrogradely labeled neurons bilaterally in the central zone of area dorsalis telencephali and ipsilaterally in the torus longitudinalis, various thalamic, pretectal, and tegmental nuclei, some nuclei in the torus semicircularis, the nucleus of the locus coeruleus, the nucleus isthmi and the superior reticular formation, basal cells in the ipsilateral valvula cerebelli, and eurydendroid cells in the contralateral lobe C4 of the corpus cerebelli. Weaker contralateral projections were also observed to arise from the ventromedial thalamus and various pretectal and tegmental nuclei, and from the locus coeruleus and superior reticular formation. Tectal injections anterogradely labeled various pretectal nuclei bilaterally, as well as ipsilaterally the dorsal preglomerular and dorsal posterior thalamic nuclei, some nuclei in the torus semicircularis, the dorsal tegmental nucleus, nucleus isthmi, and, again bilaterally, the superior reticular formation. A comparison of retinal, cerebellar, tectal, and telencephalic connections in Gnathonemus with those in nonelectrosensory teleosts reveals several points: (1) the visual area of the diencephalon is highly reduced in Gnathonemus, (2) the interconnections between the preglomerular area and telencephalon in Gnathonemus are unusually well developed compared to those in other teleosts, and (3) two of the three corpopetal diencephalic nuclei are homologues of the central and dorsal periventricular pretectum in other teleosts. The third is a subdivision of the preglomerular area, rather than an accessory optic or pretectal nucleus, and is related to electroreception. The preglomerulo-cerebellar connections in Gnathonemus are therefore interpreted as uniquely derived characters for mormyrids.     

Anomalous retrograde labeling of brain cells from the eye in the goldfish: evidence for long distance growth of sprouted neurites

We have used retrograde labeling with horseradish peroxidase (HRP) and a wheat germ agglutinin conjugate of HRP (WGA:HRP) to investigate the projections of the nucleus postglomerulosus (nPg) both in normal goldfish and in animals which had undergone retinal removal. In normal animals, our evidence indicates that nPg projects only to the optic tectum. Using small HRP and WGA:HRP application sites in the tectum, we have shown that nPg cells have broadly spread terminals in the tectal neuropil and that there is no obvious correspondence between the rostrocaudal axis of the nPg and the deployment of the terminal arbors of its cells along the rostrocaudal axis of the tectum. In addition, we found no evidence for an nPg projection to the eye in normal animals. After retinal removal we found that nPg cells were more readily backfilled from small tectal applications of HRP. However, our most interesting observation was that at 4-6 weeks and more after ocular surgery, we could retrogradely label the cells of the nPg with intraocular or retroocular injections of WGA:HRP. At the same postoperative times, we were also able to label neurites in the atrophied optic nerve by microinjecting WGA:HRP into the contralateral midbrain tegmentum. Finally, we found that the cells of the nPg undergo a hypertrophic response, similar to that seen in other neurons after axotomy, following retinal removal or section of the dorsomedial brachium of the optic tract. Thus, these cells respond to retinal denervation of the tectum with a response characteristic of axotomized cells although their axons have not been cut. Similar changes were also seen in the nucleus isthmi on both sides of the brain following retinal removal. We interpret our data to indicate that cells of the nPg can respond to optic (and thus heterotypic) denervation of their terminal field by sprouting processes which grow away from the terminal field, through denervated optic pathways, to the retinaless eye. This interpretation requires that the sprouted processes grow for several millimeters.     

Anatomy and physiology of a binocular system in the frograna pipiens

The locations of tectal neurons projecting to nucleus isthmi (n. isthmi) were found by iontophoretic injection of horseradish peroxidase (HRP) into n. isthmi. After retrograde transport, stained tectal somata are found to lie almost exclusively in layer 6 and below of the ipsilateral tectum. Many cells are colored throughout the extent of their dendrites into the fine rami, giving the appearance of a Golgi stain. Nucleus isthmi receives projections from the ipsilateral tectum and from no other region. Nucleus isthmi units recorded electrically respond to visual stimuli and are arranged in a topographic map of the visual field. There are two types of receptive fields, those with small centers and those with large centers. The small centers are about 3-5 degrees in diameter, similar to type 2 optic nerve fibers. Their response is to many of the same geometric features of stimulus as excite type 2 fibers. The large centers are at least 7-10 degrees in diameter and respond to many of the same features as excite types 3 and 4 optic nerve fibers. The responsiveness of small and large center n. isthmi units is very similar to the elements of the ipsilateral visual field projection onto tectum, i.e. the neuropilar units recorded in layers A and 8 of the tectum when the contralateral eye is occluded. These are in strong contrast to those of tectal cells of layer 6 and below, which have large receptive fields, show far less vivacious response, adapt extremely rapidly to repeated stimuli and are hard to describe in terms of characteristic stimuli because they are unresponsive most of the time. We suggest, therefore, that the axons of tecto-isthmic cells are quite active and that their cell bodies, located in layer 6 and below, only fire occasionally on the firing of their axons.     

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.     

Tests for neuroplasticity in the anuran retinotectal system

Mapping of retinotectal projections in the tree frog Hyla regilla was carried out by both behavioral and electrophysiological recording techniques following tectal ablations, with and without optic nerve regeneration. Scotomata produced by unilateral and bilateral half tectum ablations and by unilateral rectangular midtectal excisions were found to remain essentially unaltered in all cases through recovery periods up to 334 days. Similarly, electrophysiological mapping of the rostral half tectum separated by Gelfilm implants from the caudal tectum for up to 191 days yielded a normal rostral visual field. The stability of the retinotectal projection in adult Hylidae observed in these experiments contrasts with the plastic readjustments obtained in young goldfish in which the retinotectal system is still probably growing and presumably capable of field regulation. The results are taken to support the original explanatory model for developmental patterning of retinotectal connections in terms of selective cytochemical affinities between retinal and tectal neurons.     

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.     

The development of the nucleus isthmi in Xenopus laevis. I. Cell genesis and the formation of connections with the tectum

The nucleus isthmi (NI) of the amphibian relays visual input from one tectum to the other tectum and thus brings a visual map from the eye to the ipsilateral tectum. This isthmotectal visual map develops slowly; it is first detected electrophysiologically at stages 60-62, the age at which the eyes begin their dorsalward migration and the region of binocular overlap beings to increase in extent. During this critical period of life, normal binocular visual input is required for establishment of normal topographic isthmotectal projections. In this study, we have used anatomical methods to trace cell birth, cell death, and formation of connections by the nucleus isthmi during the critical period. Tritiated thymidine labelling demonstrates that cells in the nucleus isthmi are generated throughout most of tadpole life (stages 29-62). Most cells conform to an orderly ventrodorsal gradient starting from stage 29 and extending to stages 56; later cells are inserted at apparently random locations in the nucleus. We have re-examined the hypothesis of Tay and Straznicky ('80) that the order of cell genesis in the NI and tectum could help establish proper isthmotectal connections, and we find that a timing mechanisms does not explain the two-dimensional topography of the isthmotectal map but that timing may aid in proper mediolateral positioning of isthmotectal axons at the points where they first enter the tectum. Horseradish peroxidase labelling was used to investigate whether anatomical projections from tectum to NI and from NI to tectum are present prior to the onset of eye migration. The results show that there are tectoisthmotectal projections by stage 52. Moreover, isthmotectal axons grow into as yet monocular tectal regions prior to the onset of eye migration. At stage 60, when binocular overlap begins, isthmotectal axons are visible throughout the tectum but are densely branched only at the rostral tectal margin, the location where they are predicted to occur on the basis of electrophysiological maps.     

Cytoarchitecture and fiber connections of the superficial pretectum in a teleost,Navodon modestus

Fiber connections of the so-called nucleus geniculatus lateralis (or the nucleus pretectalis superficialis pars parvocellularis) in a teleost, Navodon modestus, were examined by means of the horseradish peroxidase (HRP) tracing method. The nucleus receives fibers from the contralateral retina, ipsilateral optic tectum and nucleus isthmi, and projects bilaterally to the nucleus intermedius of Brickner and ipsilaterally to the optic tectum and raphe nuclei. The fiber connections suggest that the nucleus relays mainly visual information to the inferior lobe (hypothalamus) but not to the telencephalon. The nucleus is not a homologous structure to the lateral geniculate nucleus in other vertebrate classes.     

Relationship between isthmotectal fibers and other tectopetal systems in the leopard frog

We studied the relationship of isthmotectal input to other tectal afferent fiber systems in three ways. 1) Using horseradish peroxidase (HRP) histochemistry, we determined the nonretinal inputs to the superficial tectum. In different sets of animals we a) applied HRP to the tectal surface; b) inserted HRP crystals into the tectum; c) injected small volumes of HRP solutions into the superficial tectum. N. isthmi accounts for more than 65% of the nonretinal extrinsic input in the superficial tectal layers. One set of fibers from the contralateral n. isthmi projects to the most superficial layer. Fibers from posterior thalamus and tegmentum project to both superficial and deeper layers in the tectum, but not to the most superficial layer. The ipsilaterally projecting isthmotectal fibers terminate in the deeper superficial layers. 2) We investigated the relationship between retinofugal and contralaterally projecting isthmotectal pathways. We orthogradely labelled n. isthmi fibers by unilateral HRP injections into n. isthmi, and we also labelled retinal fibers by injecting tritiated l-proline into both eyes. In such animals contralaterally projecting isthmotectal fibers cross in the dorsal posterior region of the optic chiasm. From the chiasm to the tectum isthmotectal fibers and retinofugal fibers are admixed. 3) We determined whether other fiber systems cross with contralaterally projecting isthmotectal fibers. We cut the posterior part of the optic chiasm and applied HRP crystals to the cut. Only n. isthmi and retina are retrogradely labelled.     

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.     

Connections of the octopus optic lobe: An HRP study

The major visual centers of the octopus central nervous system are the paired optic lobes. Bidirectional transport of horseradish peroxidase (HRP) was used to determine connections of the optic lobe. Cells afferent to the optic lobe were identified by retrograde HRP transport in the following lobes of the central ganglia: anterior basal, median basal, dorsal basal, interbasal, subvertical, precommissural, brachial, and magnocellular. Labeled cells were also observed within the contralateral optic lobe, various optic tract lobes bilaterally, and in photoreceptors of the ipsilateral retina. Additionally, individual fibers, in part originating from cells in the posterior subvertical lobe, were labeled within the central neuropil core of various vertical lobules. Differences in results between superficial and deep optic lobe medulla injections indicate that some afferent projections from central sources may terminate on cell populations at specific depths within the lobe. Efferent optic lobe fibers into the superior frontal and lateral basal lobes were labeled by anterograde transport. Other possible optic lobe efferent projections terminated in supraesophageal lobes and the magnocellular lobe. The many inputs to the optic lobe from higher motor and associative centers in the central ganglia emphasize that the medulla region of the optic lobe is an exceptionally complex integrative area.     

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.     

Development of the retinotectal system in normal quail embryos: cytoarchitectonic development and optic fiber innervation

The development of the optic tectum and the establishment of retinotectal projections were investigated in the quail embryo from day E2 to hatching day (E16) with Cresyl violet-thionine, silver staining and anterograde axonal tracing methods. Both tectal cytodifferentiation and retinotectal innervation occur according to a rostroventral-caudodorsal gradient. Radial migration of postmitotic neurons starts on day E4. At E14, the tectum is fully laminated. Optic fibers reach the tectum on day E5 and cover its surface on day E10. 'Golgi-like' staining of optic fibers with HRP injected in vitro on the surface of the tectum reveals that: growing fronts are formed exclusively by axons extending over the tectal surface; fibers penetrating the outer tectal layers are always observed behind the growing fronts; the penetrating fibers are either the tip of the optic axons or collateral branches; as they penetrate the tectum, optic fibers give off branches which may extend for long distances within their terminal domains; the optic fiber terminal arbors acquire their mature morphology by day E14. The temporal sequence of retinotectal development in the quail was compared to that already established for the chick, thus providing a basis for further investigation of the development of the retinotectal system in chimeric avian embryos obtained after xenoplastic transplantation of quail tectal primordia into the chick neural tube.     

Relative number of cells projecting from contralateral and ipsilateral nucleus isthmi to loci in the optic tectum is dependent on visuotopic location: horseradish peroxidase study in the leopard frog.

The leopard frog optic tectum is the principal target of the contralateral retina. The retinal terminals form a topographic map of the visual field. The tectum also receives bilateral topographic input from a midbrain structure called nucleus isthmi. In this study we determined the relative strength of n. isthmi projections to different loci in the tectum. Horseradish peroxidase (HRP) was applied at single superficial tectal locations in a series of leopard frogs. The application sites were distributed across the tectum. Retrogradely filled cells were counted in ipsilateral and contralateral nucleus isthmi. Although all regions of the tectum receive input from both n. isthmi, the relative number of labeled cells in the two n. isthmi is dependent on visuotopic location. Input to the rostromedial tectum representing the visual field ipsilateral to the labeled tectum comes primarily from the contralateral n. isthmi. Input to the caudolateral tectum representing the visual field contralateral to the labeled tectum originates mostly from the ipsilateral n. isthmi. Tectal application sites representing the visual midline had approximately equal numbers of labeled cells in the two n. isthmi. The results are similar at postapplication survival times ranging from 2 to 14 days. Using application of HRP to rostral tectum and application of nuclear yellow to caudal tectum, we show that the anisotropy in isthmi labeling is not due to take up of these labels by isthmotectal fibers passing through the application sites that terminate elsewhere.     

Plasticity in the developing chick visual system: topography and maintenance of experimentally induced ipsilateral projections

The present work examines the topography of the contra- and ipsilateral centrifugal projections from the isthmooptic nuclei (IONs) to the remaining retina in monocular chick embryos. After removal of the left eyecup at embryonic day (E)1.5, the IONs were investigated at various embryonic stages by the retrograde transport of fluorescent dyes and horseradish perioxidase (HRP) injected into the remaining eye. The projection of the ipsilateral ION was consistently found at E13 and frequently disappeared by E18 to E19. Selective regional labeling of the remaining retina in monocular embryos with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate) revealed that the retinotopic order of the enhanced projection from the ipsilateral ION corresponded precisely to the normal one from the contralateral ION. The formation of the projection from the retina to the ipsilateral tectum was also investigated at E18 to E19 by means of intravitreally injected HRP or rhodamine-B-isothiocyanate (RITC) in monocular embryos after early eyecup removal. In cases with persistent ION, the eye enucleations resulted in ipsilateral retinotectal projections consisting of varying numbers of retinofugal fibers. The data are consistent with the view that there is a certain degree of plasticity in the embryonic development of the chick visual system. If an ION projection to the ipsilateral retina is strongly developed, it is retinotopically organized and probably influences the maintenance of the ipsilateral retinotectal projection. The stabilization of the otherwise transiently formed ipsilateral retinotectal projection may be influenced by the tectal neurons which receive retinal input and are efferently connected with persisting ION neurons.