The retinofugal pathways in the primitive African bony fish Polypterus senegalus (Cuvier, 1829)

Retinal projections were studied using Fink-Heimer and radioautographic methods in Polypterus senegalus, a species which is representative of a small group of African fresh-water bony fish often considered to be very primitive.The large optic nerve showed partial decussation at the chiasm. Two major contralateral tracts were observed: the axillary and marginal optic tracts, with the latter being subdivided posteriorly into the tractus opticus medialis and tractus opticus lateralis. The retina projected onto the: (1) hypothalamus (area optica postoptica); (2) thalamus (nucleus opticus dorsolateralis thalimi, nucleus dorsomedialis thalami, corpus geniculatum laterale, area optica dorsolateralis thalami, area optica ventrolateralis thalami); (3) pretectum (nuclei commissurae posterioris, pretectalis ventralis, pretectalis dorsalis); and (4) optic tectum (stratum marginale, stratum opticum, stratum griseum et fibrosum superficiale, stratum griseum et album centrale, stratum griseum et fibrosum periventriculare). Ipsilateral retinal projections were demonstrated to the same 4 levels and more precisely to the nucleus opticus dorsolateralis thalami, area optica dorsolaterale thalami, nucleus commissurae posterioris, stratum marginale and stratum griseum et album centrale. The existence of a retinal projection to the mesencephalic tegmentum is discussed.Comparing the primary optic system of Polypterus with that of other jawed vertebrates, and particularly with that of other bony fish, indicated that this species possesses a combination of characteristics which are both actinopterygian and sarcopterygian. The phylogenetic significance of this mozaic anatomical arrangement is discussed.     

Diameters and terminal patterns of retinofugal axons in their target areas: an HRP study in two teleosts (Sebastiscus and Navodon)

Studies in various vertebrate classes, particularly amphibians and mammals, have revealed that retinal ganglion cells with different functional properties project by means of axons of correspondingly different diameters onto specific target regions. Whether a similar pattern exists in teleosts is partly investigated in the present study. HRP was injected into the optic nerve of Sebastiscus and Navodon. The calibers of intraretinal HRP-labeled axons were classed as fine (ca. 0.8 micron), medium (ca. 1.3 micron), and coarse (ca. 2.5 microns). The calibers of HRP-labeled retinofugal axons were then determined in their target areas, and these can be summarized as follows: Optic hypothalamus: fine, medium. Lateral geniculate nucleus: fine. Dorsolateral thalamic nucleus: fine, medium. Area pretectalis: fine. Nucleus of the posterior commissure: fine, medium. Area ventralis lateralis, contralateral: fine, medium, coarse; ipsilateral: coarse. Optic tectum, stratum opticum: fine, medium; stratum fibrosum et griseum superficiale: fine, medium, coarse, segregated in sublayers; stratum album centrale: fine, medium, coarse. Therefore, fine fibers were found to reach all target areas except the ipsilateral area ventralis lateralis, and these were the only fibers found in the lateral geniculate nucleus, area pretectalis, and stratum griseum centrale of the optic tectum. Coarse fibers, on the other hand, were found only in the area ventralis lateralis and the optic tectum (stratum fibrosum et griseum superficiale and stratum album centrale). Terminal patterns of these fibers were also studied. Most fine fibers take tortuous courses giving off a few branches and terminate with many varicosities, and medium and coarse fibers give off several finer branches and terminate with bulbous swellings. The physiological significance of these findings is discussed. In addition, retrogradely labeled (retinopetal) cells were found in the olfactory bulb and the area ventralis pars ventralis of the telencephalon, as well as in the preoptic area and the dorsolateral thalamic nucleus.     

Tectal projection neurons to the retinopetal nucleus in the filefish

Following horseradish peroxidase (HRP) injections into the preoptic retinopetal nucleus (PRN), neurons in the ipsilateral optic tectum were labeled retrogradely. Labeled neurons exhibited a 'Golgi-like' appearance, somata of these neurons were pyriform or round, and most of them were located in the stratum album centrale (SAC) or the stratum periventriculare (SPV). These neurons had a long apical dendrite, which ramified in the upper-half of SGC into horizontally arborized dendritic fields. The main trunk of the apical dendrites also gave off several branches in the stratum fibrosum et griseum superficiale (SFGS) and reached the stratum opticum (SO). These neurons resemble the 'large pyriform neurons' of Vanegas et al. (Vanegas, H., Laufer, M. and Amat, J., The optic tectum of a perciform teleost. I. General configuration and cytoarchitecture, J. Comp. Neurol., 154 (1974) 43-60) except that in the tecto-PRN neurons the axons originates from the apical dendritic shaft at or near the level of the SAC. Judging from their dendritic patterns, the tectal neurons projecting indirectly to the retina may receive non-retinal inputs besides the retinal input.     

Calretinin-like immunoreactivity in the optic tectum of the tench (Tinca tinca L.)

The distribution of calretinin-like immunopositive cells and fibers in the optic tectum of the tench (Tinca tinca) was studied by using a polyclonal antibody and the avidin-biotin-peroxidase technique. A clear laminated pattern of calretinin-like immunoreactivity was observed. The stratum periventriculare demonstrated a large number of strongly labeled cells whereas in the strata album centrale and griseum centrale, and at the boundary between the strata griseum centrale and fibrosum et griseum superficiale, some scarce, weakly immunostained cells were observed. No immunoreactive cells were seen in the strata fibrosum et griseum superficiale, opticum and marginale. Cells belonging to neuronal types X and XIV, previously characterized using Golgi impregnation, were found to be calretinin-like immunoreactive. Most calretinin-like immunopositive fibers were found in the strata fibrosum et griseum superficiale and opticum with a distribution pattern similar to retinotectal axons in these layers. In agreement with previous biochemical studies, our data suggest that, by contrast to all other classes of vertebrates, instead of calretinin and calbindin D-28k, only one protein is present in teleosts. Nevertheless, the calretinin-like immunostaining pattern in the teleost optic tectum was more complex than that previously described for calbindin D-28k. When compared to the calretinin-immunostaining in the rat superior colliculus, it is evident the presence in both amniotes and anamniotes of calretinin-immunopositive retinotectal axons. However, the distribution patterns of intrinsic calretinin-immunoreactive cells were different. Immunolabeled cells have been described in all layers of the superior colliculus, whereas the cells containing calretinin were restricted to the three deep strata of the tench optic tectum, a more similar distribution to what has been reported in the chick optic tectum.     

Immunohistochemical localization of nicotinic acetylcholine receptor subunits in the mesencephalon and diencephalon of the chick (Gallus gallus).

Monoclonal antibodies against two alpha-bungarotoxin-binding subunits (alpha 7 and alpha 8) of the nicotinic acetylcholine receptors (nAChRs) were used as immunohistochemical probes to map their distribution in the chick diencephalon and mesencephalon. The distribution of the alpha 7 and alpha 8 nAChR subunits was compared to the distribution of immunoreactivity produced by a monoclonal antibody against the beta 2 structural subunit of the nAChRs. Structures that contained high numbers of alpha 7-like immunoreactive (LI) somata included the intergeniculate leaflet, nucleus intercalatus thalami, nucleus ovoidalis, organum paraventricularis, nucleus rotundus, isthmic nuclei, nucleus trochlearis, oculomotor complex, nucleus interstitio-pretecto-subpretectalis, stratum griseum centrale of the optic tectum, and nucleus semilunaris. Neuropil staining for alpha 7-LI was intense in the nucleus dorsomedialis hypothalami, nucleus geniculatus lateralis ventralis, griseum tecti, isthmic nuclei, nucleus lentiformis mesencephali, nucleus of the basal optic root, and stratum griseum et fibrosum superficiale of the tectum. High numbers of alpha 8-LI somata were found in the stratum griseum et fibrosum superficiale of the tectum and the nucleus interstitio-pretecto-subpretectalis, and intense neuropil staining for alpha 8-LI was found in the dorsal thalamus, nucleus geniculatus lateralis ventralis, lateral hypothalamus, griseum et fibrosum superficiale of the tectum. High numbers of beta 2-LI somata were found only in the nucleus spiriformis lateralis, whereas neuropil staining for beta 2-LI was intense in the nucleus geniculatus lateralis ventralis, nucleus suprachiasmaticus, nucleus lateralis anterior, nucleus habenularis lateralis, area pretectalis, griseum tecti, nucleus lentiformis mesencephalis, nucleus externus, and nucleus interpeduncularis, and in the stratum griseum centrale, stratum griseum et fibrosum superficiale, and stratum opticum of the tectum. These results indicate that there are major disparities in the localization of the alpha-bungarotoxin-binding alpha 7 and alpha 8 nAChR subunits and the beta 2 structural nAChR subunit in the chick diencephalon and mesencephalon. These nAChR subunits appear, however, to coexist in several regions of the chick brain.     

Hereditary visual impairment in a new mutant strain of chicken, GSN/1

Visual pathways of a new mutant of chicken (GSN/1) with hereditary visual impairment were ophthalmologically, electrophysiologically, and histopathologically examined. Clinically, GSN/1 chickens drooped and had decreased locomotor activity. As an indicator of visual acuity, the spatial frequency characteristics of GSN/1 chickens showed poor scores at high frequency. No abnormal findings were observed ophthalmoscopically in the fundus of them. The amplitudes of a and b waves of the electroretinogram in GSN/1 chickens revealed no abnormal findings for xenon flash stimuli with different intensities. However, responses of the visual evoked potential in GSN/1 chickens were insensitive to xenon flash stimuli. Histologically, the retina of GSN/1 chickens was slightly hypoplastic and the retinal ganglion cells decreased in number, although there were no degenerative or reactive changes. The optic tectum, especially the stratum opticum and the stratum griseum et fibrosum superficiale were hypoplastic and contained reduced numbers of optic nerve fibers. The sublayers of the stratum griseum et fibrosum superficiale were disorganized, in which axons of the optic nerve were distributed irregularly. These findings suggest that visual impairment observed in the new mutant GSN/1 chicken may be related to developmental defects in the visual pathways, especially in the optic tectum.     

A quantitative study of the relative contribution of different retinal sectors to the innervation of various thalamic and pretectal nuclei in goldfish

The contribution of retinal ganglion cells situated in different retinal quadrants to the innervation of eight nontectal, retinorecipient targets was examined in goldfish. In some fish, cobaltous-lysine was used to selectively fill severed intraretinal ganglion cell axons and the number of filled axons within each nucleus was determined. In other fish, either the dorsal or ventral or nasal or temporal retina was ablated and the remaining axons from the intact retina were filled with cobalt. The density of the cobalt-filled axons within the retinorecipient targets was quantified with a microdensitometer. All of the eight targets received different degrees of innervation when the contributions from dorsal and ventral retina were compared. The suprachiasmatic nucleus received axons from ventral, but not from dorsal, retinal ganglion cells (RGCs), while the nucleus opticus dorsolateralis, nucleus opticus commissurae posterior, and nucleus opticus pretectalis dorsalis received more axons from ventral than from dorsal RGCs. The tuberal region, nucleus corticalis, and the accessory optic nucleus received axons from dorsal, but not from ventral, RGCs. The nucleus opticus pretectalis ventralis received more axons from dorsal then from ventral RGCs. Only one target, nucleus corticalis, appeared to receive more axons from nasal than from temporal RGCs. In general, those nuclei that were closest to the dorsal optic tract were innervated exclusively or predominantly by ventral RGC axons, whereas those nuclei that were closest to the ventral optic tract were innervated exclusively or predominantly by dorsal RGC axons. These data indicate that in this particular vertebrate, the dorsal and ventral retinal regions are not homogeneous with respect to their projections to nontectal nuclei. The possible role that the nontectal nuclei play in determining the course of optic axons is discussed.     

A comparative neuroanatomic study of retinal projections in two fishes: Astyanax hubbsi (the blind cave fish), and Astyanax mexicanus.

Retinofugal projections in the blind cave fish A. hubbsi and in the highly visual A. mexicanus were studied with both reduced silver and autoradiographic methods. Contrary to what has been reported for other teleosts, ipsilateral, as well as the generally accepted contralateral, projections were found in A. mexicanus. Bilateral retinofugal projections were traced to the dorsolateral thalamic nucleus and area pretectalis. Contralateral projections were traced to the lateral geniculate nucleus, nucleus pretectalis, accessory optic nucleus, nucleus corticalis, nucleus opticus hypothalamicus and the superficial layers of the optic tectum (strata opticum, fibrosum and griseum superficiale, and the cellular zone of griseum centrale). Retinal efferents in the blindfish, A. hubbsi, are sparse and totally crossed. Areas receiving a retinal projection include nucleus opticus hypothalamicus, lateral geniculate and the superficial layers of the medial third of the optic tectum. Preliminary behavioral studies are described and discussed in relation to the possible visual potential of this teleost.     

Growing and regenerating axons in the visual system of teleosts are recognized with the antibody RT97

We have analyzed the immunolabeling with the antibody RT97, a good marker for ganglion cell axons in several species, in the normal and regenerating visual pathways of teleosts. We have demonstrated that RT97 antibody recognizes several proteins in the tench visual system tissues (105, 115, 160, 200, 325 and 335 kDa approximately). By using immunoprecipitation and Western blot we have found that after crushing the optic nerve the immunoreactivity to anti RT97 increased markedly in the optic nerve. In immunohistochemical analysis we also found a different pattern of labeling in normal and regenerating visual pathways. In normal tench RT97 is a good marker for the horizontal cells in the retina, for growing ganglion cell axons which run along the optic nerve from the retina to the optic tectum and of the axon terminals in the stratum opticum and stratum fibrosum and griseum superficiale in the optic tectum. After optic nerve crush, no immunohistochemistry modifications were observed in the retina. However, in accordance with Western blot experiments, in the optic nerve intensely stained groups of regenerating axons appeared progressively throughout the optic nerve as far as the optic tectum. We conclude that the antibody RT97 is an excellent marker of growing and regenerating axons of the optic nerve of fish.     

Glutamate-immunoreactivity in the retina and optic tectum of goldfish

Glutamate was immunohistochemically localized in the goldfish retina and tectum at the light and electron microscopic (E.M.) levels using double affinity purified antisera against glutaraldehyde conjugated L-glutamate. In retina, glutamate-immunoreactivity (Glu+) was observed in cone inner segments, cone pedicles, bipolar cells, a small number of amacrine cells and the majority of cells in the ganglion cell layer. The latter were shown to be ganglion cells by simultaneous retrograde labeling. Centrally, Glu+ was observed in axons in the optic nerve and tract, and in stratum opticum and stratum fibrosum et griseum superficialis (SFGS) of the tectum. The Glu+ in the optic pathway disappeared four days after optic denervation and was restored by regeneration without affecting the Glu+ of intrinsic tectal neurons. In tectum, Glu+ was also observed in torus longitudinalis granule cells, toral terminals in stratum marginale, some pyramidal neurons in the SFGS, multipolar and fusiform neurons in stratum griseum centrale, large multipolar and pyriform projection neurons in stratum album centrale, and many periventricular neurons. Glu+ was also localized within unidentified puncta throughout the tectum and within radially oriented dendrites of periventricular neurons. At the E.M. level, a variety of Glu+ terminals were observed. Glu+ toral terminals formed axospinous synapses with dendritic spines of pyramidal neurons. Ultrastructurally identifiable Glu+ putative optic terminals formed synapses with either Glu+ or Glu- dendritic profiles, and with Glu- vesicle-containing profiles, presumed to be GABAergic. These findings are consistent with the hypothesis that a number of intrinsic and projection neurons in the goldfish retinotectal system, including most ganglion cells, may use glutamate as a neurotransmitter.     

A radioautographic study of retinal projections in Type I and Type II lizards

The retinofugal projections of 5 species (Acanthodactylus boskianus, Scincus scincus, Tarentola mauritanica, Uromastix acanthinurus and Zonosaurus ornatus) belonging to 5 different families of Type I and Type II lizards have been examined by means of the radioautographic method. In the 5 species the retinal ganglion cells project to the contralateral hypothalamus (nucleus suprachiasmaticus), thalamus (nucleus geniculatus lateralis pars ventralis, nucleus geniculatus lateralis pars dorsalis), pretectum (nuclei lentiformis mesencephali, geniculatus pretectalis, postero-dorsalis griseus tectalis), tectum opticum (layer 2 to layer 6 of the stratum griseum et fibrosum superficiale) and tegmentum mesencephali (nucleus opticus tegmenti). Ipsilateral optic fibers were never observed in Uromastix acanthinurus, whereas an uncrossed quota was visible in both nucleus geniculatus lateralis pars dorsalis and nucleus postero-dorsalis in the other species. An ipsilateral retinotectal projection was observed only in Tarentola mauritanica. With the exception of the nucleus griseus tectalis the contralateral optic centers identified in this material have to a large extent been observed in other reptiles belonging to the different orders. The presence in reptiles of a general pattern of contralateral visual projections indicates that these were established very clearly in the course of evolution. Similarities become apparent when this plan is compared with that observed in birds. In marked contrast the ipsilateral component in reptiles is unstable and mutable in nature. This ipsilateral retinotectal projections do not appear to be a feature restricted to Type I lizards. On the other hand, the presence of this optic component cannot be linked solely to nocturnal habits.     

The projection of the retina upon the optic tectum of the pigeon.

The projection of the retina onto the optic tectum of the pigeon has been investigated using silver impregnation methods for degenerating axons and terminals, autoradiographic tracing, and the Golgi methods. Degenerating optic afferents distribute to sublaminae a-d and f of the stratum griseum et fibrosum superficiale over the whole tectum, but two major fields are observed. One occupies the caudal and ventral tectum (in which region laminar cytoarchitecture is marked), and the other rostral and dorsal tectum (where demarcation of cell laminae is relatively poor). Degeneration in the latter field is coarse and clearly distributes in a distinctly laminated fashion within the stratum griseum et fibrosum superficiale. In contrast, degeneration in the caudo-ventral region is fine, and laminated distribution less clear. The evolution of the degeneration pattern over survival periods from 3 to 56 days suggests that these laminar distributions reflect the existence of several different types of optic terminal ramification present in all parts of the tecum. A selective laminar distribution of such optic afferent types correlates well with our own and other Golgi studies. The results of the autoradiography experiments are broadly compatible with these findings.     

Quantitative histological studies of the optic tectum in six species of Notropis and Cyprinella (Cyprinidae, Teleostei).

Significant differences in stratification and size of the visual layers of the optic tectum were found between three clear-water minnows (Notropis amabilis, N. boops, Cyprinella venustas) and three turbid-water minnows (N. atherinoides, N. bairdi, and C. lutrensis). Correlations among a variety of neural structures suggested the importance of stratum marginale (SM), stratum opticum (SO), and stratum fibrosum et griseum superficiale (SFGS), stratum griseum centrale (SGC) and stratum periventriculare (SPV) in vision, of stratum album centrale (SAC) and SGC for olfaction, and of SPV for the processing of acoustico-lateral information.     

Topography of the retinal projection to the superficial pretectal parvicellular nucleus of goldfish: a cobaltous-lysine study

The retinal projection to the superficial pretectal parvicellular nucleus (SPp) of goldfish was examined by filling select groups of optic axons with cobaltous-lysine. The tracer was applied intraocularly to peripheral retinal slits in some fish. In other fish, it was applied to optic axons from an intact hemiretina after one-half of the retina was ablated and the corresponding optic axons had degenerated. The results indicated that SPp is a folded structure, having a dorsal surface innervated by axons from temporal retinal ganglion cells and a ventral surface innervated by axons from nasal retinal ganglion cells. Peripheral retina innervates the anterodorsal and anteroventral edges of SPp, while central retina innervates the posterior genu. Dorsal retina innervates lateral SPp and ventral retina innervates medial SPp. Thus, although SPp is a folded nucleus, the topography of the retino-SPp projection is similar to the topography of the retinotectal projection. That is, the relative position of optic axons within SPp mirrors the retinal location of the ganglion cells that project to SPp. Retino-SPp axons occupy the center of the main optic tract before it divides into the two optic brachia. These axons are topographically arranged, with temporal retino-SPp axons being flanked on both sides by nasal retino-SPp axons. Retino-SPp axons arborize within SPp and then continue to enter the superficial tectal retino-recipient lamina. Thus, these axons innervate both SPp and the optic tectum. These findings are discussed with respect to chemospecific and morphogenetic views of visual system topography.     

A reconsideration of the primary visual system of the turtle Emys orbicularis.

The retinocerebral projections of Emys orbicularis were investigated by means of [3H]-proline or HRP, administered by intraocular injection. Two newly-hatched, two juvenile and seven adult specimens were examined. The results reveal contralateral retinal projections to fifteen sites: two in the hypothalamus (the nuclei suprachiasmaticus and periventricularis), five in the thalamus (the nuclei ovalis, geniculatus lateralis ventralis, geniculatus laleralis dorsalis, dorsolateralis anterior and ventrolateralis), five in the pretectal region (the nuclei geniculatus pretectalis, opticus pretectalis ventrolateralis, lentiformis mesencephali, posterodorsalis and griseus tectalis), two in the optic tectum (the stratum opticum and the stratum fibrosum et griseum superficiale), and one in the tegmentum (the nucleus opticus tegmenti). Ipsilateral projections to nine of these sites at thalamic, pretectal, tectal and tegmental levels, while weak, could be clearly demonstrated. These results differ considerably from those obtained in a previous investigation using a Nauta-paraffin technique; it is suggested that the differences are due to limitations of the latter technique. A review of the existing literature on the Chelonian primary visual system reveals considerable terminological diversity, and a standard nomenclature for the primary visual centres of turtles is proposed.     

Depth segregation of retinal ganglion cells projecting to mouse superior colliculus

The retinotectal projections in the mouse were analyzed with injections of horseradish peroxidase into the superior colliculus and of radioactive amino acids into the eye. At least 70% of the ganglion cells, and possibly all of them, were found to project to the superior colliculus, including ganglion cells of all sizes. Small injections revealed that ganglion cells of different sizes terminate at different levels in the superior colliculus. The small ganglion cells that form the vast majority of all cells project predominantly to the upper stratum griseum superficiale. A small population of mainly medium-sized and large ganglion cells project to the deep stratum griseum superficiale and to the stratum opticum. The ipsilateral projection is restricted to the deep stratum griseum superficiale and stratum opticum and consists predominantly of medium-sized and large ganglion cells.     

Microglia in normal and regenerating visual pathways of the tench (Tinca tinca L., 1758; Teleost) : a study with tomato lectin

We have studied the microglial cells in the normal and regenerating visual pathways of Tinca tinca (Cyprinid, Teleost) by using the lectin from Lycopersicum esculentum (tomato), which, in our case, has been demonstrated as a specific marker for teleost microglia. In the normal fish, there are tomato lectin positive microglial cells in the retina, optic nerve, and optic tectum. Following optic nerve crush, we observed a more extensive labeling of the microglia in the crushed optic nerve and in the contralateral optic tectum affecting the stratum opticum and stratum fibrosum et griseum superficiale. In both cases, there was an increase of rounded and Icss ramified microglial cells, and granular cells. This response of a more extensive labeling of microglial cells increases to a maximum at 2–3 weeks after the crush; the density of labeled microglial cells decreases after 3 months after crushing. However, in the retina no changes were observed after optic nerve crush. These results suggest that the microglial cells could play an important role in regeneration of fish optic pathway, as other neuroglial cells do.     

Trajectories of regenerating retinal axons in the goldfish tectum: II. Exploratory branches and growth cones on axons at early regeneration stages

HRP was applied to small sites in the dorsotemporal or dorsonasal retina in fish at 10-36 days after optic nerve section. The anterogradely labeled axons were visualized in tectal whole mounts. Axons traveled through all regions of the tectum in various abnormal routes. Misrouted axons were also seen to alter their orientation and to direct their course toward their target. At all regeneration stages the majority of dorsotemporal axons coursed and achieved target-related orientations preferentially within the rostral tectal half whereas dorsonasal axons proceeded into the caudal tectum. The growing axons exhibited various morphologies. All axons in the superficial fascicle layer stratum opticum (SO) and some in the synaptic layer stratum fibrosum et griseum superficiale (SFGS) were unbranched and tipped with a leading growth cone. Other axons in the synaptic layer carried one to several growth cones at their ends and often filopodia proximal to the growth cone, or they had sprouted numerous side branches with growth cones and filopodia on the shaft and on branches. Some axons at retinotopic or ectopic sites gave rise to several long branches of several hundred microns in length, with growth cones and filopodia. From 32 days onward axons ending in terminal arbors at retinotopic sites became apparent. Thus, numerous axons at early regeneration stages go through a phase of exploratory growth on their way toward their target sites.     

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.     

Optic tectum of the eastern garter snake, Thamnophis sirtalis. II. Morphology of efferent cells

Tectal efferent neurons were retrogradely filled from extracellular injections of horseradish peroxidase (HRP) into pathways efferent from the tectum. Tectorotundal neurons have cylindrical dendritic trees, 80-100 microns in diameter, that extend vertically across the central and superficial tectal layers. Apical and basal dendrites are laden with complex appendages. The axon gives rise to an intratectal, collateral arbor that extends horizontally into the stratum griseum centrale beyond the cell's dendritic tree. The parent axon exits the tectum laterally in the tectothalamic tract. Tectogeniculate neurons also have narrow, radially oriented, and highly branched apical dendrites, but their basal dendrites are infrequently branched and lack appendages. An intratectal axon collateral forms a small, spherical arbor overlapping the apical dendrites in sublayer c of the stratum fibrosum et griseum superficiale. The parent axon ascends vertically and just below the stratum opticum turns rostrad to follow the optic fibers to the diencephalon. Tectoisthmi neurons have small somata and thin, radial dendrites that arborize below the pial surface in the stratum zonale. An intratectal axon collateral forms a spatially restricted arbor ventral to the soma in register with the dendritic tree. Tectoisthmobulbar neurons have dendrites that arborize extensively in sublayer a of the stratum fibrosum et griseum superficiale. The axon exits the tectum without collateralizing and joins a small-caliber component of the ventral tectobulbar tract. Ipsilateral tectobulbar neurons have stellate dendritic fields, 150-250 microns in diameter, that are restricted to the deep layers of the tectum. Sparsely branched dendrites are appendage-free but bear many short, fine spicules. The axon initially ascends from the soma and recurves into the stratum album centrale without collateralizing before joining a medium-caliber component of the ventral tectobulbar tract. Crossed tectobulbar neurons have large, stellate dendritic trees with diameters ranging from 200 to 500 microns. Like ipsilateral tectobulbar neurons, their dendrites are appendage-free but bear spicules. Their thick-caliber axons exit the tectum without collateralizing and course deep in the stratum album centrale to reach the dorsal tectobulbar tract.