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.     

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.     

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.     

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.     

An experimental study of the retinal projections of the European eel (Anguilla anguilla), carried out at the catadromic migratory silver stage

A radioautographic study of the European eel (Anguilla anguilla) was carried out in ten female specimens at the catadromic migratory silver stage. Terminal arborizations of contralaterally projecting visual fibres were identified in ten hypothalamic structures (area optica preoptica ventralis and the nuclei suprachiasmaticus, opticus hypothalamicus ventromedialis, preopticus magnocellularis lateralis, posterioris lateralis, posterioris dorsalis periventricularis posterioris dorsalis lateralis, posterioris dorsalis medialis, posterioris ventralis lateralis, and posterioris ventralis periventricularis), ten thalamo-pretectal structures (Areas C1 and C2, area optica tractus opticus ventrolateralis and the nuclei dorsolateralis thalami, ventrolateralis thalami pars ventralis, opticus ventralis thalami, geniculatus lateralis, opticus pretectalis partes dorsalis et ventralis, and opticus commissurae posterioris), and in the tectal strata opticum partes externa et interna, fibrosum et griseum superficiale, griseum centrale and album centrale. An accessory optic system was identified, and a contralateral retinal projection to the anterior region of the anterior semicircular torus (n. opticus dorsolateralis mesencephali) was identified. Ipsilateral projections to hypothalamic and thalamopretectal structures were also observed. Apart from the retinal projection to the preoptic area, which is exceptionally important in the silver eel, the general plan of organization of the primary visual centres of this form is comparable to that described in other species of teleost. However, the architecture of some primary visual centres shows characteristics similar to those described in more primitive Actinopterygians.     

Immunohistochemical localization of choline acetyltransferase in the chicken mesencephalon

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

Localization of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin binding to nicotinic sites in the chicken forebrain and midbrain.

We have previously localized cholinergic cell bodies and fibers within the midbrain of the chicken with choline acetyltransferase immunohistochemistry. In a continuing effort to characterize the central cholinergic system, the present study examines the distribution of various nicotinic acetylcholine receptors in the forebrain and midbrain of the chicken. The binding of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin was localized by film autoradiography in adjacent sections of the adult chicken brain, allowing a comparison of the distribution of different classes of nicotinic binding sites within the brain. Although all three ligands were often co-localized, there were areas that bound 3H-nicotine but not the 125I-neurotoxins, or vice versa. Very high densities of all three ligands were found in the hyperstriatum ventrale; the nucleus geniculatus lateralis, pars ventralis; the griseum tectale; the nucleus dorsolateralis anterior thalami; the nucleus lentiformis mesencephali, pars lateralis and pars medialis; the periventricular organ; and the stratum griseum et fibrosum superficiale, layer f of the optic tectum. The nucleus spiriformis lateralis had the highest levels of 3H-nicotine binding in the chicken brain, but it did not bind either of the two snake neurotoxins. On the other hand, high levels of both 125I-alpha-bungarotoxin and 125I-kappa-bungarotoxin binding were found in the nucleus semilunaris and the nucleus ovoidalis, but these areas contained little or no 3H-nicotine binding. No unique 125I-kappa-bungarotoxin sites, unrecognized by 125I-alpha-bungarotoxin, were identified by the low resolution autoradiography performed in this study. In general, nicotinic receptors were found in areas that have been reported to contain cholinergic cell bodies or fibers. Comparison of our results with the expression of neuronal nicotinic receptor subunits, as determined by in situ hybridization, suggests that many of the high affinity 3H-nicotine sites are localized presynaptically, as, for example, in the retinorecipient nuclei and the nucleus interpeduncularis. The lack of 125I-kappa-bungarotoxin binding in the presence of alpha-bungarotoxin indicates that the chicken brain has only very low levels of a unique kappa-bungarotoxin site. This is in marked contrast to chicken, frog, and rat autonomic ganglia, where a unique kappa-neurotoxin-sensitive receptor has been identified and shown to mediate nicotinic neurotransmission.     

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.     

Retinal projections in the lizard Podarcis hispanica

The retinofugal of the lizard Podarcis hispanica has been examined by means of enzymatic method with horseradish peroxidase (HRP). The retinal ganglion cells project contralaterally to thalamus (nucleus geniculatus lateralis pars dorsalis, nucleus geniculatus lateralis pars ventralis and nucleus ventrolateralis pars ventralis and nucleus ventrolateralis pars ventralis), pretectum (nucleus lentiformis mesencephali, nucleus geniculatus pretectalis and nucleus posterodorsalis) and optic tectum (layers 14 and 12, mainly, and layers 13 and 11). A small ipsilateral tract has been observed. Some of these fibers project to the lateral geniculate complex and the nucleus ventrolateralis pars ventralis. Most of the ipsilateral fibers have been observed in the neuropil of nucleus geniculatus pretectalis and the layer 14 of the optic tectum. The ipsilateral component, an inconstant structure in reptiles, presents an important development in Podarcis hispanica, although the number of its fibers is relatively small.     

Retinofugal and retinopetal projections in the cichlid fish Astronotus ocellatus

The retinofugal and retinopetal projections of the cichlid fish Astronotus ocellatus were studied by applying cobaltous-lysine to the optic nerve. Retinal axons terminate bilaterally in a preoptic-suprachiasmatic region between the base of the third ventricle and the anterior genu of the horizontal commissure and among periventricular cells along the sides of the ventricle. Other retinal axons innervate the tuberal region of the hypothalamus anterior to the infundibulum. Targets innervated in the pretectum include nucleus lateralis geniculatus and dorsal, medial, and ventral pretectal nuclei. Three other targets (nucleus opticus dorsolateralis, nucleus opticus commissurae posterior, nucleus opticus ventrolateralis) are innervated by fibers that leave the medial edge of the dorsal optic tract. Two other targets (basal optic nucleus and accessory optic nucleus) are innervated by fibers from the ventral optic tract. These retinal projections are similar to those previously reported for goldfish in an experiment that used the cobaltous-lysine method (Springer and Gaffney, J. Comp. Neurol. 203:401-424, '81). Retinotectal optic axons were found in a superficial lamina just above the stratum opticum, in the stratum opticum, in three layers of the stratum fibrosum et griseum superficiale, in a lamina just beneath the stratum fibrosum et griseum superficiale, and in the stratum album centrale just above the stratum periventriculare. This result is similar to that previously reported for goldfish; however, the spatial relationships between the various retinorecipient laminae differ for goldfish and Astronotus ocellatus. Efferents to the retina originate in two nuclei and both project contralaterally. The first is the nucleus olfactoretinalis, which is located ventrally between the olfactory lobe and telencephalon. It consists of about 400 cells, of which, approximately 200 cells project to the retina. The second retinopetal nucleus, nucleus thalamoretinalis, is a diffuse group of about 200 cells that project to the retina.     

Quantitative analysis of the localization of adrenergic binding sites in chick brain

In the present work [3H]-WB4101, [3H]-DHA, and [3H]-clonidine were used for the study of the localization of alpha 1, alpha 2, and beta adrenergic receptors in the chick brain. The highest concentration of [3H]-WB4101 was observed in the nucleus pretectalis, followed by the nucleus brachium conjunctivum descendens. The superficial layers of stratum griseum fibrosum superficiale, the nucleus mesencephalis lateralis pars dorsalis, and the locus coeruleus showed concentrations of [3H]-WB4101 binding higher than 300 fmoles/mg protein. Concentrations of [3H]-DHA binding higher than 300 fmoles/mg protein were observed in the paleostriatum, the external part of nucleus pretectalis, the nucleus isthmi parvocellularis, the nucleus mesencephalis lateralis pars dorsalis, the dorsal nucleus of oculomotor center, and the molecular layer of cerebellum. Locus coeruleus was the only area of chick brain which showed concentration of [3H]-clonidine binding higher than 300 fmoles/mg protein. With few exceptions, [3H]-clonidine binding was very low and in the telencephalon it was undetectable.     

Distribution of Met-enkephalin, Leu-enkephalin, substance P, neuropeptide Y, FMRFamide, and serotonin immunoreactivities in the optic tectum of the Atlantic salmon (Salmo salar L.)

The distribution of the neuropeptides methionine- and leucine-enkephalins, substance P, FMRFamide, neuropeptide Y, and vasoactive intestinal peptide, as well as the biogenic amine serotonin was studied in the optic tectum of the Atlantic salmon by means of immunocytochemistry. Peroxidase-antiperoxidase and indirect immunofluorescence methods were used to compare the differential laminar distribution of each of these substances. Nine parts of the optic tectum were selected for analysis on frontal sections: median, dorsolateral, and ventrolateral areas at rostral, medial, and caudal levels. Methionine- and leucine-enkephalin immunoreactive fibers were found in discrete sublayers in the following strata: stratum opticum, stratum fibrosum et griseum superficiale, stratum griseum centrale, stratum, and album centrale. Most of the substance P-, serotonin-, and vasoactive intestinal peptide-immunoreactive fibers were found in the stratum album centrale, whereas the FMRFamide- and neuropeptide Y-immunoreactive fibers were more or less randomly distributed within most of the strata of the optic tectum. Neuropeptide Y-immunoreactive cell bodies were located in the stratum periventriculare. We suggest an extrinsic origin for most of the immunoreactive fibers observed in the optic tectum, except for the neuropeptide Y-immunoreactive fibers that probably originate in the periventricular neurons. Although retinal peptidergic input to the optic tectum has been proposed in other vertebrates, there is no evidence that any of the neuropeptidelike or serotonin immunoreactive fibers in the optic tectum of the salmon should be of retinal origin. Differences and similarities with the distribution of neuropeptides in the optic tectum in representatives of other vertebrate classes are discussed.     

Comparative neurology of the optic tectum in ray-finned fishes: patterns of lamination formed by retinotectal projections

Retinotectal projections were studied in 33 different species of Actinopterygii, the ray-finned fishes, with horseradish peroxidase and cobalt tracing techniques. The distribution of retinorecipient layers in the contralateral optic tectum was analyzed. In addition, the degree of differentiation of the stratum periventriculate, and the presence of ipsilateral retinotectal projections was examined. Retinofugal fibers are labeled in the stratum opticum (SO), stratum fibrosum et griseum superficiale (SFGS), stratum griseum centrale (SGC), stratum album centrale (SAC) and stratum periventriculare (SPV). Some species lack the projection to the SO, others lack the projection to the SGC, and a third group of fishes lack both projections. Five different patterns of retinorecipient tectal strata are distinguished. These patterns correlate with the species' taxonomic position. Evolutionary trends of tectal lamination and retinotectal innervation are described. The retinotectal projection patterns provide a useful indicator of phylogenetic relationships. Some of our data suggest different relationships between actinopterygian species than hitherto believed.     

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

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

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.     

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

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

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.     

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.     

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

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