Single retinal ganglion cells sending axon collaterals to the bilateral superior colliculi: a fluorescent retrograde double-labeling study in the Japanese monkey (Macaca fuscata).

Single retinal ganglion cells projecting bilaterally to the superior colliculi (SC) by way of axon collaterals were revealed in the Japanese monkey (Macaca fuscata). After injecting Fast blue into the SC on one side and Diamidino yellow into the SC on the opposite side, some retinal ganglion cells were double-labeled with both tracers. Most of them were large cells (more than 25 microns in diameter), and were localized in a narrow strip around the vertical meridian of the retina on each side. This retinal area roughly corresponds to the reported strip of nasotemporal overlap, where both crossed and uncrossed retinofugal projections arise.     

Intrinsic determinants of retinal axon collateralization and arborization patterns.

Permanent, novel retinal projections to the principal thalamic somatosensory (ventrobasal) or auditory (medial geniculate) nuclei can be produced in adult hamsters if the superior colliculus is ablated bilaterally and the somatosensory and auditory lemniscal axons are transected unilaterally on the day of birth. We studied the development of those novel projections by labeling retinal axons with the fluorescent tracer 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate to examine the relative roles of intrinsic factors and axon-target interactions in the specification of retinal axon connections. Our principal findings are as follows: (1) In hamsters operated on the day of birth to produce the novel retinal projections, retinal ganglion cell axons projecting to the ventrobasal or medial geniculate nuclei develop in three morphologically distinct stages, i.e., elongation, collateralization, and arborization, as do retinal axons projecting to the dorsal lateral geniculate nucleus, the principal thalamic visual nucleus, in normal hamsters. (2) In both the ventrobasal and medial geniculate nuclei of operated hamsters, as in the dorsal lateral geniculate nucleus of normal hamsters, collateral branches were initially formed by retinal ganglion cell axons in both the superficial and internal components of the optic tract and only collaterals from the superficial component formed permanent projections. (3) The retinofugal axon terminal arbors in the ventrobasal and medial geniculate nuclei of mature, operated hamsters resemble the same three morphologic classes that are observed in the lateral geniculate nucleus of normal hamsters, although their absolute size appears to be altered. These data suggest that both superficial and internal optic tract axons can produce thalamic collaterals during development but that only superficial optic tract axons can permanently retain thalamic collaterals. Furthermore, the same morphologic types of retinofugal axons appear to contribute to normal and surgically induced retinal projections.     

The retinal projection to the superior colliculus in the cat: A quantitative study with HRP

The projections of cat retinal ganglion cells to the superior colliculus (SC) were examined using the method of retrograde axonal transport of horseradish peroxidase (HRP). Several injections of HRP were made in a single SC after the visual projection to the injection sites had been established physiologically. The HRP injections resulted in a homogeneous distribution of labelled ganglion cells in whole mount preparations of the retinae of both eyes. In the eye contralateral to the injected colliculus, ganglion cells with a crossed projection were labelled in both nasal and temporal retina; in the ipsilateral eye, ganglion cells with uncrossed projection were labelled only in the temporal retina. Analysis of the counterstained retinal whole mounts indicated that at least 50% of all ganglion cells in the nasal retina and 26% in the temporal retina have a crossed projection to SC, and that 24% of all ganglion cells of the temporal retina have an uncrossed projection to the SC. The morphological classes of retinal ganglion cells have different patterns of crossed/uncrossed decussation and they participate in varying proportions in the retino-tectal projection. Almost all Alpha cells in the retina send axon collaterals to the SC. Probably only about 10% of the Beta cells project to the SC and at least 80% of all Gamma cells send axons to the SC.     

Central projections of melanopsin-expressing retinal ganglion cells in the mouse

A rare type of ganglion cell in mammalian retina is directly photosensitive. These novel retinal photoreceptors express the photopigment melanopsin. They send axons directly to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus (OPN), thereby contributing to photic synchronization of circadian rhythms and the pupillary light reflex. Here, we sought to characterize more fully the projections of these cells to the brain. By targeting tau-lacZ to the melanopsin gene locus in mice, ganglion cells that would normally express melanopsin were induced to express, instead, the marker enzyme beta-galactosidase. Their axons were visualized by X-gal histochemistry or anti-beta-galactosidase immunofluorescence. Established targets were confirmed, including the SCN, IGL, OPN, ventral division of the lateral geniculate nucleus (LGv), and preoptic area, but the overall projections were more widespread than previously recognized. Targets included the lateral nucleus, peri-supraoptic nucleus, and subparaventricular zone of the hypothalamus, medial amygdala, margin of the lateral habenula, posterior limitans nucleus, superior colliculus, and periaqueductal gray. There were also weak projections to the margins of the dorsal lateral geniculate nucleus. Co-staining with the cholera toxin B subunit to label all retinal afferents showed that melanopsin ganglion cells provide most of the retinal input to the SCN, IGL, and lateral habenula and much of that to the OPN, but that other ganglion cells do contribute at least some retinal input to these targets. Staining patterns after monocular enucleation revealed that the projections of these cells are overwhelmingly crossed except for the projection to the SCN, which is bilaterally symmetrical.     

The Termination of Retinofugal Fibers in the Crab-Eating Monkey (Macaca irus): an Autoradiographic Study

Abstract: In the crab-eating monkey ( Macaca irus ), the subcortical projection from the retina was studied by means of the autoradiographic method. Retinofugal fibers terminated bilaterally in the suprachiasmatic, pregeniculate, lateral geniculate, olivary, pretectal, lateral terminal and dorsal terminal nuclei and the superior colliculi. In the ipsilateral lateral geniculate nucleus, the retinofugal fibers terminated on laminae 0, 2, 3, and 5; in the contralateral nucleus, they ended in laminae 1, 4, and 6. In the superior colliculi, retinal terminals were aggregated in the stratum griseum superficiale. The ipsilateral suprachiasmatic nucleus showed a heavier labeling than the contralateral nucleus.     

Lucifer yellow, retrograde tracers, and fractal analysis characterise adult ferret retinal ganglion cells.

The dendritic morphology of retinal ganglion cells in the ferret was studied by the intracellular injection of lucifer yellow in fixed tissue. Ganglion cells were identified by the retrograde transport of red or green fluorescent microspheres that had been injected into different target nuclei, usually the lateral geniculate nucleus or superior colliculus. This approach allows the comparison of dendritic morphologies of ganglion cells in the same retina with different central projections and also identifies cells with branching axons. The digitised images of dendritic arbors were analysed quantitatively by a variety of measures. Dendritic complexity was assessed by calculating the fractal dimension of each arbor. The ferret has distinct alpha, beta, and gamma morphological classes of cells similar to those found in the cat. The gamma cell class was morphologically diverse and could be subdivided into "sparse," "loose," and "tight" groups, reflecting increasing dendritic complexity. Whereas the beta cell projection was limited to the lateral geniculate nucleus alone, alpha and gamma cells could project to either or both nuclei. Retinal ganglion cells labelled from the pretectal nuclei formed a morphologically distinct class of retinal ganglion cells. The ipsilateral projection lacked alpha cells and the most complex, "tight" gamma cells. However, ipsilaterally projecting "loose" gamma cells overlapped alpha cells in both soma and dendritic dimensions. Different morphological classes of retinal ganglion cells hence show characteristic axon behaviour both in their decussation at the chiasm and in which targets they innervate. Fractal measures were used to contrast variation within and between these identified classes.     

Depletion of cortical target induced by prenatal ionizing irradiation: effects on the lateral geniculate nucleus and on the retinofugal pathways.

Studies using neonatal surgical lesions to reduce the target area of the retina have supported the idea that developing axons show only a limited specificity in their targeting. This investigation tested whether retinogeniculate axons adjust for partial target depletion by repositioning of axons. We used adult Swiss mice exposed to gamma rays at the time when layer IV cells are generated in the ventricular zone (16 days of gestation). Nissl-stained brain sections were used for histological analyses in thalamus and cortex. Retinal ganglion cells were backfilled from the optic tract with horseradish peroxidase. Intraocular injections of horseradish peroxidase were used to study the retinal projections. In the posterior cortex there was a nearly complete absence of layer IV. The irradiated animals showed a 75% reduction of the dorsal lateral geniculate nucleus. The ventral division, superior colliculus, and other visually related nuclei were not affected. The loss in the ganglion cells (15.7%) was significant but clearly smaller than that observed in the dorsal lateral geniculate nucleus (75%). Therefore, the shrinkage of the dorsal lateral geniculate nucleus led to a reduction in the area available for retinal projections. Despite partial target loss, pattern of retinal projections did not differ from that of the controls. The effect on the dorsal lateral geniculate nucleus is discussed in the light of differences between prenatal and neonatal damage of the presumptive visual cortex. The absence of aberrant retinal projections suggests that repositioning of axons is not the first mechanism employed by retinal axons to match connections in numerically disparate populations.     

The morphology, number, distribution and central projections of Class I retinal ganglion cells in albino and hooded rats

Class I retinal ganglion cells have been identified in wholemounts of rat retinae following injections of horseradish peroxidase (HRP) into retino-recipient nuclei. Class I cells are characterized by relatively large somata, 3-7 fairly frequently branching large-gauge primary dendrites and relatively thick axons. Cells with a very similar morphology have been visualized in the ganglion cell layer of retinal wholemounts using a neurofibrillar stain. The size of the somata and dendritic trees of Class I cells is affected by the density of all classes of ganglion cells: both somata and dendritic trees of Class I cells located in the region of peak density are smaller than those located in medium- and low-density ganglion cell regions. The mean numbers of Class I ganglion cells labelled following massive injections of HRP into retino-recipient nuclei were 876 (in albino rats) and 944 (in hooded rats), while the mean number of cells stained with the neurofibrillar method in albino retinae was 791. Thus, with the total number of positively identified retinal ganglion cells being 110,000-115,000 [Potts et al., 1982; Perry et al., 1983], Class I cells in both strains of rat constitute less than 1% of all retinal ganglion cells. Nevertheless the dendritic fields of Class I cells cover the entire retina. Although Class I cells are distributed relatively evenly across the retina, the density is slightly greater in the lower temporal retina where the bulk of the ipsilaterally projecting fibres originates. While Class I cells represent up to 10% of ipsilaterally projecting retinal ganglion cells in both strains of rat, fewer Class I cells project ipsilaterally in albinos than in hooded rats. All contralaterally projecting Class I cells appear to send branching axons to the superior colliculus and dorsal lateral geniculate nucleus. Class I cells represent a larger proportion of the ganglion cells projecting to the dorsal lateral geniculate nucleus (4-5%) than that of ganglion cells projecting to the superior colliculus (about 1%). The morphology, numbers, distribution and the pattern of the central projections of Class I retinal ganglion cells in rats suggest that they are likely to be homologues of the alpha-type ganglion cells distinguished in carnivores.     

Dorsal raphe nucleus projecting retinal ganglion cells: Why Y cells?

Retinal ganglion Y (alpha) cells are found in retinas ranging from frogs to mice to primates. The highly conserved nature of the large, fast conducting retinal Y cell is a testament to its fundamental task, although precisely what this task is remained ill-defined. The recent discovery that Y-alpha retinal ganglion cells send axon collaterals to the serotonergic dorsal raphe nucleus (DRN) in addition to the lateral geniculate nucleus (LGN), medial interlaminar nucleus (MIN), pretectum and the superior colliculus (SC) has offered new insights into the important survival tasks performed by these cells with highly branched axons. We propose that in addition to its role in visual perception, the Y-alpha retinal ganglion cell provides concurrent signals via axon collaterals to the DRN, the major source of serotonergic afferents to the forebrain, to dramatically inhibit 5-HT activity during orientation or alerting/escape responses, which dis-facilitates ongoing tonic motor activity while dis-inhibiting sensory information processing throughout the visual system. The new data provide a fresh view of these evolutionarily old retinal ganglion cells.     

Collateral innervation of the inferior colliculus in the North American opossum: A study using fluorescent markers in a double-labeling paradigm

The innervation of the opossum inferior colliculus was investigated using the retrograde transport of fluorescent markers in a double-labeling paradigm. True Blue was injected into one inferior colliculus while Nuclear Yellow was placed into the other. Many single-labeled neurons were found in all of the brainstem and cortical areas previously labeled by comparable injections of HRP. When the two injections of fluorescent markers were bilaterally symmetrical within the inferior colliculi, double-labeled neurons were numerous in the medial and lateral superior olivary nuclei. Only a few double-labeled neurons were found in the auditory cortex (AC), the dorsal nucleus of the lateral lemniscus (DNLL), the nucleus reticularis gigantocellularis pars ventralis (RGcv), and the dorsal column nuclei (DColN) and the spinal trigeminal complex (TrS). These data suggest that two patterns of innervation are seen for afferent fibers from regions with bilateral connections to the IC. The first pattern, typified by projections from AC, DNLL, RGcv, the DColN and TrS, primarily consists of neurons with unilaterally directed axons, extending to either the ipsilateral or contralateral IC. The second pattern, typified by projections from LSO and MSO, displays a great number of neurons whose axons apparently innervate both inferior colliculi, presumably through a process of axon collateralization. The bilateral projection of axons from individual neurons in these two nuclei possibly reflects their involvement in the processing of sensory information from the two cochleas and may represent at least one pathway whereby binaural information is relayed to both inferior colliculi.     

Retinal ganglion cell projections to the hamster suprachiasmatic nucleus,intergeniculate leaflet,and visual midbrain: bifurcation and melanopsin immunoreactivity

The circadian clock in the suprachiasmatic nucleus (SCN) receives direct retinal input via the retinohypothalamic tract (RHT), and the retinal ganglion cells contributing to this projection may be specialized with respect to direct regulation of the circadian clock. However, some ganglion cells forming the RHT bifurcate, sending axon collaterals to the intergeniculate leaflet (IGL) through which light has secondary access to the circadian clock. The present studies provide a more extensive examination of ganglion cell bifurcation and evaluate whether ganglion cells projecting to several subcortical visual nuclei contain melanopsin, a putative ganglion cell photopigment. The results showed that retinal ganglion cells projecting to the SCN send collaterals to the IGL, olivary pretectal nucleus, and superior colliculus, among other places. Melanopsin-immunoreactive (IR) ganglion cells are present in the hamster retina, and some of these cells project to the SCN, IGL, olivary pretectal nucleus, or superior colliculus. Triple-label analysis showed that melanopsin-IR cells bifurcate and project bilaterally to each SCN, but not to the other visual nuclei evaluated. The melanopsin-IR cells have photoreceptive characteristics optimal for circadian rhythm regulation. However, the presence of moderately widespread bifurcation among ganglion cells projecting to the SCN, and projection by melanopsin-IR cells to locations distinct from the SCN and without known rhythm function, suggest that this ganglion cell type is generalized, rather than specialized, with respect to the conveyance of photic information to the brain.     

Ventral lateral geniculate nucleus projects to the dorsal lateral geniculate nucleus in the cat

The lamina C3 of the dorsal lateral geniculate nucleus of the cat does not receive retinal projections but instead receives visual information from the small subpopulation of W-type ganglion cells via the upper substratum of the stratum griseum superficiale of the superior colliculus. We herein report a projection from the lateral division of the ventral lateral geniculate nucleus into the lamina C3 of the dorsal lateral geniculate nucleus. As the lateral division receives projections from the contralateral retina and the ipsilateral upper stratum griseum superficiale of the superior colliculus, we suggest that these regions make up a small cell type W-cell neuronal network that provides visual information to layer I of the striate cortex via the lamina C3.     

Lack of intralaminar sprouting of retinal axons in monkey LGN.

In the dorsal lateral geniculate nucleus (LGN) of the adult cat there is no evidence for translaminar sprouting of retinal axons to fill sites freed of retinal endings from the other eye. We tested the possibility that retinal axons will sprout to fill denervated retinal sites within laminae of the monkey LGN. In 4 monkeys, retinal ganglion cell axons from either the upper or lower half of the retina were destroyed. To maximize the potential for sprouting in the LGN, on one side of the brain the LGN cells to which the remaining retinal axons normally project were removed by ablation of the appropriate portion of the striate cortex. Three months later the eye receiving the retinal lesion was injected with [3H]proline and the retinal projection to the LGN on both sides of the brain was studied using autoradiography. We found no evidence of intralaminar sprouting of retinal axons either in the normal LGN or in the LGN in which the usual targets of retinal axons had been removed.     

Retinal projections to the superior colliculus and dorsal lateral geniculate nucleus in the tammar wallaby (Macropus eugenii): II. Topography after rotation of an eye prior to retinal innervation of the brain

Retinal projections to visual centers in a marsupial mammal, the tammar wallaby (Macropus eugenii), have been investigated after an eye rotation prior to retinal innervation of the brain. Retinal topography to the superior colliculus and dorsal lateral geniculate nucleus was mapped by using laser lesions of the retina and horseradish peroxidase histochemistry. Despite the change in orientation of optic axon outgrowth from the developing eye after rotation, retinal ganglion cells made orderly connections in the colliculus and geniculate according to their original retinal position within the eye and not their rotated position. Axons must have corrected their pathways at some point between the back of the eye and their targets. The optic chiasm was one such site. Optic axons from the rotated eye took an abnormal course at the caudal end of the chiasm. Growth of optic axons through aberrant pathways in the brain did not preclude specific innervation of targets. When by chance optic axons entered through the oculomotor nerve root they specifically innervated their correct visual centers, albeit in reduced density, and did not innervate inappropriate targets. These results support the idea of specific interactions between growing axons, the pathways they grow along, and their targets.     

Development of retinotopy in projections from the eye to the dorsal lateral geniculate nucleus and superior colliculus of the wallaby (Macropus eugenii)

The development of retinotopy in projections from the eye to the dorsal lateral geniculate (dLGN) and superior colliculus (SC) has been studied in the marsupial wallaby. Discrete retinal lesions were made and the remaining retinal projections were traced with horseradish peroxidase in animals at stages ranging from just after optic innervation of the dLGN and SC to the time when the projections are mature. Topographically organised projections could be recognized a few weeks after axons first reached the dLGN and SC with a topographically discrete projection from nasoventral retinal recognized later than from dorsal, dorso-temporal, temporal, and temporoventral retina. Over time there was an increase in precision of the retinotopy as judged by an increase in sharpness of the borders of filling defects in the projection labelled with horseradish peroxidase. Refinement of the projection from temporal retina preceded that from nasal retina in both the dLGN and SC and in the former occurred concomitantly with the segregation of eye-specific terminal bands. Refinement was complete 16 weeks after birth, prior to eye opening at around 20 weeks after birth. Inequalities in retinal representations in both nuclei were present from the time retinotopy could first be detected. This was before the inequalities in retinal ganglion cell distribution, which underly these representations in the adult, were obvious. Retinotopy and inequalities in retinal representation characteristic of the adult are present from a very early stage in the protracted development of visual projections in the wallaby. Refinement may involve death of inappropriately projecting cells, pruning of inappropriately projecting axon arborizations or could be achieved by growth of the retinorecipient neuropil. Temporonasal differences in the time course of refinement may reflect gradients of maturation in the retina.     

Axonal Target Choice and Dendritic Development of Ferret Beta Retinal Ganglion Cells

We have investigated the relationship between axon targeting and dendritic morphology in beta retinal ganglion cells in the postnatal ferret. Axonal projections were assessed by making separate injections of different fluorescent retrograde tracers into either the superior colliculus or lateral geniculate nucleus in viva The dendritic morphology of retrogradely labelled cells was revealed by the in vitro intracellular injection of Lucifer yellow in fixed retina. In this way, 405 retinal ganglion cells were triple- or double-labelled and characterized by their dendritic branching styles. Both the distinct dendritic morphology of beta cells and the characteristic restriction of their adult axonal terminals to the lateral geniculate nucleus emerge postnatally. Beta cell dendritic morphology is established between postnatal days 5 and 9. As in the cat (Ramoa et al, 1989), beta cells extend and then retract a projection to the superior colliculus as part of their normal development. Transient beta axonal collaterals to the superior colliculus persist beyond the period of cell death, but nearly all are withdrawn by postnatal day 15. No dendritically distinct beta cell projects to the superior colliculus alone, at any age. Heterochronic injections of different colours of retrograde tracer into the superior colliculus were used to study changes in the complement of the retinocollicular projection over time. A significant proportion of cells (58%) labelled at postnatal day O from the superior colliculus, which subsequently survived the period of cell death, were found to be beta cells that could no longer be demonstrated to have a retinocollicular axon. Neonatal decortication, which reduced the volume of the adult lateral geniculate nucleus by 36–86%, resulted in the limited stabilization of this normally transient beta cell projection to the superior colliculus. The fundamental dendritic branching style of beta ganglion cells is unchanged in decorticate ferrets, suggesting that it develops independently of their ultimate axonal target choice.     

Autoradiographic study of the retinal projections in the Chinese pangolin, Manis pentadactyla

Autoradiography was used to investigate the optic system of the Chinese pangolin, Manis pentadactyla. The pattern of retinal projections in the Chinese pangolin is similar to that described in other mammals. Each retina projects bilaterally to the suprachiasmatic nucleus, dorsal and ventral lateral geniculate nuclei, pretectal area, and superior colliculus (SC). Only contralateral projections are found to the medial, lateral, and dorsal accessory optic nuclei. The large lateral nucleus receives a dense projection from the retina and forms a compact mass on the dorsolateral area of the cerebral peduncle. The lamination of the SC could not be clearly demonstrated in the brain of the Chinese pangolin.     

The projections of different morphological types of ganglion cells in the cat retina.

The central projections of the retinal ganglion cells of the cat were examined using the method of retrograde transport of horseradish peroxidase. Peroxidase was injected into the lateral geniculate nucleus and into the superior colliculus by means of a recording micropipette. After injections at retinotopically homologous points in these two structures in separate animals, tha patterns of retinal ganglion cell labeling were compared. We found that there were three populations of ganglion cells: small cells, that projected predominantly to the superior colliculus; medium-sized cells, that projected predominantly to the lateral geniculate nucleus; and large cells, some of which projected to both structures, and some of which projected to the lateral geniculate nucleus alons. Quantitative studies showed that the average size of the cells in each population was smaller at the area centralis than in the periphery. These results could be directly related to physiological classifications of retinal ganglion cells proposed by other authors.     

The retinal projection to the thalamus in the cat: a quantitative investigation and a comparison with the retinotectal pathway

The projection of cat retinal ganglion cells to the thalamus was examined using the method of retrograde axonal transport of horseradish peroxidase (HRP). After the injection site was determined physiologically, HRP was applied by one of three methods: iontophoretic injection of minimal amounts, single pressure injections and multiple pressure injections. Iontophoretic injections into single laminae of the dorsal part of the lateral geniculate nucleus (LGNd) revealed that laminae A and A1 receive almost exclusively axon terminals from alpha and beta cells. Single pressure injections elucidated the retinotopic organization of the LGNd. Multiple injections lead to HRP uptake in the whole LGNd including parts of adjacent thalamic nuclei and revealed that at least 77% of all retinal ganglion cells project to the thalamus. This pathway is made up of all alpha cells, all beta cells and almost half of the gamma cells. The thalamus receives its visual input predominantly from the ipsilateral temporal and the contralateral nasal retina; some alpha cells were also labeled in the contralateral temporal retina. The shape of the decussation line was analyzed and its width was found to be proportional to the average ganglion cell spacing along the dorsoventral axis of the retina. From a comparison of the retinothalamic and retinotectal pathways, an estimate of the number of cells with bifurcating axons could be given. The axons of all alpha cells, 10% of the beta cells, and every second gamma cell bifurcate; this amounts to 30% of the retinal ganglion cells.