An autoradiographic investigation of the subcortical visual system in chimpanzee.

Based on one adult chimpanzee monocularly injected with radioactive proline, retinofugal fibers were found to terminate bilaterally in the suprachiasmatic, pregeniculate, lateral geniculate, olivary, pretectal and lateral terminal nuclei, and the superior colliculi; the existence of a dorsal terminal nucleus of the accessory optic system is in doubt. In the ipsilateral geniculate nucleus, the fibers terminate in layers 2, 3 and 5; in the contralateral nucleus, they end in layers 1, 4 and 6. Midway through the geniculate nucleus, layers 3 and 4 split medially into two daughter layers each. In the superior colliculi, most of the retinal terminals are aggregated superficially in a band located in the stratum griseum superficiale. The contralateral band is interrupted by gaps; the ipsilateral band has fewer gaps, is slightly thicker and located more deeply. There is a limited second tier of terminals in the contralateral superficial gray.     

Postnatal development of primary visual projections in the tammar wallaby (Macropus eugenii)

The time course and pattern of retinal innervation of primary visual areas was traced in pouch-young wallabies. Tritiated proline was injected into one eye of animals ranging in age from 1 to 72 days after birth. These results are compared to the 11 primary visual areas found in the adult wallaby, seven of which receive binocular input while four are monocular. At birth retinal ganglion cell axons have not reached any visual areas. Two to 4 days after birth, all of the axons are crossing to the contralateral optic tract. Nine to 12 days after birth axons begin to invade the contralateral lateral geniculate nucleus, the superior colliculus, and the medial terminal nucleus. Twenty to 21 days after birth, ipsilateral axons invade the lateral geniculate nucleus and superior colliculus. The contralateral projection precedes the ipsilateral projection in all binocular visual areas. By 25 days, ipsilateral and contralateral afferents share common territory in the lateral geniculate nucleus; however, afferents from each eye are initially concentrated in appropriate areas. Between 52 and 72 days, afferents to the dorsal lateral geniculate nucleus are gradually segregated into nine terminal bands. Four are contralateral while five are ipsilateral. By 72 days, the ipsilateral component to the superior colliculus is clustered beneath the contralateral projection a deeper layer. Projections to four monocular visual areas--lateral posterior nucleus, dorsal terminal nucleus, lateral terminal nucleus, and nucleus of the optic tract--are established later than binocular visual areas, except the suprachiasmatic nucleus. The suprachiasmatic nucleus is the last to be bilaterally innervated even though it is situated closest to the optic chiasm. At the light microscope level a mature pattern of visual development is emerging by 72 days, although the eyes do not open until 140 days.     

The retinofugal fibers and their terminal nuclei in Galago crassicaudatus (primates)

Control and experimental brains of Galago were used to study the subcortical optic nuclei and their retinal input. Weil, Nissl and Fink-Heimer methods were employed. The pregeniculate nucleus receives predominantly contralateral retinal input. Terminal degeneration is found only in its pars grisea, but not in its pars fibrosa. The lateral geniculate nucleus exhibits seven demonstrable laminae. Laminae 0, 2, 3 and 4 receive ipsilateral retinal input, whereas laminae 1, 5 and 6 receive contralateral input without overlap. In Fink-Heimer preparations only the ipsilateral lamina 4 and the contralateral lamina 5 show clusters of argyrophilic spherules which probably indicate axo-dendritic termination of retinofugal fibers. The nucleus praetectalis posterior and nucleus tractus optici receive predominantly contralateral retinal input. In all likelihood, only the stratum griseum superficiale of the superior colliculus receives retionfugal fibers. On the basis of the very remarkable pattern of terminal degeneration, we divide this stratum into substrata A, B and C. The ipsilateral superior colliculus receives retinal input to its substratum B and the contralateral superior colliculus receives retinal input to its substratum A only. Substratum C receives probably no retinal input.     

An autoradiographic study of the projections from the lateral geniculate body of the rat.

The projections from the lateral geniculate body of the rat were followed using the technique of autoradiography after injections of [3H] proline into the dorsal and/or ventral nuclei of this diencephalic structure. Autoradiographs were prepared from either frozen or paraffin coronal sections through the rat brain. The dorsal nucleus of the lateral geniculate projected via the optic radiation to area 17 of the cerebral cortex. There was also a slight extension of label into the zones of transition between areas 17, 18 and 18a. The distribution of silver grains in the various layers of the cerebral cortex was analyzed quantitatively and showed a major peak of labeling in layer IV with minor peaks in outer layer I and the upper half and lowest part of layer VI. The significance of these peaks is discussed in respect to the distribution of geniculocortical terminals in other mammalian species. The ventral nucleus of the lateral geniculate body had 5 major projections to brain stem structures both ipsilateral and contralateral to the injected nucleus. There were two dorsomedial projections: (1) a projection to the superior colliculus which terminated mainly in the medial third of the stratum opticum, and (2) a large projection via the superior thalamic radiation which terminated in the ipsilateral pretectal area; a continuation of this projection passed through the posterior commissure to attain the contralateral pretectal area. The three ventromedial projections involved: (1) a geniculopontine tract which coursed through the basis pedunculi and the lateral lemniscus to terminate in the dorsomedial and dorsolateral parts of the pons after giving terminals to the lateral terminal nucleus of the accessory optic tract, (2) a projection via Meynert's commissure to the suprachiasmatic nuclei of both sides of the brain stem as well as to the contralateral ventral lateral geniculate nucleus and lateral terminal nucleus of the accessory optic tract, and (3) a medial projection to the ipsilateral zona incerta. The results obtained in these experiments are contrasted with other data on the rat's central visual connections to illustrate the importance of these connections in many subcortical visual functions.     

The tecto-thalamic connections in the brain of the rabbit

We have correlated the tectal connections and cytoarchitecture of regions in the rabbit's midbrain and caudal thalamus. The inferior colliculus projects ipsilaterally to the central gray, superior colliculus, and via the brachium of the inferior colliculus to its interstitial nucleus and the parabrachial region of the midbrain tegmentum. From the brachium, fibers fan out to the principal and internal divisions of the medial geniculate. A smaller contralateral pathway sweeps into the contralateral inferior colliculus and in its brachium to the interstitial nucleus, the parabrachial region, and the internal and principal divisions of the medial geniculate. The superior collicular projection is mainly ipsilateral. Medially, fibers terminate in the central gray and pretectal area. Laterally, fibers ascend in the superior brachium to parabrachial region, suprageniculate pretectal nucleus, posterior complex, caudodorsal internal division of the medial geniculate, and to a discrete part of the ventral nucleus of later geniculate. A component of the commissure of Gudden originates in the rostral superior colliculus and terminates in the contralateral ventral lateral geniculate, posterior complex, pretectal area and midbrain tegmentum. Interconnections between the colliculi and overlap of their projections in the parabrachial region, the central gray, and the internal division of the medial geniculate are described.     

Retinal projections in the ringtailed possum Pseudocheirus peregrinus.

The retinal projections in the ringtailed possum, Pseudocheirus peregrinus were determined using Fink-Heimer material and autoradiography. At least seven regions in the brain receive retinal projections. These are (1) the suprachiasmatic nucleus of the hypothalamus (2) the dorsal lateral geniculate nucleus (3) the ventral lateral geniculate nucleus (4) the lateral posterior nucleus (5) the pretectum (6) the superior colliculus, and (7) the accessory optic system. The accessory optic system and lateral posterior nucleus receive a contralateral retinal projection only and the other five regions receive a bilateral retinal projection. The dorsal lateral geniculate nucleus consists of two parts: an outer alpha division of closely packed cells and an inner beta division containing loosely scattered cells. There are no cell layers apparent within the alpha division in Nissl sections. The autoradiographs and Fink-Heimer material reveal four concealed laminae within the alpha division. Lamina 1, which is adjacent to the optic tract and lamina 3 receive a predominantly contralateral input. Laminae 2 and 4 receive a predominantly ipsilateral input. The beta segment contains a fifth lamina which receives contralateral retinal input.     

The lateral geniculate nucleus of the marsupial phalanger, Trichosurus vulpecula. An experimental study of cytoarchitecture in relation to the intranuclear optic nerve projection fields

From a cytoarchitectural viewpoint four laminae are apparent in the normal dorsal lateral geniculate nucleus of the marsupial phalanger. The contra- and ipsilateral Nauta preterminal degeneration patterns produced by previous unilateral optic nerve section have been precisely delineated and related to these laminae. The terminations of the optic nerve fibers fractionate the nucleus into six laminae of which the crossed and uncrossed optic fibers each mainly supply three; crossed fibers terminate in laminae 2a, 3 and 6, uncrossed fibers terminate in laminae 1, 2b and 4+5. The “crossed” and “uncrossed” laminae alternate. Concealed lamination is a prominent and hitherto unsuspected characteristic of the organisation of the phalanger lateral geniculate nucleus. The most superficial lamina (1) is unusual in so far as it is mainly “uncrossed”. No other reported mammal except the primitive primate Tupaia shows this feature. Although the two intranuclear optic nerve projection fields are largely segregated, the segregation is only complete in laminae 3 and 4+5. Binocular overlap occurs in the remaining laminae, particularly in lamina 2b. The ventral lateral geniculate nucleus and the pretectal nuclear complex are confirmed as primary optic centers supplied by both crossed and uncrossed optic nerve fibers. Within the superior colliculus cells of the zonal, superficial gray and optic layers are in synaptic relationship with crossed fibers while the ipsilateral optic nerve fibers mainly terminate in a circumscribed region of the optic layer. There is no evidence that any component of the lateral thalamic nuclear complex is a terminal center for nerve fibers of retinal origin.     

Retinofugal projections in a marsupial, Tarsipes rostratus (honey possum)

We examined the mature retinal projections of the honey possum, Tarsipes rostratus, an Australian, diprotodont marsupial, using uni-ocular injections of horseradish peroxidase and tetramethylbenzidine processing. The suprachiasmatic nucleus, medial, lateral and dorsal terminal nuclei of the accessory optic tract, pretectal nuclei and superior colliculus received bilateral retinal input. Contralateral input only was observed in the lateral posterior nucleus. The pattern of input to these regions was essentially similar to that seen in other marsupials. Cyto-architectural examination of the dorsal lateral geniculate nucleus (dLGN) indicated that 5 laminae were present in the alpha-segment, but the beta-segment appeared to lack lamination. Input to the dLGN was bilateral, overlapping considerably, and was organised in a laminar fashion with 7 and 3 terminal bands in the alpha- and beta-segments, respectively. The monocular segment accounted for 12% of the total volume of the dLGN. In the alpha-segment, 2 terminal bands each received exclusively contralateral or ipsilateral input and 3 bilateral input. In the beta-segment, 2 terminal bands received bilateral and 1 contralateral input. The volumes of the nucleus receiving contralateral and ipsilateral input were 77 and 54% of the total, respectively. A marked overlap of input from the two eyes is an unusual feature for a diprotodont marsupial and has previously been seen only in the feathertail glider. Our findings for the dLGN are of interest in the light of recent serological and taxonomic studies which suggest a close link between the feathertail glider and the honey possum.     

Pattern of retinal projections in the California ground squirrel (Spermophilus beecheyi): anterograde tracing study using cholera toxin

The retinofugal pathways in the California ground squirrel, Spermophilus beecheyi, were mapped after intravitreal injections of cholera toxin B-subunit. The results of the current study are consistent with work in other mammals and provide new details relevant to the organization and evolution of the visual system. All retinorecipient nuclei received bilateral input, with a contralateral predominance. The suprachiasmatic nucleus is heavily innervated, and sparse terminals were noted in other hypothalamic areas. In addition to the interstitial, medial, lateral, and dorsal terminal nuclei, a few fibers of the accessory optic tract may enter the ventral lateral geniculate and the nucleus of the optic tract, though this innervation may not derive from the same ganglion cells innervating the accessory optic nuclei. Retinal terminals are found in the intergeniculate leaflet and the "dorsal cap" of the ventral lateral geniculate. Retinal fibers pass rostrally from the dorsal cap toward the anterodorsal thalamus, confirming a projection described in the tree shrew and monkeys. Retinal termination patterns in the dorsal lateral geniculate reveal a hexilaminate organization of alternating ipsilateral and contralateral input. Variations in terminal morphology suggest that sublayers receive input from distinct ganglion cell types and that laminar comparisons can be made with primates. Finally, terminal patterns in the superior colliculus reveal a dense, highly ordered columnar organization supporting functional properties of tectal receptive fields. All the visual structures in the ground squirrel are large and well differentiated, making the sciurid visual system an accessible rodent model for comparing visual processing with that in other diurnal vertebrates.     

Ascending brain stem projections of the anteroventral cochlear nucleus in the rhesus monkey

In a series of seventeen rhesus monkeys attempts were made to produce discrete stereotaxic lesions in the anteroventral cochlear nucleus (Av). Anterograde degeneration was described in detail in four cases with lesions confined within the cochlear complex to Av. Fibers decussating at pontine levels coursed exclusively in the trapezoid body. Degenerated fibers projected: ipsi-laterally to the lateral superior olivary nucleus; bilaterally to the preolivary nuclei; to the lateral side of the ipsilateral medial superior olive and the medial side of the contralateral medial superior olive; and to the contralateral medial trapezoid nucleus. A topographic projection upon the medial superior olive was demonstrated. Projections were bilateral but mainly crossed to the nuclei of the lateral lemniscus and central nucleus of the inferior colliculus; the posterior end of the ipsilateral ventral nucleus of the lateral lemniscus contained an island of profuse degeneration. A few fibers crossed in the commissure of the inferior colliculus. Few if any fibers from Av projected to the contralateral magnocellular medial geniculate.     

Neuropeptide Y immunoreactivity in the hamster geniculo-suprachiasmatic tract

The distributions of neuropeptide Y (NPY) and avian pancreatic polypeptide (APP) immunoreactivity were examined in the suprachiasmatic nucleus and the geniculate area of male golden hamster brains. In some cases, colchicine was injected intraventricularly to aid in visualization of immunoreactive cell bodies. A group of hamsters were given bilateral or unilateral radiofrequency lesions of the geniculate area and neuropeptide Y immunoreactivity was examined in the suprachiasmatic nucleus after survival times varying between 8 and 300 days. Another group of hamsters received unilateral intraocular injections of anterograde tracers and the overlap of NPY-immunoreactive cells in the geniculate area and labeled retinal afferents was assessed. It was found that NPY- and APP-immunoreactive fibers formed a dense plexus in the ventro-lateral suprachiasmatic nucleus. NPY-immunoreactive cell bodies were observed in the intergeniculate leaflet as well as in the external lamina of the anterior portion of the ventral lateral geniculate nucleus. Unilateral lesions of the geniculate produced a relative reduction in neuropeptide Y immunoreactivity in the ipsilateral suprachiasmatic nucleus whereas bilateral lesions produced a reduction of neuropeptide Y immunoreactivity in both suprachiasmatic nuclei. All NPY-immunoreactive cells in the intergeniculate leaflet were overlapped by bilateral retinal afferents. In the ventral lateral geniculate nucleus, all NPY-immunoreactive cells were overlapped by contralateral retinal afferents; however, not all such cells were in areas receiving ipsilateral retinal afferents. These results indicate that the hamster geniculo-suprachiasmatic tract originates in part from NPY-immunoreactive cell bodies and that these cells lie in areas receiving direct retinal afferents.     

Projections from the lateral geniculate nucleus to the hypothalamus of the Mongolian gerbil (Meriones unguiculatus): an anterograde and retrograde tracing study.

The lateral geniculate nucleus of the thalamus sends efferents to the hypothalamic suprachiasmatic nucleus, which is involved in generation and entrainment of several circadian rhythms. It seems reasonable to believe that the lateral geniculate conveys visual information about the length of the photoperiod to the circadian oscillator. In order to study in more detail the topographical relationship between the lateral geniculate and the suprachiasmatic nucleus, anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde tracing with wheatgerm agglutinin coupled to horseradish peroxidase (WGA-HRP) were performed in the gerbil. After iontophoretic injections of PHA-L in the lateral geniculate, a large number of PHA-L-immunoreactive fibers and nerve terminals were observed in the ventrolateral part of the suprachiasmatic nucleus. Nerve fibers were also present in the ventromedial and dorsolateral portions, particularly in the caudal half of the nucleus. PHA-L-immunoreactive nerve fibers continued outside the borders of the suprachiasmatic nucleus to the adjacent anterior hypothalamic, the periventricular, and the subparaventricular areas. A moderate number of fibers entered the lateral hypothalamic area and the tuber cinerum via the optic tract and chiasm. Moreover, the paraventricular nucleus, the supraoptic nucleus, the medial preoptic area, the lateral preoptic area, and the supramammillary nucleus contained a few labeled fibers. In all parts of the hypothalamus receiving an input from the lateral geniculate, fine beaded immunoreactive fibers with varicosities and nerve terminals were observed, some of which were found in close apposition to hypothalamic neurons. Only after labeling of neurons in the intergeniculate leaflet of the lateral geniculate nucleus, fibers were found in the hypothalamus. This topographical organization of the geniculohypothalamic pathway was supported by retrograde tracing after injections of WGA-HRP in the suprachiasmatic area. In these experiments, retrograde labeled neurons were observed in the intergeniculate leaflet and, in agreement with the anterograde studies, most of labeling was observed in the ipsilateral side. These results confirm that the suprachiasmatic nucleus receives a substantial input from the intergeniculate leaflet of the lateral geniculate. Moreover, the present data demonstrate that the suprachiasmatic nucleus is not the only nucleus that receives a direct visual input. Thus other hypothalamic areas might be influenced by a direct rhythmic neuronal input as well.     

Retinal projections in the house musk shrew, Suncus murinus, as determined by anterograde transport of WGA-HRP.

Retinal projections in the house musk shrew (Suncus murinus) were determined by the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Unilateral injection of WGA-HRP into the vitreous body resulted in the terminal labeling of the optic projections in the suprachiasmatic nucleus (SCH), the ventral (CGLv) and dorsal (CGLd) lateral geniculate nuclei, the intergeniculate leaflet (IGL), the pretectum, the superficial layers of the superior colliculus (CS), and the dorsal terminal nucleus (DTN) of the accessory optic system (AOS). Labeling of the SCH was bilateral, with ipsilateral predominance, and covered the whole dorsoventral extent of the nucleus. Immunohistochemical studies revealed that VIP-like immunoreactive neurons and fibers were present in almost all parts of the SCH. No hypothalamic regions other than the SCH received the optic fibers. The ipsilateral projections to the CGLv, CGLd, and IGL were sparse, and a considerable number of uncrossed retinal fibers were found in the pretectal olivary nucleus. No retinal projections to the lateral posterior thalamic nucleus (LP) were found. Ipsilateral optic fibers projected sparsely to the medial part of the CS. The AOS consisted of a small DTN with a very few crossed retinal projections but no lateral and medial terminal nuclei. In addition, the AOS had no inferior fascicle.     

Altered retinal projection in brushtailed possum,trichosurus vulpecula,following removal of one eye

One eye was removed from ten brushtailed prossums aged 18 to 102 days. The possums were kept until they reached 8-13 months of age and the remaining eye was injected with ³H leucine to show the retinal projections. Retinal projections were also mapped in five normal possums. In the one-eyed possums we found an altered retinal projection to the medial terminal nucleus (MTN), the dorsal lateral geniculate nucleus (LGNd) and the superior colliculus (SC). In normal possums the MTN receives an contralateral retinal projection. The LGNd has a laminar structure, there being three contralateral laminae and five ipsilateral laminae. In normal possums there is a contralateral retinal input to the superficial layers of the SC and in the front half of the SC there is an ipsilateral input top the deeper layers. In the one-eyed possums we observed a bilateral retinal projection to the MTN. This was best developed in possums which had lost one eye at an early age (about 25 days). In the one-eyed possums there was an extensive retinal projection to the superficial layers of the ipsilateral SC in addition to the normal retinal projection to the contralateral SC. In animals which had lost one eye at about 25 days the retinal fibers covered about three-quarters of the ipsilateral SC, whereas in animals which had been operated at about 80 days the retinal fibers occupied only part of the front half of the ipsilateral SC. Animals which had lost one eye at about 80 days showed some growth of retinal axons into the deafferented layers of the LGNd and these layers were thinner than the layers with a normal nerve input. In animals which had lost one eye at 25 to 50 days retinal fibers form the remaining eye covered the whole of the LGNd on both sides. The possum which lost one eye at age 102 days, when its eyes were open, had retinal pathways which were nearly normal.     

Single retinal ganglion cells projecting in bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals in the cat

In most mammals with frontalized eyes, retinal ganglion cells in the nasal or temporal retina send their axons to the contralateral or ipsilateral half, respectively, of the brain. Previous studies in the cat, however, have indicated a retinal region of “nasotemporal overlap” from which arise the retinal projections to both the contralateral and ipsilateral halves of the brain. The present study thus examined in the cat whether any retinal ganglion cells give rise to bifurcating axons that innervate both halves of the brain. By employing fluorescent retrograde double labeling, we investigated whether or not single retinal ganglion cells project bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals. After Fast Blue was injected into the lateral geniculate nucleus on one side and Diamidino Yellow was injected contralaterally into the lateral geniculate nucleus, 100–200 ganglion cells in each retina were double labeled with both tracers. These double-labeled cells were distributed primarily in the temporal retina, including the region around the vertical meridian and, additionally, in the nasal retina. About 60–80% of the double-labeled cells had large cell bodies (more than 25 μm in diameter), and the others had medium-sized ones (15–25 μm in diameter). The pattern of distribution of double-labeled cells, which was observed after the combined injection into both superior colliculi, was similar to that seen after the combined injection into both lateral geniculate nuclei; more than 9% of double-labeled cells, however, were large. The results indicate that a certain population of ganglion cells in the cat retina send their axons bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals. The bilaterally projecting ganglion cells are mostly large, corresponding probably to α cells (the morphological counterparts of Y cells). In comparison with the patterns of bilateral projections of single retinal ganglion cells in the rat and monkey, the pattern of the bilateral retinofugal projections in the cat could represent an intermediate between those in the rat and monkey. © 1994 Wiley-Liss, Inc.     

Evidence for cerebellar efferents to the ventral lateral geniculate nucleus and the lateral terminal nucleus of the accessory optic system in the rabbit. A morphological study with comments on the organizational features of visuo-oculomotor-trunco-cerebellar loops

The efferent ascending connections of the cerebellar nuclei and afferent optic projections to the ventral lateral geniculate nucleus and the terminal nuclei of the accessory optic tract were traced in the 26 rabbits using the technique of experimental anterograde degeneration. Following eyeball enucleation, within the ventral lateral geniculate nucleus terminal degeneration was found mostly contralaterally and was restricted to both the sublayers (external and internal) of the lateral division, while ipsilaterally only scanty and confined to the dorsal region of the external sublayer of the lateral (sector alpha) division. After cerebellar lesions degeneration was found within the ventral region of the medial division (sector gamma) of the contralateral LGv and within contralateral LTN. From the localization of the lesions in the cerebellar nuclei, as well as from the distribution of degenerations in the area of the LGv, it was postulated that the parent neurons for the cerebello-LGV fibers are located in the contralateral posterior interposed nucleus, although the anteroventral lateral cerebellar nucleus, the Y group and the infracerebellar nucleus have been not excluded. Within the all terminal nuclei of the accessory optic tract the retinal fibers were found to terminate bilaterally with contralateral preponderance, mostly in the MTN, while ipsilateral fibers terminate most extensively in the lateral terminal nucleus of the accessory optic tract (LTN). In this means the retinal afferents of both sides seem to subserve the contralateral lateral cerebellar nucleus control. Taken together, the findings indicate that the extrageniculate visual inputs might be subjected to direct reciprocal cerebello-nuclear control. The visual extrageniculate cerebellopetal pathways and their correlations with the vestibulo-ocular and optokinetic reflex loops are discussed.     

Prenatal development of retinogeniculate projections in the rabbit: an HRP study

The prenatal development of the rabbit's retinal projections to the dorsal lateral geniculate nucleus (dLGN) was studied by using anterograde axonal transport of HRP injected intraocularly. Further, the ontogenesis of the dLGN's alpha and beta sectors was studied. Fetuses aged embryonic day 18 (E18) to E29 were examined. Gestation in the rabbit is 30-31 days. On E18 the future dorsal lateral and medial geniculate nuclei appear as a continuous strip of cells along the lateral margin of the dorsal thalamus. On E21 labelled retinal fibers are invading the lateral margin of the dLGN contralateral, but not ipsilateral, to an injected eye. At this age the dorsal lateral and medial geniculate nuclei are separating. By E23 contralateral fibers occupy the entire presumptive alpha sector, while ipsilateral fibers are invading the caudal half of the sector, overlapping the contralateral fibers. At this age the alpha and beta sectors begin to differentiate. On E25 contralateral fibers are more densely distributed throughout the alpha sector and the ipsilateral fibers are concentrated dorsally within the caudal three-quarters of the sector. By E27 contralateral fibers begin to withdraw from a medial zone of the alpha sector, while ipsilateral fibers remain densest in this zone and begin to withdraw from more lateral and caudal aspects of the sector; contralateral fibers, but not ipsilateral fibers, invade the beta sector. At this age the alpha and beta sectors acquire an adult-like appearance. By E29 the contralateral fibers vacate the beta sector and the medial zone of the dLGN and the ipsilateral fibers are restricted to this zone. Thus, 1 or 2 days before birth, the locations of the ipsilateral and contralateral retinal projections to the dLGN resemble those seen in the adult. The early overlapping projections of ipsilateral and contralateral retinal fibers within the dLGN and their eventual segregation in the fetal rabbit are consistent with the development of these projections in other mammalian orders. Further, the brief invasion of the beta sector by the contralateral fibers resembles the transient occupation of the carnivores' perigeniculate nucleus by developing retinal fibers. In addition, direct comparisons of temporal and spatial events during retinal innervation of the dLGN and the superior colliculus indicate several developmental differences between the two nuclei.     

Retinogeniculate projections in the rabbits of the albino allelomorphic series

Retinogeniculate pathways have been studied by fiber degeneration and autoradiographic methods in rabbits that are homozygous for alleles of the albino series of genes. It has been found that albino and Himalayan rabbits, which both lack all melanin pigment in the eye, have a similar abnormality of the retinogeniculate pathway. The number of ipsilateral optic fibers going to the lateral geniculate nucleus is reduced in these rabbits, the ipsilateral projection forms a discontinuous terminal zone instead of the normal continuous zone, and some of the ipsilateral axons terminate in an inappropriate part of the nucleus, so that regions receiving a crossed input in normal rabbits receive an uncrossed input in the abnormal rabbits. Chinchilla rabbits show a slightly reduced fur pigmentation but have a normal distribution of pigment in the retinal pigment epithelium and these rabbits have normal retinogeniculate pathways. In addition, the normal retinogeniculate pathway was studied. Autoradiographic methods show that the β segment of the lateral geniculate nucleus receives a contralateral input. Hence, earlier views that this segment projects to the visual cortex but receives no retinal input, are untenable. Further, in the autoradiographic material it was not possible to identify separate ipsilateral laminae and it was concluded that in the normally pigmented rabbit the ipsilateral retinogeniculate projection forms one relatively continuous group.     

Retinal projections to the superior colliculus and dorsal lateral geniculate nucleus in the tammar wallaby (Macropus eugenii): I. Normal topography

The topography of retinal projections to the superior colliculus and dorsal lateral geniculate nucleus of a wallaby, the tammar (Macropus eugenii), was investigated by an anatomical method. Small laser lesions were made in the retinas of experimental animals, and the remaining retinal projections were visualized by means of horseradish-peroxidase histochemistry. The position of each lesion was correlated with the position of the filling defects in the terminal label. The whole of the retina projects to the contralateral superior colliculus. The nasal retina is represented caudally, and the temporal retina rostrally. The ventral retina is represented medially, and the dorsal retina laterally. There is a projection to the ipsilateral superior colliculus, but it is patchy and its topography could not be determined by this method. The retinotopic map in the contralateral dorsal lateral geniculate nucleus has the nasal retina represented rostrally and the temporal retina caudally in the nucleus. The dorsal retina is represented ventrally, and the ventral retina is represented dorsally. It appears that the whole of the retina projects contralaterally, and in addition the temporal retina projects ipsilaterally. The maps of visual space through the two eyes were shown to be in topographic register in the binocular region by making a deposit of HRP in the visual cortex. This resulted in a column of retrogradely labeled cells in the nucleus. This column crossed the laminae, which are innervated by the ipsilateral and contralateral eye at right angles.     

An experimental anatomical study of the optic nerve fibers in the rat: Courses of the accessory optic tract

By means of silver impregnation and an HRP method, courses of the accessory optic tract were examined in albino and pigmented rats. The accessory optic tract is composed of 3 fasciculi: anterior, lateral and dorsal. The anterior fasciculus gives off fibers to the subthalamic nucleus and terminates in the medial terminal nucleus. The lateral fasciculus branches from the main optic tract at the level of the ventral nucleus of the lateral geniculate body and descends the lateral surface of the crus cerebri to enter the medial terminal nucleus after contributing a few fibers to the lateral terminal nucleus. The dorsal fasciculus originates from the brachium colliculi superioris and descends the posterior surface of the medial geniculate body and the posterolateral surface of the crus cerebri as an independent fasciculus to enter the medial terminal nucleus. This fasciculus supplies many fibers to the dorsal terminal nucleus.