Postnatal development of retinal projections in the brushtailed possum, Trichosurus vulpecula

High precision systems require high precision "blueprints": a new view regarding the formation of connections in the Mammalian visual system

Abstract It is well established that early in development interconnections within the mammalian visual system are often more widespread and less precise than at maturity. The literature dealing with the formation of visual connections has largely ignored differences in developmental specificity among species differing in their phylogenetic status and/or the visual ecological niche that they occupy. Based on a review of the available evidence, we have formulated an hypothesis to account for the varying degrees of developmental specificity that characterize different visual systems. It is suggested that extremely precise systems required for high-acuity binocular vision exhibit fewer presumed developmental errors than do visual systems characterized by poorer acuity and relatively crude depth perception. The developmental implications of the hypothesis are considered, and specific experiments are proposed to further test its validity.

Synaptogenesis in retino-receptive layers of the superior colliculus of the opossum Didelphis marsupialis

The maturation of the neuropil and synapse formation were examined in the retino-receptive layers of the superior colliculus (SCr-r) in the opossum from a period prior to the onset of arborization of retinocollicular fibers (postnatal day 22 - P22), at 44% of the coecal period (CP), to the end of the fast phase of optic fiber myelination and weaning time (P81 - 118% CP). Development of the SCr-r neuropil follows a protracted time course and can be divided into three broad stages, which are characterized by (I) Large extracellular spaces, numerous growth cones that participate rarely in synaptic junctions, vesicles-poor immature synapses (P22-P30), (II) Synapses of varied morphology with abundant synaptic vesicles, and small terminals with dark mitochondria and round synaptic vesicles (RSD terminals) synapsing mostly onto dendritic shafts, flat-vesicles (F) terminals (P40-P56), (III) Sequential appearance of retinal (R) and pleomorphic-vesicles (P) terminals and of RSD terminals synapsing onto spine or spine-like processes, appearance of glomerulus-like synaptic arrays (synaptic islets) (P61-P81). The advancement of synaptogenesis in SCr-r from stage I to II and from stage II to III correlates closely with the differentiation of astrocytes and oligodendrocytes, respectively.

The initial stages of development of the retinocollicular projection in the wallaby (Macropus eugenii): distribution of ganglion cells in the retina and their axons in the superior colliculus

The time course of ingrowth of retinal projections to the superior colliculus in the marsupial mammal, the wallaby (Macropus eugenii), was determined by anterograde labelling of axons from the eye with horseradish peroxidase, from birth to 46 days, when axons cover the colliculus contralaterally and ipsilaterally. The position of retinal ganglion cells giving rise to these projections over this period was determined in fixed tissue by retrograde labelling from the colliculus with a carbocyanine dye. Axons first reach the rostrolateral contralateral colliculus 4 days after birth and extend caudally and medially, reaching the caudal pole at 18 days and the far caudomedial pole at 46 days. The first contralaterally projecting cells are in the central dorsal and temporal retina, followed by cells in the nasal and finally the ventral retina. They are distributed closer to the periphery with increasing age. The first sign of a visual streak appears by 18 days. Axons reach the ipsilateral colliculus a day later than contralateral axons and come from a similar region of the retina. The sparser ipsilateral projection reaches the caudal and medial collicular margins by 46 days but by 16-18 days, ganglion cells giving rise to this transient projection are already concentrated in the temporoventral retina. The orderly recruitment of ganglion cells from retinotopically appropriate regions of the retina as axons advance across the contralateral colliculus suggests that the projection is topographically ordered from the beginning. The ipsilateral projection is less ordered as cells are located in the temporoventral crescent at a time when their axons are still transiently covering the colliculus prior to becoming restricted to the rostral colliculus. Features of mature retinal topography such as the visual streak and the location of ipsilaterally projecting cells begin to be established very early in development, before the period of ganglion cell loss and long before eye opening at 140 days.

Rapid plastic response following early retinal lesions in rats

Following an early retinal lesion, aberrant uncrossed projections from the opposite, undamaged, retina form in the target visual nuclei. The present study has examined the development of such aberrant projections by making retinal lesions in newborn rat pups, and then examining the nature of the uncrossed retinocollicular projection at different ages following the lesion. Intravitreal injections of horseradish peroxidase were made into the intact eye, and the uncrossed projection was subsequently revealed histochemically. A mature aberrant projection forms as early as postnatal day 9. On postnatal days 5 and 2, aberrant projections are discernable amongst the exuberant uncrossed terminals of normal developing rats, although the former have not matured to form the dense terminal fields characteristic of older projections. Aberrant projections were also detectable as early as 12 h following the lesion, revealed as a relative increase in the density of uncrossed label. These results indicate that lesion-induced plastic responses by intact retinal arbors are initiated shortly after the insult, and they caution the use of retinal lesions in studies of normal retinotopic connectivity during development.

Generation and death of cells in the dorsal lateral geniculate nucleus and superior colliculus of the wallaby, Setonix brachyurus (quokka)

To study postnatal cell generation in primary visual centres of the quokka, tritiated thymidine was injected into pouch-young aged postnatal day (P)1-P85. Brains were examined at P100, just before eye-opening, when primary visual projections are essentially mature. Neurons in the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) were generated at P1-P10 and P1-P18 respectively. Peak numbers of labelled cells were seen at P3 and P5 in the dLGN and SC. Cell death was assessed in the dLGN and SC of young aged P10-P150. Low numbers of dying cells were seen in the dLGN throughout this period, with a small peak at P85. A more substantial peak of cell death was seen in the SC, also at P85. In the quokka, the time interval between the peaks of cell generation and of cell death in the dLGN and SC is 70-80 days, considerably longer than the interval of 40 days between birth and death of retinal cells.

Cytogenesis in the monkey retina

Time of cell origin in the retina of the rhesus monkey (Macaca mulatta) was studied by plotting the number of heavily radiolabeled nuclei in autoradiograms prepared from 2- to 6-month-old animals, each of which was exposed to a pulse of 3H-thymidine (3H-TdR) on a single embryonic (E) or postnatal (P) day. Cell birth in the monkey retina begins just after E27, and approximately 96% of cells are generated by E120. The remaining cells are produced during the last (approximately 45) prenatal days and into the first several weeks after birth. Cell genesis begins near the fovea, and proceeds towards the periphery. Cell division largely ceases in the foveal and perifoveal regions by E56. Despite extensive overlap, a class-specific sequence of cell birth was observed. Ganglion and horizontal cells, which are born first, have largely congruent periods of cell genesis with the peak between E38 and E43, and termination around E70. The first labeled cones were apparent by E33, and their highest density was achieved between E43 and E56, tapering to low values at E70, although some cones are generated in the far periphery as late as E110. Amacrine cells are next in the cell birth sequence and begin genesis at E43, reach a peak production between E56 and E85, and cease by E110. Bipolar cell birth begins at the same time as amacrines, but appears to be separate from them temporally since their production reaches a peak between E56 and E102, and persists beyond the day of birth. Müller cells and rod photoreceptors, which begin to be generated at E45, achieve a peak, and decrease in density at the same time as bipolar cells, but continue genesis at low density on the day of birth. Thus, bipolar, Müller, and rod cells have a similar time of origin. The maximal temporal separation of cell birth is between cones and amacrine cells so that cell generation exhibits two relatively distinct phases: the first phase gives rise to ganglion, horizontal, and cone cells, and the second phase to amacrine, bipolar, rod, and Müller cells. In addition, cells of the first phase are generated faster than the second phase cells, and there are differences in the topography of spread of labeled cells between the two phases. Each cell class displays a central-to-peripheral gradient in genesis, although the spatiotemporal characteristics of the gradients differ between the classes.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of connections to and from the visual cortex in the wallaby (Macropus eugenii)

The time course of the development of connections between the visual cortex and the main subcortical visual structures, as well as intrahemispheric and interhemispheric connections, has been studied in the marsupial wallaby (Macropus eugenii) to compare its development with that of placental mammals. Pouch young are born prior to retinal innervation of the primary visual centers and spend a protracted period of development in the pouch, making them ideal for visual, developmental studies. Horseradish peroxidase conjugated to wheatgerm agglutinin was injected into either the presumptive visual cortex or the superior colliculus in young of varying ages. Thalamocortical projections from the dorsal lateral geniculate and lateral posterior nuclei reach the presumptive visual cortex between 12 and 15 days after birth. Descending cortical connections form later. Corticogeniculate axons are first detected in the geniculate and lateral posterior nucleus at 48 days after birth, while corticocollicular axons first reach the superior colliculus at 71 days and, by 81 days, have innervated the superficial layers. Intrahemispheric and interhemispheric connections form even later. By 99 days intrahemispheric axons from area 17 have accumulated in visual association areas but are yet to invade layers III and IV, their major termination zones in adult, while axons projecting back to area 17 have also reached their target area. At this time interhemispheric axons from area 17 have begun to accumulate in the opposite visual cortex, although they have not invaded the cortical layers. By 111 days cortical cells projecting to the opposite visual cortex are first labelled. These have a more widespread distribution in area 17 at 111 and 122 days compared to the adult, where they are confined to the 17/18 border. The results show that the marsupial wallaby has a timetable of similar sequence, but different relative timing, in the formation of cortical connections compared to that of placental mammals. In the first half of the period between conception and eye opening, the timing in the wallaby precedes considerably that in placental mammals. Ascending connections from the thalamus develop relatively earlier in the wallaby but descending collicular connections are delayed until the same relative time that they appear in placental mammals.

Entrainment of circadian phase in developing gray short-tailed opossums: mother vs. environment

In a marsupial species, the gray short-tailed opossum (Monodelphis domestica), the suprachiasmatic nuclei (SCN), the site of a circadian clock, are formed postnatally and begin oscillating as a circadian clock on day 20. In this study, we examined how the timing (phase) of the SCN clock in the developing opossum is coordinated to the environmental light-dark cycle. When pups were reared from birth in darkness by intact dams, the circadian phases in SCN metabolic activity (monitored by 2-deoxy-D-[14C]glucose autoradiography) in 27-day-old pups were desynchronized. When pups were reared in a light-dark cycle that was 12 h out of phase with the circadian time of blinded dams, the pattern of SCN metabolic activity on day 20 was rhythmic and in phase with the light-dark cycle but out of phase with the circadian time of the dam. On day 20, retina-mediated light activation of SCN metabolic activity was also demonstrated, and anterograde tract-tracing studies revealed the presence of the retinohypothalamic tract within the SCN. These results show there is no influence of the opossum dam on the timing of the pup's biological clock. Instead, from the inception of the daily rhythm in SCN metabolic activity, its timing is regulated by retina-mediated light-dark entrainment.

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.

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.

Developmental changes in the pattern of retinal projections in pigmented and albino rabbits

The distribution of retinal axons and/or terminals in the retino-recipient nuclei of pigmented and albino rabbits varying in age from the 24th postconceptional day (24PCD) to adulthood was examined following unilateral intraocular injections of the enzyme horseradish peroxidase. Both in pigmented and albino rabbits contralateral retinal axons and/or terminals in the dorsal and ventral lateral geniculate nuclei (DLG and VLG), superior colliculi (SC), pretecta (PT) and accessory optic tract nuclei (AON) were already present on 24PCD. In the period 26-30PCD the contralateral projection occupied the entire volume of the DLG, VLG and SC. Although 32PCD (the day of birth) the proportions of the volumes of DLG and VLG occupied by the contralateral projections were slightly reduced, their volume continued to increase in absolute terms up to adulthood. In pigmented rabbits the ipsilateral projections to all retino-recipient nuclei were most dense and extensive on 26PCD. From 26PCD, the relative extent of the ipsilateral projections was gradually reduced, but a reduction in their absolute extent did not become evident until 32PCD. By 32PCD the ipsilateral projection to the AON had disappeared completely. The distribution of ipsilateral axons and/or terminals and the relative proportion of the nuclei occupied by the ipsilateral projection in all other retino-recipient nuclei had become adult-like by 34PCD. In albino rabbits only a sparse ipsilateral projection to the presumptive superficial collicular layers was present on 24PCD. In the remaining retino-recipient nuclei an ipsilateral projection was present on 26PCD. From 26PCD the relative extent and from 30PCD the absolute extent of ipsilateral retinal axons and/or terminals was gradually reduced. The relative extent of the ipsilateral projection had become almost adult-like by 34PCD. Throughout development ipsilateral projections in albinos were consistently less dense and less extensive than those in pigmented rabbits, and unlike in pigmented rabbits, the ipsilateral projections to the VLG and PT were only transient. The differences between the two strains in the pattern of retinofugal projections were further enhanced during the period of segregation of the ipsilateral and contralateral projections. Considering the fact that in both strains there is a partial correspondence between the period in which the spatial extent of the ipsilateral projections is reduced and the period of retinal ganglion cell (RGC) death, it is likely that RGC death plays a role in the process of segregation of the retinal afferents into ocular domains. However, our data suggest that other mechanism(s) also play an important role in the process.

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

The prenatal development of the rabbit's retinal projections to the superior colliculus (SC) was studied by using anterograde transport of horseradish peroxidase injected intraocularly. Fetuses aged embryonic day 21 (E21) to E29 and an adult rabbit were examined. Gestation in the rabbit is 30-31 days. On E21 contralaterally projecting retinal fibers invade across the entire SC. Their distribution is initially diffuse within the superficial laminae, but by E29 they have a distinct stratified appearance. Ipsilaterally projecting retinal fibers invade the rostral half of the SC on E21. By E23 they cover the entire SC and overlap the contralateral fibers both tangentially and radially. The ipsilateral fibers for the most part are sparsely distributed, but they form a dense focal distribution in the rostrolateral quarter of the SC. This focus straddles the stratum griseum superficiale/stratum opticum (SGS/SO) border. On E25 the ipsilateral fibers maintain their widespread distribution and focal rostrolateral concentration. By E27 they are excluded almost entirely from the caudal half of the SC and are reduced in density in the rostromedial quarter of the nucleus. On E29 the ipsilateral terminal field forms distinct patches and bands that are restricted to the rostrolateral quarter of the SC and are confined to the SGS/SO border. Thus, a few days before birth the pattern and location of the ipsilateral retinocollicular projection resemble those seen in the adult. The early widespread distribution of the ipsilaterally projecting retinal fibers to the SC and their eventual restriction in the fetal rabbit are consistent with the development of this projection in other mammalian orders.

Expanded retinofugal projections to the dorsal lateral geniculate nucleus and superior colliculus after unilateral enucleation in the wallaby Setonix brachyurus, quokka

We removed one eye of quokkas either neonatally, before retinal innervation of visual centres, or at 35-40 days postnatal, when projections overlap bilaterally and are more widespread than in the adult. Retinal projections to the dorsal lateral geniculate nucleus and superior colliculus at postnatal day 100 were demonstrated following anterograde transport of horseradish peroxidase. There were significant reductions in the size of the dorsal lateral geniculate nucleus and superior colliculus ipsilateral to the remaining eye. However, the extent of retinofugal projections was markedly expanded in comparison to the normal input from one eye. Unexpectedly, projections were expanded to similar extents in the two series of enucleated animals although ipsilateral labelling appeared more dense after neonatal enucleation. In controls, label was restricted to eye-specific regions but in enucleated animals there were no label-free zones. Nevertheless the alpha laminae remained distinct in the dorsal lateral geniculate nucleus of enucleated animals. Our findings suggest that binocular interactions are necessary for the segregation and refinement of visual projections but not for the formation of the alpha laminae.

Retinal ganglion cell number is unchanged in the remaining eye following early unilateral eye removal in the wallaby Setonix brachyurus, quokka

The expanded visual projections which develop after unilateral eye removal have been associated in some studies, but not in others, with the survival of more ganglion cells than normal in the remaining eye. We have addressed this issue using the small wallaby Setonix brachyurus, quokka. Moreover to determine whether more ganglion cells survive when the eye is removed at a very early stage, we have compared the effect of enucleations at two ages. These were within 3 days of birth, before optic fibres innervate visual centres, and at 35-40 days postnatal, when visual projections are exuberant. At 100 days postnatal, retinal ganglion cells were retrogradely labelled from primary visual centres and tracts with horseradish peroxidase, allowing 24 h for transport. Numbers of ganglion cells were similar between animals enucleated as neonates (X = 231,000, n = 3) and at 35-40 days postnatal (X = 218,000, n = 4). These results were comparable to those of controls (X = 227,000, n = 5). Distributions of ganglion cells were also essentially similar in experimental and control series. However, mean ganglion cell soma diameter was significantly greater than normal in both the area centralis and temporal retina after neonatal enucleation. Our results indicate that in enucleated quokkas increased ganglion cell numbers do not underlie enhanced retinofugal projections.

Effects of very early monocular and binocular enucleation on primary visual centers in the tammar wallaby (Macropus eugenii)

The role of retinal afferents and their binocular interactions in the development of mammalian primary visual centers has been studied in the marsupial wallaby. Monocular and binocular enucleation was performed prior to any retinal innervation of the visual centers. After monocular enucleation retinal projections were traced by horseradish peroxidase histochemistry and compared with those in normal animals and those during development. The topography of retinal projections to the superior colliculus and the dorsal lateral geniculate nucleus after monocular enucleation was determined by making retinal lesions and tracing the remaining projections with horseradish peroxidase. The position and nature of the filling defects in terminal label were compared with controls with similarly placed lesions. The superior colliculus and dorsal lateral geniculate nucleus ipsilateral to the remaining eye were shrunken. Projections to the ipsilateral superior colliculus, ipsilateral accessory optic nuclei, and ipsilateral suprachiasmatic nucleus, although enlarged, never approached the density contralaterally, as was also the case during normal development. The expanded projection in the ipsilateral superior colliculus came primarily from temporal and ventral retina. In the dorsal lateral geniculate nucleus, terminal bands and cellular laminae, although not identical to normal, did develop. During normal development overlap of afferents from the two eyes occurs in the binocular region. The decrease in volume of the nucleus ipsilateral to the remaining eye after monocular enucleation suggests that the monocular region disappears in the absence of appropriate input and the binocular region survives. Contralaterally there was no decrease in volume, compatible with this idea. The topography of retinal projections supports this interpretation. It was normal contralaterally while ipsilaterally it was appropriate for the normal binocular region. There was an expansion of the projection along the lines of projection in what would normally be binocular regions of the nucleus, where retinal afferents failed to segregate in the absence of binocular competition. After binocular enucleation the alpha and beta segments of the dorsal lateral geniculate nucleus were still recognizable but cell-sparse zones were absent, as was the characteristic orientation of primary dendrites of geniculocortical cells. There are rigid developmental constraints operating on the innervation of territory by retinal afferents from the two eyes, and many features of the mature pattern arise without binocular interactions during development.

The accessory optic system of the wallaby, Setonix brachyurus: anatomy in normal animals and after early unilateral eye removal

We have traced primary visual projections to nuclei of the accessory optic system in the mature wallaby, Setonix brachyurus, the "quokka," following unilateral intraocular injections of horseradish peroxidase. The organization of pathways and nuclei is similar to that of other marsupials and to that of eutherian mammals. The dorsal, lateral and medial terminal nuclei receive bilateral input, though nuclei ipsilateral to the injected eye are weakly labelled in comparison with their contralateral counterparts. We also report on the accessory optic system in animals which were unilaterally enucleated neonatally or at postnatal day 35. At maturity in enucleated animals, ipsilateral projections to all nuclei of the accessory optic system are more densely labelled than normal. This exuberance is more pronounced in neonatally enucleated animals than in those enucleated at the later stage.

Comparative anatomy of the mammalian hypothalamic suprachiasmatic nucleus

A detailed analysis of the cytoarchitecture, retinohypothalamic tract (RHT) projections, and immunohistochemical localization of major cell and fiber types within the hypothalamic suprachiasmatic nuclei (SCN) was conducted in five mammalian species: two species of opossum, the domestic cat, the guinea pig, and the house mouse. Cytoarchitectural and immunohistochemical studies were conducted in three additional species of marsupial mammals and in the domestic pig. The SCN in this diverse transect of mammalian taxonomy bear striking similarities. First, the SCN are similar in location, lying close to the third ventricle (3V) dorsal to the optic chiasm (OC), with a cytoarchitecture characterized by small, tightly packed neurons. Second, in all groups studied, the SCN receive bilateral retinal input. Third, the SCN contain immunohistochemically similar elements. These similarities suggest that the SCN developed characteristic features early in mammalian phylogeny. Some details of SCN organization vary among the species studied. In marsupials, vasopressin-like immunoreactive (VP-LI) and vasoactive intestinal polypeptide-like immunoreactive (VIP-LI) cells codistribute primarily in the dorsomedial aspects of the SCN, while in eutherians, VP-LI and VIP-LI cells are separated into SCN subnuclei. Furthermore, the marsupial RHT projects to the periventricular dorsomedial region, whereas the eutherian RHT projects more ventrally in the SCN into the zone that typically contains VIP-LI perikarya.

Axon trajectories and pattern of terminal arborization during the prenatal development of the cat's retinogeniculate pathway

In this study we have examined the trajectories taken by populations of ganglion cell axons and the spatial gradients of terminal arbor maturity within the lateral geniculate nucleus (LGN) during the prenatal development of the cat's visual system. To do so, an in vitro method of labeling optic tract axons from fetal brains between embryonic day 37 (E37) and postnatal day 2 (P2) with horseradish peroxidase (HRP) was used. At the earliest ages studied (E37-E53), optic axons leave the optic tract to run across the LGN toward their sites of termination in straight trajectories parallel to each other. At later ages (E57-P2), however, axons with abrupt changes in their course across the nucleus can be clearly identified. When the detailed terminal arbor morphology of the set of retinogeniculate axons filled with HRP at a given age was examined, two different spatial gradients of maturation could be detected. The terminal arbors of axons within LGN layer A are always more mature than those ending in layer A1, an observation consistent with previous findings that axons from the contralateral eye arrive within the LGN several days before those from the ipsilateral eye. Moreover, the terminal arbors of axons projecting to the medial portions of each layer are always more mature than their more lateral counterparts. These gradients are likely to be a direct reflection of the central-first, peripheral-last gradient associated with the neurogenesis of the retinal ganglion cells themselves. In the oldest animals studied (E58-P2), a remarkable periodic pattern of terminal arbor labeling was seen following a localized HRP injection into the optic tract. Within the labeled portions of the LGN, densely filled axon terminal arbors are separated by unlabeled gaps of similar width. This pattern of labeling could reflect local topographic disorder within the optic tract or could arise if axons of different classes of retinal ganglion cells run in separate portions of the optic tract. Taken together, all of these observations suggest that there may be a fair degree of topographic order in the retinogeniculate projection within the cat's LGN early on in development. However, when topographic errors are present, some can be corrected by minor readjustments in axonal trajectories.

The ipsilateral retinocollicular projection in the rabbit: an autoradiographic study of postnatal development and effects of unilateral enucleation

The postnatal development of the ipsilateral retinocollicular projection in the rabbit and the effects of unilateral enucleation (performed on the day of birth, day 0) on that development were studied by using the anterograde axonal transport of tritiated proline injected intraocularly. Material from 1-, 6-, and 10-day-old (i.e., at days 1, 6, and 10) and adult animals was examined. On day 1, autoradiographically labelled optic fibers from the ipsilateral eye formed distinct patches and bands within the superior colliculus (SC), which were restricted primarily to the lateral one-half and anterior one-third to one-half of the nucleus. At subsequent ages no major changes in the location of this projection were found for normal animals or animals enucleated on day 0 (0-DE animals). From dorsal-view reconstructions, the pattern of the ipsilateral projection appeared wedge-shaped with a broad base aligned with the lateral SC border for all normal and 0-DE animals at the various postnatal ages examined. In normal animals the surface area of this projection increased with age and maintained a constant proportion of the increasing surface area of the total SC. In 0-DE animals the surface area of the projection initially increased more rapidly than in normal animals. Thus, by day 6 the area was already within the normal adult range but did not exceed this range at later postnatal ages. The only obvious difference in the appearance of the ipsilateral retinocollicular projection between normal and 0-DE animals at corresponding ages was an enhanced radial distribution of the projection across laminae in the 0-DE animals. Taken together these findings suggest that, in the rabbit, once topographically appropriate connections are established between the SC and the ipsilateral retinal projection, they are maintained regardless of substantial postnatal growth of the SC and removal of the contralateral retinal projection to the SC.

Development of visual projections in the marsupial, Setonix brachyurus

Retinal projections to the primary visual centres were studied following injection of tritiated proline into one eye in the Marsupial, Setonix brachyurus between 10 and 100 days postnatal and in adults. Initially, projections from the two eyes overlapped extensively, particularly between 20 and 50 days. There was a gradual refinement thereafter, including a segregation of inputs from the two eyes within both the lateral geniculate nucleus (LGN) and superior colliculus (SC) by 70 days. Such refinement in visual centres is discussed in relation to the concurrent emergence of retinal ganglion cell density gradients, a decrease in ganglion cell numbers, cell death in the ganglion cell layer and loss of optic axonal profiles.

The effects of neonatal monocular enucleation on the organization of ipsilateral and contralateral retinothalamic projections in the rabbit

Autoradiographic methods were used to compare the ipsilateral and contralateral retinothalamic projections in pigmented Dutch-Belted rabbits that had neonatal monocular enucleation with the projections found in normally reared rabbits. In the normal adult rabbit, there is dense label throughout the dorsal lateral geniculate nucleus (LGd) except for a decreased label density in the region corresponding to the ipsilateral input. Following neonatal monocular enucleation, the contralateral projection fills in the part of the LGd corresponding to the ipsilateral input. Our data indicate that following monocular enucleation, two processes occur: an arrest of the segregation process and an expansion of the contralateral projection into the space normally containing the terminals of the ipsilateral projection. In addition, this filling in of the terminal space occurs relatively rapidly and is completed by day 14. No changes, however, were observed in the ipsilateral projection to the LGd. Unlike the LGd, the ventral lateral geniculate nucleus and the intergeniculate leaflet showed increases in the size of the ipsilateral projection region, and no changes in the contralateral projection. The present findings suggest that there may be different mechanisms governing whether alterations in the distribution of retinothalamic projections will occur in either the ipsilateral or contralateral nucleus.

The topography of expanded uncrossed retinal projections following neonatal enucleation of one eye: differing effects in dorsal lateral geniculate nucleus and superior colliculus

The topographic organization of the uncrossed retinal projections to the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) was studied in normal adult hooded rats and in rats subjected to unilateral ocular enucleation on the day of birth. Sections were stained for anterograde degeneration products following discrete retinal lesions at various locations. The projection from the temporal crescent to the dLGN in neonatally enucleated rats had an expanded but topographically normal organization, with the nasotemporal and dorsoventral retinal axes displaying polarities identical to those in normal adults. Neonatal enucleation permits the remaining uncrossed retinogeniculate projection to extend primarily along the "lines of projection" into neuropil normally recipient of binocularly conjugate crossed projections. In the SC, the dorsoventral axis of the temporal crescent showed a normal polarity, but the nasotemporal axis failed to display any topographic organization. Retinal loci in the temporal crescent projected throughout the rostrocaudal extent of the ipsilateral SC. Retinal lesions placed outside the temporal crescent failed to produce any substantial degeneration in ipsilateral dLGN or SC. These topographically distinct effects in dLGN and SC following unilateral eye removal on the day of birth are discussed in the context of differing constraints upon axonal ingrowth and connectivity during early development, which may normally bring about the characteristically distinct features of retinogeniculate and retinocollicular organization.

An exuberant retinocollicular pathway in Siamese kittens: effects of competition and abnormal activity on its maturation

Retinocollicular pathways were studied in normally pigmented and Siamese adult cats and newborn kittens. In addition, retinocollicular pathways were studied in Siamese cats which were unilaterally enucleated on the day of birth and in Siamese cats which were reared in a stroboscopically illuminated environment. In adult Siamese cats the ipsilateral retinocollicular pathway is spatially less extensive than it is in adult normally pigmented cats. In contrast, the ipsilateral retinocollicular pathway in newborn Siamese kittens is widespread, while that of newborn normally pigmented kittens is restricted, as it is in normally pigmented adults. This comparison indicates that the spatial restriction of the retinocollicular pathway occurs after birth in Siamese cats. After enucleation or stroboscopic rearing the ipsilateral retinocollicular pathway in Siamese cats remains widespread. These results demonstrate the importance of interactions with afferents from other sources and the requirement for appropriate neural activity in the normal maturation of this initially exuberant pathway.

The morphology of relay neurons in the dorsal lateral geniculate nucleus of the marsupial brush-tailed possum (Trichosurus vulpecula)

The retinal terminal zones and the morphology of relay neurons within the dorsal lateral geniculate nucleus (LGNd) of the brush-tailed possum (Trichosurus vulpecula) have been investigated with horseradish peroxidase (HRP) tracing techniques. Anterograde transport of HRP from the retina confirmed previous descriptions of the laminar distribution of retinal afferents in this nucleus. In addition, it was found that lamina III consists of two adjacent bands (IIIa and IIIb) of contralateral retinal input, separated by a terminal-free zone 20-40 micron wide. This zone is not apparent with Nissl or fibre stains. Relay neurons in the LGNd were retrogradely filled following cortical injections of HRP, and two classes (A and B) were distinguished. Class A neurons are found in the alpha portion of the LGNd (laminae I, II, III, and IV) and class B neurons in the beta portion (laminae V, VI, and VII). Class A cells are more densely packed and have shorter and more numerous dendrites, less-extensive dendritic arbors, and thicker axons than class B cells. No significant differences were found between the two classes in perikaryal size or thickness of proximal dendrites. Neurons in each lamina of the nucleus have dendritic arbors which ramify extensively within adjacent laminae, except cells in lamina IIIb, which have relatively few dendrites that cross into the cell-free zone and lamina IV.

Retinal development in humans: the roles of differential growth rates, cell migration and naturally occurring cell death

The distribution of ganglion cells throughout the retinal ganglion cell layer is non-uniform in adult mammals. This paper reviews some of our data describing the development of retinal ganglion cell topography in the human fetus. Results indicated that early in the fetal period the distribution of cells in the ganglion cell layer is almost uniform, but by the end of gestation there is a gradient in cell density of about 10:1 (central:peripheral). Peripheral retina grows more rapidly than the central retina prior to about 23 weeks gestation, but this differential growth rate apparently has little effect on the development of a centro-peripheral density gradient. The gradient appears between about 18 and 30 weeks gestation, and during this period there appears to be a greater rate of cell death in the ganglion cell layer of the peripheral retina. Cell density at the developing fovea is less than the perifoveal cell density at all ages, suggesting that ganglion cells migrate from foveal into perifoveal regions throughout the fetal period.

Presence of retinogeniculate fibers is essential for initiating the formation of each interlaminar space in the lateral geniculate nucleus

We demonstrated in a previous study that, following neonatal bilateral enucleation in the tree shrew, interlaminar spaces (ILSs) in the dorsal lateral geniculate nucleus fail to form. In the present study we sought to determine which aspects of ILS formation are dependent upon retinal input. Accordingly, we studied the degree of ILS formation in tree shrews which were bilaterally enucleated either during ILS formation on postnatal day 3 (P3) or just after all ILS were apparent but before they had reached a mature width (P15). Our results indicate that retinal input is necessary for the initial formation of each ILS, but that it is not required for the maturation or maintenance of ILSs which have already begun to form.

Growth and restriction of the ipsilateral retinocollicular projection in the opossum

The distribution of optic nerve fibers and terminals in the superior colliculus (SC) was followed throughout its development in pouch young opossums in order to establish the normal sequence of events leading to the formation of mature patterns. Up to 7 days of life in the pouch, labeled fibers can be followed only as far as the rostral aspect of the optic tract. The earliest evidence for crossed retinal projections in the SC is found at 10 days of age. In parasagittal sections, the label extends along the rostrocaudal tectal axis from the rostral border to the presumptive caudal pole of the SC. Unequivocal evidence for ipsilateral retinocollicular projection is found at 15 days extending to all but the caudal 5th of the rostrocaudal extent of the SC. The projections from both eyes overlap extensively in the SC at 22 days and after this age significant changes occur, mostly at the ipsilateral side: a sub-pial tier of fine label develops excluding both rostral and caudal collicular poles; a deeper tier of coarse label extends from the rostral to the caudal pole and a third, patchy tier of label is found at the prospective strata griseum superficiale and griseum intermediate. By 47 and 60 days the tangential distribution of the projections is virtually indistinguishable from the adult pattern although laminar segregation does not seem as sharp as in the adult. Comparisons of the changeable patterns of ipsilateral retinocollicular projections from 22 to 34 days with the invariant, aberrant pattern in adult animals submitted to uniocular enucleation at either age suggests that the preservation of a juvenile pattern does not provide a comprehensive explanation for the formation of aberrant projections.

Synaptic organization of the dorsal lateral geniculate nucleus in the adult hamster. An electron microscope study using degeneration and horseradish peroxidase tracing techniques

The synaptic organization of the alpha sector of the dorsal lateral geniculate nucleus has been examined by electron microscopy in normal adult hamsters and in adult hamsters subjected to unilateral eye enucleation or intravitreal injection of horseradish peroxidase. Two types of neuropil are apparent. Islands of complex neuropil partially enclosed by astrocyte processes (synaptic glomeruli) are surrounded by a sea of simpler non-glomerular neuropil. The latter is dominated by small axon terminals with spherical synaptic vesicles and Gray type 1 axodendritic contacts (SR-boutons) and also contains axon terminals with flattened synaptic vesicles (F-boutons). The glomerular neuropil contains exclusively postsynaptic dendrites and dendritic protrusions of presumptive projection cells; pre- and postsynaptic pleomorphic-vesicle-containing P-boutons (interpreted as appendages of the dendrites of interneurons); large axon terminals containing spherical synaptic vesicles and large pale mitochondria (R-boutons) which were experimentally identified as retinal terminals and which are presynaptic to both projection cell dendrites and P-boutons at Gray type 1 contacts; F-boutons (minority component). F-boutons and P-boutons are presynaptic to both projection cell dendrites and P-boutons and P-boutons are the intermediate elements of various serial synapses including triplet (triadic) synapses. Medium-large terminals with spherical synaptic vesicles and dark mitochondria (RLD-boutons) which were commonly invaginated by dendritic spines of projection cells in small glomerulus-like formations were also identified. The origin of RLD-boutons is unknown but SR-boutons probably derive chiefly from ipsilateral visual cortex and possibly also from superior colliculus, and non-glomerular F-boutons probably originate in the ipsilateral thalamic reticular nucleus. No differences in synaptic organization were found between the part of the nucleus which receives uncrossed retinal input and the part which receives crossed input, nor were differences seen in the size, fine structure or relationships between the terminals of identified crossed and uncrossed retinal axons.

Normal postnatal development of retinogeniculate axons and terminals and identification of inappropriately-located transient synapses: electron microscope studies of horseradish peroxidase-labelled retinal axons in the hamster

Axons from the eyes reach the dorsal lateral geniculate nucleus of the hamster at birth and both crossed and uncrossed axons spread throughout the nucleus within which they overlap extensively between postnatal days 2-6, before segregating to terminate in different parts of the nucleus by days 8-10 [So, Schneider and Frost (1978) Brain Res. 142, 343-352]. We have labelled retinal axons and their terminations between the day of birth (day 0) and day 6 by injecting one eye with horseradish peroxidase a few hours prior to sacrifice. Labelled profiles were then systematically sought, identified and their position determined, by electron microscope study of large frontal thin sections of both dorsal lateral geniculate nuclei. Labelled crossed and a few labelled uncrossed axons were present at day 0 and became progressively more common over the following few days; appropriately-located labelled uncrossed axons and terminals in the centromedial part of the nucleus (future ipsilateral sector) were relatively less common than labelled crossed axons in the ventrolateral part of the nucleus (part of the future contralateral sector), particularly between days 0 and 3. Synaptic contacts established by such labelled axons were characterized by predominantly electron-lucent spherical presynaptic vesicles and a prominent postsynaptic density. At day 4, labelled uncrossed axons made synaptic contact in the future contralateral sector (which is devoid of uncrossed input after days 8-10) and a few crossed axons made synaptic contacts in the future ipsilateral sector (devoid of crossed input after days 8-10). Such terminals and their synaptic contacts, were identical to appropriately-located ones in the same material. Inappropriately-located terminals were not found in the future contralateral sector at day 6, or in adults. No specialized contacts were observed between inappropriately-located axons or terminals and either other axon terminals or glial cell processes. Thus, during the development of the hamster retinogeniculate projection, inappropriately-located axons establish transient synaptic contacts with geniculate cells, and these contacts are lost as the segregated adult pattern of projections is established.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Binocular interaction in the fetal cat regulates the size of the ganglion cell population

During fetal development of the cat's visual system there is a marked overproliferation of optic nerve axons. In utero binocular interaction contributes to the severity of fiber loss since removal of an eye during gestation attenuates axon loss in the remaining optic nerve. The purpose of the present study was to determine whether this reduced loss of optic nerve fibers is due to a failure of retraction by supernumerary axon branches or to a reduction in ganglion cell death. To resolve this issue, we compared the number of ganglion cells and optic nerve fibers in adult cats which had one eye removed at known gestational ages. Retinal ganglion cells were backfilled with horseradish peroxidase and counts were made from retinal wholemounts. The axon complement was assessed with an electron microscopic assay. In the retinas of a normal cat we estimated 151,000 and 152,000 ganglion cells. The optic nerves of two other normal cats contained approximately 158,000 and 159,000 axons. In comparison, an animal enucleated on embryonic day 42 had 180,000 ganglion cells and 178,000 optic nerve fibers, while in an animal enucleated on embryonic day 51 the corresponding estimates were 182,000 and 190,000. The close agreement between cell and fiber counts indicates that axonal bifurcation does not contribute appreciably to the axon surplus in the optic nerve of prenatally enucleated cats. These results demonstrate that prenatal binocular interaction regulates the size of the mature retinal ganglion cell population.

Observations on postnatal neurogenesis in the superior colliculus and the pretectum in the opossum

Postnatal neurogenesis has been detected in the superior colliculus (CS) and caudal pretectum of the opossum in the period ranging from 2 to 13 days ( PND2 to PND13 ) of life in the pouch. Examination of the pattern of labeling in specimens exposed to a pulse of tritiated thymidine ( [3H]T) in PND4 or PND7 and allowed 1.5 h survival reveals that postnatal cell proliferation for the CS is virtually confined to the ventricular zone with no evidence for in situ[3H]T uptake in the collicular plate. Semi-quantitative analysis in long survival animals shows that postnatal neurogenesis peaks later in the CS ( PND7 ) than in the caudal pretectum ( PND4 ) and also persists longer in the former than in the latter. Comparisons of the numerical density of heavily labeled neurons suggest the occurrence of ventro-dorsal and rostro-caudal gradients of neurogenesis in the CS. Separate analysis of superficial, intermediate and deep layers shows, in addition, a combined rostrolateral-to-caudomedial gradient of neurogenesis in the superficial layers. Comparisons of the time schedules of neurogenesis for the superficial layers and of the deployment of optic fibers suggest that migration of neurons to their eventual destination is completed at or after the arrival of afferents.

The normal and abnormal postnatal development of retinogeniculate projections in golden hamsters: an anterograde horseradish peroxidase tracing study

In adult hamsters, the dorsal lateral geniculate nucleus (LGd), which lacks a noticeable pattern of cellular lamination, receives fibers predominantly from the contralateral eye except for a medial segment which receives fibers from the ipsilateral eye. Using the method of anterograde transport of horseradish peroxidase (HRP), it is shown in this study that the contralateral and ipsilateral retinogeniculate fibers innervate the LGd by day 0 with the development of the contralateral fibers slightly ahead of the ipsilateral ones. The entire contralateral LGd is filled with retinal fibers by day 1. The ipsilateral LGd is almost completely covered with retinal fibers on day 2 but with the fiber density much higher in the dorsal half of the nucleus. Thus, fibers from both eyes overlap with each other completely beginning on day 2 in the LGd. The segregation of these fibers becomes obvious on day 6 as indicated by a decrease in the density of ipsilateral fibers in the ventral portion of the LGd while the ipsilateral projection continues to concentrate in the dorsal half of the nucleus. A low density area in the dorsomedial part of the contralateral LGd is observed on day 7. By day 8, the segregation of the contralateral and ipsilateral projections has achieved an adult-like pattern. Thus, there seems to be two phases for the normal development of the retinogeniculate fibers. In the first phase, axons from both eyes grow in and occupy the entire LGd. In the second phase, those axons occupying inappropriate areas of the LGd are eliminated to form the adult pattern. The effect of unilateral eye removal at birth on the development of the retinogeniculate projection from the remaining eye was also studied with the anterograde HRP method. The ipsilateral fibers in the experimental animals are distributed in the lateral portion of the nucleus in the first two postnatal days. The entire LGd is not filled with ipsilateral fibers until day 4. From day 6 onwards, the ipsilateral fibers are more extensive than those of the normal animals. In addition to a dense projection to the dorsal two-thirds of the LGd, moderate amount of ipsilateral axons can be detected in the remaining ventral portion of the nucleus in day 6 and older experimental animals. The development of the contralateral retinal fibers in the experimental animals is similar to that of the normal from day 1 to day 6, i.e. the entire LGd is densely filled with crossed optic axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of the retinal pathway to the pretectum of the cat

The development of retinal projections to the pretectal complex of prenatal and early postnatal cats has been examined using the anterograde transport of horseradish peroxidase and tritiated amino acids. As early as embryonic day 38, the entire dorsal pretectum is penetrated by retinal ganglion cell axons. At this stage the bilateral complement of retinal efferents appears to be dispersed uniformly within the pretectal anlage. A week later, on embryonic day 46, indistinct foci of peroxidase reaction product can be discerned within 2 of the primordial nuclei: the nucleus of the optic tract and the olivary nucleus. By embryonic day 56, five distinct bilateral fields of retinal fiber termination are apparent within the following regions: (i) the nucleus of the optic tract; (ii) the pretectal olivary nucleus; (iii) the posterior pretectal nucleus; (iv) the anterior pretectal nucleus; and (v) the medial pretectal nucleus. Four days before birth, on embryonic day 61, crossed and uncrossed retinal arbors are partially segregated within the nucleus of the optic tract and the pretectal olivary nucleus. The early postnatal retinal connection to the pretectum has an overall pattern virtually indistinguishable from that of the mature cat. The ontogeny of the retinal influx to the pretectum is similar to that of the retinocollicular projection. However, the development of retinal projections to the pretectum and superior colliculus appears to lag behind those to the dorsal lateral geniculate nucleus. These differences may reflect temporal and spatial gradients in the maturation of three major classes of retinal ganglion cells.