Cellular degeneration and synaptogenesis in the developing retina of the rat.

We have investigated the time course and magnitude of cellular degeneration in the ganglion cell layer and the presumptive amacrine and bipolar regions of the inner nuclear layer during the development of the retina in the rat. Pyknotic profiles are present in the ganglion cell layer during the first 2 postnatal weeks, reaching peak numbers during the first 4 postnatal days (corresponding to the time of greatest loss of ganglion cells and their axons: Potts et al., '82; Lam et al., '82; Perry et al., '83). Two observations suggest that the majority of pyknotic profiles present in the ganglion cell layer during the second postnatal week are not ganglion cells. First, following injection of kainic acid into one superior colliculus, degenerating ganglion cells in the contralateral retina are cleared within 24-48 hours. Therefore, since most ganglion cell and axon loss occurs within the first postnatal week, few of the pyknotic profiles present in the second week are likely to be ganglion cells. Second, the time course of cellular degeneration in the ganglion cell layer during the second postnatal week follows a very similar pattern to that seen in the presumptive amacrine sublayer of the inner nuclear layer. Such a correspondence suggests that two phases of cell death occur in the ganglion cell layer: during the first postnatal week the majority of dying cells are ganglion cells, and in the second, most cell death is due to a loss of displaced amacrine cells. In the inner nuclear layer pyknotic profiles are most numerous in the presumptive amacrine region on postnatal days 6 and 7, and in the presumptive bipolar region on day 10. Synaptogenesis in the inner plexiform layer occurs later but reflects the order of cell death. Thus, conventional (presumed amacrine) synapses were first observed on day 11 and synaptic ribbons (indicative of bipolar synapses) on day 13. These observations suggest that amacrine and bipolar cells initiate synapses only after their numbers have stabilized.

The early development of the optic nerve and chiasm in embryonic rat.

To establish the time course and major features of the development of the optic nerve and chiasm in the embryonic rat, the growth of axons from the retina to the brain has been studied by light and electron microscopy. On embryonic day 14 (E14), the first axons are generated by retinal ganglion cells. Fascicles of axons can be detected in the optic stalk at E14.5 and in the diencephalon by E15.0. In the vitreal retina and optic fissure, large extracellular spaces resemble the oriented channels previously described in the mouse. They form approximately 12 hours before the invasion of optic axons and contain hyaluronic acid. In the optic stalk and diencephalon of the rat, similar spaces are not present, but the timed autolysis of neuroepithelial cells could provide a pathway of minimal resistance for the earliest axons. Degenerating cells are prominent in the ventral stalk and rostral diencephalon prior to the arrival of the first optic axons that preferentially invade these regions. The role of pigment in the development of visual pathways is controversial. In one strain of rat, Manchester Hooded, the retinae are heavily pigmented, but little pigment is seen at any stage in the stalk; in albinos, pigment is absent from both retina and stalk. However, the distribution of axons within the developing optic stalk is very similar in both strains, suggesting that the reduction in size of the ipsilateral pathway observed in the albino rat compared with the Manchester Hooded is not due to a lack of pigment in the optic stalk early in development. Several factors previously reported to contribute to the development of retinotopic order in other species are also present in the rat. These include the sequence in which axons grow into the stalk, and fasciculation. Intermembranous contacts observed between growth cones and adjacent tissues suggest one mechanism by which fasciculation occurs. A small group of fascicles, which may represent the ipsilateral projection, diverges from the crossing fibers on E15.5, without evidence of being deflected by any glial or other structures.

Cotransplantation of embryonic mouse retina with tectum, diencephalon, or cortex to neonatal rat cortex.

Retinae from embryonic mice were transplanted to the occipital cortex of neonatal rats together with their normal target regions, tectum or diencephalon, from embryonic mice or rats. In control experiments, retinae were cotransplanted with embryonic rat occipital cortex. In over 80% of the experimental animals, both transplants differentiated and grew. Ganglion cells in the retinae cotransplanted close to tectum or diencephalon survived for at least 15 weeks. Their survival was associated with the development of a distinct optic fiber layer and outgrowth of axons from the transplanted mouse retina. Specific innervation of distinct patches within the cotransplanted rat tectum or diencephalon was demonstrated by the use of an anti-mouse antibody. The innervated regions, which could be as far away as 1.3 mm from the retinae, were correlated with cytological features of the cotransplanted tectum or diencephalon. By contrast, the host cortex was never innervated by the transplanted retinae. In the control animals in which the retinae were cotransplanted with occipital cortex and in four animals in which the cotransplants lay more than 2.7 mm apart, no ganglion cells were identified and there was no evidence of an optic fiber layer, outgrowth of axons, or innervation. These results support the idea that in order to survive, retinal ganglion cells need to innervate an appropriate target region. Further, the specific innervation of regions within the cotransplanted tectum or diencephalon suggests that these target regions are able to exert a tropic influence on the axons of retinal ganglion cells, even in the absence of many of the normal structure cues.

Properties of neurons in cat's dorsal lateral geniculate nucleus: a comparison between medial interlaminar and laminated parts of the nucleus.

We studied the receptive field properties of 460 cells in the cat's dorsal lateral geniculate nucleus (LGNd), 108 cells were located in the medial interlaminar nucleus (MIN) and 352 in the laminated part of the LGNd. In both the MIN and laminated parts of the LGNd, relay cells belonging to all three functional classes (W, X and Y) have been identified. Of cells in the laminated LGNd, about 32.5% were Y cells, about 54.5% were X cells and about 8.5% were W cells. By contrast, in the MIN, about 84% were Y cells, only about 4.5% being X cells and about 7.5%, W cells. In the laminated LGNd, Y cells represented 25% of cells with receptive fields near the area centralis (0-3 degrees eccentricity group) and about 42% in the group of cells with the most peripherally located receptive fields (20-40 degrees eccentricity group). A similar but much weaker trend was observed in the MIN. In the laminated LGNd but not in the MIN the receptive field center sizes increased with increasing eccentricity of receptive field position. At any eccentricity, receptive field centers of MIN Y cells tended to be larger than those of Y cells in the laminated LGNd. Response latency ranges to orthodromic and antidromic stimulation were the same for cells located in the laminated LGNd and those in the MIN. However, the mean response latency to stimulation of the optic chiasm was significantly shorter for Y cells in MIN than for Y cells in the laminated LGNd. Our results suggest that the most numerous cells observed histologically in the MIN, class 1 cells of Guillery ('66) are morphological equivalents of Y cells.

Connections between the reticular nucleus of the thalamus and pulvinar-lateralis posterior complex: a WGA-HRP study.

The present study utilises the capacity of wheat germ agglutinin-conjugated horseradish peroxidase to label both afferent and efferent projections from selected regions of the thalamic reticular nucleus (TRN) to the pulvinar lateralis-posterior complex (Pul-LP) of the cat. Fourteen injections into the TRN located between anterior-posterior levels 8.5 and 4.5 were analysed. The projection of the TRN to the Pul-LP complex is roughly organised in a topographic manner and is not widespread within the thalamus. Anterograde labelling in the Pul-LP extended rostrocaudally with a slight oblique dorsoventral orientation. Projections to the medial LP were predominantly but not exclusively from rostral areas of TRN, while projections to the lateral LP were largely from caudal areas of the TRN. Projections to other areas of the Pul-LP were sparse. The connections between TRN and Pul-LP were reciprocal, although the distribution of labelled cells and anterograde labelling was not completely overlapping. Reciprocal connections with the dorsal lateral geniculate nucleus were largely with the C-laminae and the medial interlaminar nucleus. The results are discussed with reference to the corticothalamic projections and the visuotopy of the Pul-LP.

Role of target tissue in regulating the development of retinal ganglion cells in the albino rat: effects of kainate lesions in the superior colliculus.

Kainic acid or ibotenic acid was injected unilaterally into the major target regions of the axons of retinal ganglion cells--the superior colliculus (SC) or dorsal lateral geniculate nucleus (DLG)--of rat pups ranging in age from postnatal day 0 to postnatal day 10 (P0 - P10). While the collicular or geniculate neurons within the injection site died within 48 hours of the injection, damage to axons and terminals of extrinsic origin within the injected region was not apparent. The neuronal degeneration induced by the neurotoxins, observed at both the light and electron microscopic levels, resembled the neuronal degeneration that occurs in the colliculus during normal development. Macrophages were identified in the regions containing degenerating cells. Two to three weeks after the injections of neurotoxin, massive injections of the enzyme, horseradish peroxidase (HRP), were made into the retinorecipient nuclei. After about 24-hour survival time the numbers of retinal ganglion cells were estimated by counting the number of neurons containing HRP reaction products in sample areas distributed in a regular rectangular array across the entire retinal surface. In the animals in which the neurotoxin was injected into the SC during the first 4 postnatal days, there was a substantial reduction (on average 41.5%; the range: 27.5-65.5%) in the normal number (mean value of 113,000--Potts et al.: Dev. Brain Res. 3:481-486, '82) of retinal ganglion cells surviving the period of "naturally occurring ganglion cell death" in the retinae contralateral to the injected SC. By contrast, injections of neurotoxins into the DLG and/or the optic tract of newborn rats did not result in a significant reduction in the numbers of retinal ganglion cells surviving the period of naturally occurring ganglion cell death. The period of sensitivity of retinal ganglion cells to the injection of neurotoxin into the colliculi extends from birth to about the end of the first postnatal week; the greatest sensitivity seems to be restricted to the first 3-4 postnatal days. In the retinae in which the total number (and density) of ganglion cells was substantially reduced by the selective destruction of their target cells, the centro-peripheral difference in the somal diameters of the ganglion cells (apparent in normal animals) was abolished, both amongst the whole population of ganglion cells and amongst the ganglion cells with the largest somata, relatively thick axons, and large-gauge primary dendrites (Class I cells). The number and distribution of the Class I cells in the depleted retinae were, however, unaltered.(ABSTRACT TRUNCATED AT 400 WORDS)

Grafting of fetal pancreata into neonatal rat brain.

Undigested fetal pancreatic tissue has been previously shown to have immunogenic properties, even after transplantation into the adult rat brain, a relatively immunoprivileged site. In the present study, iso-, allo-, and xenografts of fetal pancreas were placed into neonatal rat brain parenchyma and ventricles in order to determine the extent and quality of its survival in this environment. Adult recipients of the same tissue types were used as controls. Selective survival of insulin-staining beta cells was observed in neonates (n = 33) over the 6-week period of the experiment. Ducts and acini were gradually destroyed in allo- and xenografts, disappearing completely by the 42nd day, while there were no such changes in the isografts. The absence of an acute inflammatory reaction was noted, but there were varying degrees of lymphocytic infiltration, though small (20 +/- 4 lymphocytes per average graft area of 0.16 mm2) in all but one graft. This infiltrate was greatest in allografts, with a significant increase observed after 14 days, corresponding to the time when the ducts started to disappear. Other structures present included fibroblasts and blood vessels. The latter increased significantly with time after transplantation. Unlike isografts placed in the parenchyma of adult rats, allo- and xenografts were rejected from the earliest time observed, 7 days postoperatively. In summary, these data show that beta cells in rat fetal pancreas will survive when grafted across major allogeneic and xenogeneic barriers for up to 6 weeks, without utilization of any form of immunosuppression, provided the recipients are neonates.

Photic responses of the lateral geniculate nucleus can be used as a stereotaxic guide for micropipette placement.

Neuroanatomical studies of the cat's thalamus require accurate placement of a micropipette for tracer injections. In order to determine interanimal differences, we describe a simple method which compares the location of the different laminae of the dorsal lateral geniculate nucleus (LGNd) with a stereotaxic atlas. The visual electrophysiological response to a flash was recorded from the LGNd with an enamelled stainless-steel electrode. The response steadily increased in amplitude through the optic radiation, lamina A and lamina A1. Further penetration through the LGNd resulted in the reversal of the recorded potential. The reversal point was lesioned and found to lie between lamina A1 and lamina C. Based on the dorsal level of laminae A, A1 and C as determined by the reversal point, these estimates of LGNd location can be compared with a stereotaxic atlas and corrections made to subsequent placement of micropipettes used for injections in other thalamic structures.

Explantation of fetal murine retinae to the chorioallantoic membrane of the chicken embryo.

A technique is here described for the culture of mammalian retinal explants on the chorioallantoic membrane of the developing chicken embryo. As an integral part of the central nervous system, the mammalian retina is characterised by its highly organised laminar structure and developmental timetable. Study of its prenatal development is, however, difficult to undertake in utero. In an attempt to render the organ of vision more accessible experimentally, fetal mouse retinae were explanted across major species barriers to the live chorioallantoic membrane of the chick. From 26 experiments, 128 explants (70% of the total) were recovered and 27 possessed a cytomorphology apparently identical to that of age-matched controls. The surviving retinae were analysed using a specifically devised set of criteria and they had developed a normal laminar structure (ganglion cell, inner plexiform, inner nuclear, outer plexiform and outer nuclear layers) but increased numbers of pyknotic profiles were present and somal sizes in the ganglion cell layer were significantly smaller. Such patterns have been obtained in other studies, both in vivo and in vitro, in which retinae had no access to their major targets in the brain, the superior colliculus and lateral geniculate nucleus. Explantation to the chorioallantoic membrane is thus a viable alternative for experiments requiring tissue isolation from natural surroundings since the explants are accessible for manipulation and observation while interacting with the host chick embryo. Furthermore, the technique allows examination of retinal differentiation, offering the opportunity to answer a number of important questions regarding development in the central nervous system.

Loss of axons from the optic nerve of the rat during early postnatal development.

The number of axons in the optic nerve of the newborn rat has been compared with the number present in the adult animal. Nerves taken from animals on the day of birth contain 242,000 +/- 29,000 (S.D.) fibres (n = 5). By the sixth postnatal day, the number of axons has fallen to the stable values of adults (99,000 +/- 3700, n = 8). Thus development of the rat's visual system during the first 5 days of life is associated with a loss of 60% of the axons present in the optic nerve at birth. Counts made on the remaining nerve after enucleation of one eye suggest that the presence of retino-retinal axons during the first 5 postnatal days cannot account for all of this reduction.

Target regions enhance the outgrowth and survival of ganglion cells in embryonic retina transplanted to cerebral cortex in neonatal rats.

In embryonic mouse retina transplanted to occipital cortex of neonatal rats, ganglion cells do not project axons into the host cortex, although they may survive up to but not beyond 6 weeks post-transplantation. By contrast, if embryonic tectum or diencephalon is transplanted along with the retina, ganglion cells exhibit vigorous outgrowth to specific regions of the co-transplant and are able to survive for at least 14 weeks.

Mode of termination of afferents from the thalamic reticular nucleus in the dorsal lateral geniculate nucleus of the rat.

The terminals of axons projecting to the dorsal lateral geniculate nucleus from the thalamic reticular nucleus were identified by electron microscopy 8-24 h after placing small lesions in the ipsilateral reticular nucleus. The terminals contained flattened synaptic vesicles and made Gray type II axo-dendritic synaptic contacts with geniculate neurons. Their identification as F-axons accords well with physiological evidence for a powerful monosynaptic inhibitory input to geniculocortical projection cells from reticular nucleus neurons.

Cortical projections to visual centres in the rat: an HRP study.

We have investigated the relationships of the visual cortex to other visual centres in the rat: namely the lateral geniculate nucleus, the visually responsive part of the thalamic reticular nucleus and the superior colliculus. We injected horseradish peroxidase iontophoretically so as to restrict the injectate to each of the regions, and reacted sections using 3 different procedures. Areas 17, 18 and 18a project to both dorsal and ventral lateral geniculate nucleus as well as to the visually responsive part of the thalamic reticular nucleus and superior colliculus. Pyramidal cells in lamina VI project to the dorsal lateral geniculate nucleus and to the thalamic reticular nucleus, whereas cells of origin of the projection to the superior colliculus lie in lamina V; cells in lamina V also project to ventral lateral geniculate nucleus. The implications of these findings are discussed, particularly in terms of the functional relationships between the visual cortex, lateral geniculate nucleus and visual thalamic reticular nucleus.

Transient outgrowth from retinae implanted in the neonatal rat cerebral cortex.

Retinae placed in the cortex of neonatal rodents show substantial outgrowth, both along the subpial margin and within the cortical white matter, which is not sustained beyond 6 days post-transplantation. These results, in combination with previous studies, suggest that optic axon outgrowth is sustained for a finite period, after which it requires target-derived influences for its long-term maintenance.

Effect of proteoglycan purified from rat superior colliculus on the survival of murine retinal ganglion cells.

Recently, Schulz and coworkers purified a chondroitin sulfate proteoglycan from the superior colliculus of the neonatal rat which promoted survival of neonatal rat retinal ganglion cells in vitro. The present work tests whether this factor supports the survival of axotomised retinal ganglion cells in vivo. To this effect, murine retinae 15 and 20 days after conception were explanted to the chorioallantoic membrane of live chicken embryos. The explants, which were left in the egg for 1, 2 or 7 days, differentiated and grew according to a normal timetable. Purified proteoglycan from neonatal rat superior colliculus was applied daily to one group of retinae while a control group received Ham's F-10 medium. Results indicated that application of proteoglycan resulted in the preferential survival of large cells in the ganglion cell layer, namely ganglion cells, for up to 7 days post-explantation. In addition, the proteoglycan had a significant short-term anti-traumatic effect on the ganglion cell layer of explants by causing a 72% decrease in the number of dead cells relative to controls 1 day post-explantation. It was concluded that the chondroitin sulfate proteoglycan purified from the superior colliculus of the neonatal rat promotes the survival of fetal and neonatal murine retinal ganglion cells in retinae explanted to the chorioallantoic membrane of the chick.

Trophic effect of collicular proteoglycan on neonatal rat retinal ganglion cells in situ.

Naturally occurring neuronal death is widespread in the central nervous system of mammals. To date, the causes and mechanisms of such death are poorly understood. A major hypothesis is that developing neurons compete for limited amounts of trophic factor(s) released from their target centres as in the case of the peripheral nervous system and nerve growth factor. The present study aims to test this 'trophic hypothesis' in the mammalian central nervous system. In the rat, more than 50% of retinal ganglion cells die in the early post-natal period. Schulz and coworkers [57] purified a potential trophic agent from their major target, the superior colliculus, which was identified as a 480 kDa chondroitin sulfate proteoglycan. This proteoglycan or control solutions were injected into the eyes of rat pups during the post-natal part of the period of naturally occurring ganglion cell death. It was found that the collicular proteoglycan prevented the death of a significant number of the ganglion cells that would normally have been lost over a post-injection period of one or two days. The effect of the proteoglycan was dose- and time-dependent. These results support the notion that trophic interactions are a determining factor in the survival of retinal ganglion cells during the period of naturally occurring cell death. It is also the first time that a proteoglycan has been shown to possess neurotrophic properties in situ.

The retinal location and fate of ganglion cells which project to the ipsilateral superior colliculus in neonatal albino and hooded rats.

We have studied the organization of the ipsilateral retinocollicular pathway in neonatal rats by injecting the enzyme horseradish peroxidase (HRP) into the superior colliculus within 24 h of birth and later examining the location of labelled cells in the contralateral and ipsilateral retinae. One day after HRP injection, regardless of the location of the injection site in the superior colliculus, the great majority (over 80%) of ipsilaterally projecting cells was located in the lower peripheral retina. Five days after injection into the posterior pole of the superior colliculus (which in adult animals does not receive input from the ipsilateral retina), there were very few labelled cells in the ipsilateral retina, but labelled cells were quite numerous in the appropriate part of the contralateral retina. These results suggest that in the neonatal rat the great majority of ipsilaterally projecting retinal ganglion cells lie in the same part of the retina as do ipsilaterally projecting cells in the adult, but that many of those cells which project to inappropriate parts of the superior colliculus die by the fifth postnatal day.