Changes in the numbers of optic nerve fibers during late prenatal and postnatal development in the albino rat

Spatiotemporal pattern of ontogenetic expression of calbindin-28/kD in the retinorecipient layers of rat superior colliculus

Using an antibody against calbindin-28kD, we have studied the spatial pattern of expression of this protein in the superior colliculi (SC) of four strains of mature laboratory rats. In all four strains, calbindin-expressing cells (CECs) formed horizontally oriented tiers in the retinorecipient and intermediate gray layers but were diffusely distributed throughout the deep layers. Ontogenetically, calbindin-28kD was expressed for the first time in the retinorecipient layers at postconceptional day 20 (PCD 20), by cells located in the rostrolateral region where the first born retinal ganglion cells (RGCs) are represented. Although on the day of birth (PCD 22/23), the CECs were distributed more widely, they were still absent in the most medial part of the SC, that is, the region where the latest born RGCs are represented. The spatial distribution of CECs became adultlike only by PCD 29, that is, at the end of the period of the naturally occurring death of the RGCs. Monocular eye enucleations on PCD 23 prevented the expression of calbindin in the medial fifth of the retinorecipient layers of the contralateral SC, while the unilateral removal of the visual cortices had no discernable effect on the numbers and distribution of the CECs in either SC. Thus, the spatiotemporal pattern of ontogenetic expression of calbindin-28kD in the retinorecipient layers of SC reflects the spatiotemporal pattern of generation of the RGCs, and the retinal input appears to induce neuronal expression of calbindin-28kD in these layers.

Retrograde fluorescent double-labeling study of bilaterally projecting retinal ganglion cells in albino rats at different stages of development

Injection of the fluorescent tracers 10% Evans blue (EB) and 4% fluoro-gold (FG) into the right and the left dorsal lateral geniculate nucleus, respectively, of albino rats at different stages of development demonstrated the presence of double-labeled retinal ganglion cells that projected bilaterally into both the dorsal lateral geniculate nuclei (dLGN). Findings confirmed that the distribution of these double-labeled cells was gradually reduced after birth, being confined to the peripheral temporoventral quarter (temporal-ventral crescent) of the retina after postnatal day 15. We estimated the proportion of double-labeled cells to total labeled cells in the same area at different stages of development (0-90 days); values ranged from 35.3% in the neonate to 5.27% in the adult rat which suggests that the majority of double-labeled cells and/or their axons were lost early in development. That a small number of ganglion cells were observed to project bilaterally in the adult rats suggested that these cells conduct the same visual information to both hemispheres throughout the animal's life.

Transfer of horseradish peroxidase from oligodendrocyte to axon in the myelinating neonatal rat optic nerve: artefact or transcellular exchange?

In this paper we make the surprising observation that intracellular injection of horseradish peroxidase (HRP) into a single myelinating oligodendrocyte also resulted in localised HRP labelling at the nodes of Ranvier of some axons of the unit. It appeared that HRP had been transferred to the nodal axoplasm from the paranodal loops of the HRP-filled oligodendrocyte. Three HRP-filled oligodendrocytes from isolated optic nerves of 14-day-old rats were analysed by serial section electron microscopy, and HRP was observed in the axonal cytoplasm at three of the nodes of Ranvier delineated by one of the cells. At labelled nodes, HRP was of a uniform intensity throughout the nodal axoplasm. Axonal labelling gradually diminished along the paranodal regions and was not evident in the contiguous internodal axoplasm beyond 20 microns from the node. The myelin sheaths, paranodal loops, and axons appeared normal at labelled nodes, and the paranodal loops and astrocyte perinodal processes adjacent to those of the HRP-filled oligodendrocyte unit did not contain HRP. There was no evidence of extracellular HRP or tissue damage in the surrounding neuropil, and axons neighbouring those enwrapped by the HRP-filled oligodendrocyte did not contain HRP. The possibility that axonal labelling was an artefact of either iontophoretic injection or tissue preparation is discussed. This provocative finding is not definite proof of exchange, but the balance of evidence supports the possibility that there was transcellular exchange of HRP at paranodes between the labelled oligodendrocyte and some of the axons in the unit. The rarity of HRP transfer to axons suggests that it may be a transient or labile event. It is not clear whether oligodendrocyte to axon macromolecular exchange has real physiological and/or pathological significance.

Target and afferents interact to control developmental cell death in the mesencephalic parabigeminal nucleus of the rat

During the period of natural cell death in the developing mammalian brain, both target cells and afferents have been shown to be important for neuronal survival. Here we demonstrate that afferents and targets have interactive roles in the maintenance of cells during development of the mesencephalic parabigeminal nucleus (PB) in rats. Pyknotic nuclei were counted in the PB of developing rats that received a bilateral lesion of the superior colliculus on the day of birth (P0). We observed that simultaneous deafferentation and deeferentation leads to a large peak of cell death at P1-2 in all three divisions of PB. Later the rate of pyknosis decreases and a second period of elevated cell death is observed just before the complete disappearance of the nucleus at P7-8. Counts of healthy neurones indicates two separate periods of increased neuronal loss. The first period occurs at P1-2, and the last and dramatic episode of cell loss at P8 leads to the disappearance of the PB. The combined effects of simultaneous target removal and deafferentation were different from the sum of the individual effects, indicating that the axonal targets and the afferents interact to control cell survival in the PB.

Widespread programmed cell death in proliferative and postmitotic regions of the fetal cerebral cortex

A key event in the development of the mammalian cerebral cortex is the generation of neuronal populations during embryonic life. Previous studies have revealed many details of cortical neuron development including cell birthdates, migration patterns and lineage relationships. Programmed cell death is a potentially important mechanism that could alter the numbers and types of developing cortical cells during these early embryonic phases. While programmed cell death has been documented in other parts of the embryonic central nervous system, its operation has not been previously reported in the embryonic cortex because of the lack of cell death markers and the difficulty in following the entire population of cortical cells. Here, we have investigated the spatial and temporal distribution of dying cells in the embryonic cortex using an in situ endlabelling technique called ‘ISEL+’ that identifies fragmented nuclear DNA in dying cells with increased sensitivity. The period encompassing murine cerebral cortical neurogenesis was examined, from embryonic days 10 through 18. Dying cells were rare at embryonic day 10, but by embryonic day 14, 70% of cortical cells were found to be dying. This number declined to 50% by embryonic day 18, and few dying cells were observed in the adult cerebral cortex. Surprisingly, while dying cells were observed throughout the cerebral cortical wall, the majority were found within zones of cell proliferation rather than in regions of postmitotic neurons. These observations suggest that multiple mechanisms may regulate programmed cell death in the developing cortex. Moreover, embryonic cell death could be an important factor enabling the selection of appropriate cortical cells before they complete their differentiation in postnatal life.

Increased serotonin in the developing superior colliculus does not alter the number or distribution of retinotectal ganglion cells

Administration of a single subcutaneous dose of 5,7-dihydroxytryptamine (5,7-DHT) to newborn hamsters results in a significant increase in the density of serotoninergic (5-HT) fibers in the superficial layers of the superior colliculus (SC) and marked abnormalities in both the crossed and uncrossed retinotectal projections when these animals reach adulthood (R. Rhoades, C. Bennett-Clarke, R. Lane, M. Leslie, and R. Mooney, 1993, J. Comp. Neurol. 334:397-409). The present study was undertaken to determine whether changes in the retinotectal projection of 5,7-DHT-treated animals were associated with alterations in the number or distribution of retinal ganglion cells in these animals. Nissl staining of retinae from normal adult and 5,7-DHT-treated hamsters revealed no differences between them in the number or average diameter of cells in the retinal ganglion cell layer. Retrograde labeling with horseradish peroxidase (HRP) demonstrated no effect of 5,7-DHT treatment on the number or distribution of ipsilaterally or contralaterally projecting ganglion cells. Neonatal 5,7-DHT administration also had no effect on the distribution of soma diameters for HRP-labeled retinal ganglion cells. Electron microscopic analysis demonstrated no significant difference between the number of optic nerve fibers in the normal and 5,7-DHT-treated hamsters. The results are consistent with the conclusion that the effect of 5,7-DHT on the retinotectal projection may primarily be a function of this toxin, or the increase in 5-HT it induces, on the terminal arbors of retinotectal axons rather than on their parent cells.

Postnatal changes in the uncrossed retinal projection of pigmented and albino Syrian hamsters and the effects of monocular enucleation

Anterograde and retrograde tracing techniques have been used to study the uncrossed retinal projection in neonatal pigmented and albino Syrian hamsters. The total number of retinal ganglion cells projecting ipsilaterally peaks at postnatal days 2-4 (P2-P4) and declines to adult values by P12. The change in cell numbers has a similar time course in albino and pigmented animals. Although the population of uncrossed cells in the temporal retina of albino hamsters is always less than that in pigmented hamsters, no difference between the colour phases was found for the population of uncrossed cells in nasal retina. Differential cell death also contributes to the adult albino decussation pattern in hamsters: The relative loss of cells from temporal retina in albinos (72%) is greater than that in pigmented animals (56%). The additional loss in albinos does not appear to depend on binocular interactions: The same proportion (30%) of uncrossed cells is "rescued" from death by neonatal monocular enucleation in both colour phases. Flat-mount preparations showing the distribution of uncrossed fibres reveal that a distinct focus of terminals emerges in rostral superior colliculus, which is topographically appropriate for a binocular mapping, at the peak of uncrossed ganglion cell numbers (P4). Comparison of uncrossed terminal distributions and ganglion cell death reveals considerable refinement of the terminals prior to the main phase of cell death. Monocular enucleations performed some time after birth have a greater effect on uncrossed terminal distributions than on cell death. These observations suggest that independent mechanisms may be involved in the regulation of terminal distributions and of cell numbers in the developing uncrossed retinal pathways.

Expression of the proto-oncogene, trk, receptors in the developing rat retina

The neurotrophins, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and NT-4/5 are important in a variety of developmental processes in the peripheral and central nervous systems. These molecules bind to a low-affinity receptor and to distinct high-affinity receptors. The high-affinity receptor for NGF is the proto-oncogene product, p140trkA(trkA). Isoforms of p140trkA, p145trkB(trkB), and p140trkC(trkC), are the primary high-affinity receptors for BDNF and NT-3, respectively. We evaluated the developmental regulation of the high-affinity neurotrophin receptors in the rat retina using polyclonal antibodies directed to a highly conserved region of the C-terminus of the p140trkA isoforms (pantrk) and antibodies directed to unique amino-acid sequences of p140trkA, p145trkB, and p140trkC. Immunoreactivities for trkA and trkB, as well as pantrk, were detected in the developing retina and showed similar distributions. At similar antibody concentrations, trkC immunoreactivity was not detected. In the embryo, immunoreactivties were present in cells located throughout the neuroblastic retina, especially in the inner retinal layers, and in fibers in the nerve fiber layer and optic nerve. In the newborn retina, immunoreactivities for these two receptor isoforms were localized to numerous somata in the inner nuclear layer (INL), as well as to cells in the ganglion cell layer (GCL) and axons in the nerve fiber layer and optic nerve. A similar pattern of immunostaining persisted throughout the first postnatal week. By postnatal day-10, immunostaining was confined to large-diameter cells in the GCL, both heavily stained and lightly stained cells in the INL and a plexus of processes in the inner plexiform layer (IPL).(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of trkB and low-affinity nerve growth factor receptor mRNA in the developing rat retina

The localization of trkB and low-affinity nerve growth factor receptor (LNGFR) mRNAs in the developing rat retina was examined by in situ hybridization. TrkB mRNA was expressed in the ganglion cell layer (GCL), in the inner border of the neuroblastic layer (NBL), and the inner border of the inner nuclear layer (INL). LNGFR mRNA was expressed in the GCL, in almost full thickness of the NBL, and in the intermediate part of the INL. Although both trkB mRNA and LNGFR mRNA were expressed in the GCL, the expression pattern was different between these mRNAs; trkB mRNA was expressed in almost all cells in the GCL uniformly and the expression of LNGFR mRNA varied greatly from cell to cell. In addition, the expression of both mRNAs, especially LNGFR mRNA seemed to be down-regulated at P7, when programmed cell death of the RGCs was prominent. These observations indicate that LNGFR may modulate the function of trkB and that trkB and LNGFR play important roles in the development and maintenance of the RGCs.

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.

Programmed cell death in development

Although cell death has long been recognized to be a significant element in the process of embryonic morphogenesis, its relationships to differentiation and its mechanisms are only now becoming apparent. This new appreciation has come about not only through advances in the understanding of cell death in parallel immunological and pathological situations, but also through progress in developmental genetics which has revealed the roles played by death in the cell lineages of invertebrate embryos. In this review, we discuss programmed cell death as it is understood in developmental situations, and its relationship to apoptosis. We describe the morphological and biochemical features of apoptosis, and some methods for its detection in tissues. The occurrence of programmed cell death during invertebrate development is reviewed, as well as selected examples in vertebrate development. In particular, we discuss cell death in the early vertebrate embryo, in limb development, and in the nervous system.

Development of abnormal lamination and binocular segregation in the retinotectal pathways of the rat

The uncrossed retinotectal pathway of pigmented rats originates from a small fraction of the retinal ganglion cell population. This projection terminates deeply in discrete patches within the upper grey layers where crossed and uncrossed inputs overlap. However, after the experimental enlargement of the uncrossed pathway, the ipsilateral fibers are also found in a superficial tier of the upper grey layers where binocular inputs segregate [36]. We studied the development of the retinotectal projections in rats after the enlargement of the uncrossed pathway as a result of a contralateral (left) optic tract lesion (OTL) made at birth. Horseradish peroxidase (HRP) was used as an anterograde tracer. An abnormal uncrossed projection from the right eye to the collicular surface appeared at postnatal day 3 (P3). Between P5 and P10, this projection developed the bilaminar pattern seen in similar operated adults. The laminar arrangement of the aberrant terminal fields did not change significantly after an ipsilateral visual cortex ablation on the day of birth. Despite the early development of the aberrant uncrossed pathway, binocular segregation was incipient at P10. At P14, 46% of the operated rats presented gaps in the terminal labeling at the tectal surface. This figure increased to 55.5% at 6 weeks, a proportion still smaller than in adult animals of the same group (69%). Eyelid suture had no effect on segregation. This projection remains plastic for at least 3 weeks, since the removal of the ipsilateral input at either P14 or P21 resulted in the absence of gaps in the contralateral projection. We conclude that the laminar selection of retinotectal projections depends on binocular interactions and that the abnormal segregation of retinal inputs to the superior colliculus has an unusually protracted development which can be reversed long after the previously defined critical period in this system.

Microglial invasion and activation in response to naturally occurring neuronal degeneration in the ganglion cell layer of the postnatal cat retina

Retinae of kittens between postnatal (P) days 2 and 10 were examined for the presence of degenerating neuronal profiles, normal nucleoli and microglia. Comparison of the numbers of degenerating profiles with numbers of axons lost from the optic nerve suggest that the majority of these profiles result from the degeneration of retinal ganglion cells. Analysis of local densities of the different profiles revealed different rates of cell loss, occurring at different times in central and peripheral retina. The period of rapid cell loss occurred between P2 and P3 in central retina compared to between P8 and P10 in peripheral retina. At both locations, these periods of rapid cell loss were accompanied by a decrease in the ratio of microglia to dying cells even though the absolute densities of microglia increased. However, calculation of the clearance times of cellular debris indicate that the speed of removal of degeneration products is greater during rapid cell loss, which suggests that cellular degeneration serves to activate the phagocytic process.

Morphology of astrocytes and oligodendrocytes during development in the intact rat optic nerve

The detailed three-dimensional morphology of macroglial cells was determined throughout postnatal development in the intact rat optic nerve, a central nervous system white matter tract. Over 750 cells were analyzed by intracellular injection of horseradish peroxidase or Lucifer Yellow to provide a new perspective of glial differentiation in situ. Retrograde analysis of changes in glial morphology allowed us to identify developmental timetables for three morphological subclasses of astrocytes and oligodendrocytes, and to estimate their time of emergence from undifferentiated glial progenitors. Glial progenitors were recognised throughout postnatal development and persisted in 35-day-old nerves, where we suggest they represent adult progenitor cells. Astrocytes were present at birth, but the majority of these cells developed over the first week as three morphological classes emerged having either transverse, random, or longitudinal process orientation. Several lines of evidence led us to believe that the majority of astrocytes in the rat optic nerve were morphological variations of a single cell type. Young oligodendrocytes were first observed 2 days after birth, indicating that they diverged from progenitors at or near this time. During early development these cells extended a large number of fine processes, which then bifurcated and extended along axons. Later, as myelination proceeded, oligodendrocytes exhibited fewer processes which grew symmetrically and uniformly along the axons, resulting in a highly stereotypic mature oligodendrocyte form. Our analysis of oligodendrocyte growth suggests that these cells did not myelinate axons in a random manner and that axons may influence the myelinating processes of nearby oligodendrocytes.

The migratory potential of vitally labelled microglial cells within the retina of rats with hereditary photoreceptor dystrophy

We employed a recently developed technique of labelling retinal microglial cells to identify these cells within the retinal parenchyma and to determine their role during degenerative diseases. In a first set of experiments we retrogradely labelled the ganglion cells from the superior colliculus of neonatal pups of the royal college of surgeons (RCS)-strain with the fluorescent dye 4Di-10ASP [N-4-4-(4-didecylaminostyryl-N-methylpropidium iodide)] at the day of birth. In a second group of young rats of the same strain retrograde staining of ganglion cells was performed by applying the fluorescent dye at the cross-sectional area of the transected optic nerve. In both experimental groups, prelabelled ganglion cells die. In the neonatal rats ganglion cells disappear because the initial neuronal cell population becomes naturally reduced by about 35% in the course of the programmed cell death, which takes place during the first 2 weeks of life. In the mature rat, ganglion cells die as a consequence of the axotomy. In both cases, the fluorescent dye, which is a lipid inserted into the cell membrane, labels the ganglion cell bodies after retrograde transport from the axons. In the course of their degradation the dye is then selectively taken up by intraretinal microglial cells in the ganglion cell and inner plexiform layers. In both groups of experiments, fluorescently labelled microglial cells were redistributed from the inner retina toward the photoreceptor cell layer, which is afflicted by chronic photoreceptor dystrophy. Similar migration does not occur when the same experiments were repeated with non-dystrophic normal rats. The experimental results suggest that intraretinal microglial cells possess a migratory potential and make use of it when the cells are exposed to degeneration-induced stimuli, the nature of which remains to be characterized.

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.

Survival of ganglion cells which form the retino-retinal projection during optic nerve regeneration in the frog

During optic nerve regeneration in the frog, axons transiently grow along the opposite optic nerve forming a retino-retinal projection. In the present study, we crushed the left optic nerve in the frog Litoria (Hyla) moorei and later applied horseradish peroxidase (HRP) or diamidino yellow (DY) to the right optic nerve. In one series, retinae were examined 3 days after application of the tracer. The retino-retinal projection was found to be maximal at 5 weeks, fell significantly by 7 weeks, and returned to close-to-normal levels by 24 weeks. In a second series, we applied DY at 5 weeks as before but did not sacrifice the frogs until 7 weeks. Numbers of labeled ganglion cells were not significantly different from those frogs in the first series labeled and examined at 5 weeks. We conclude that ganglion cells giving rise to the retino-retinal projection had not died in appreciable numbers, presumably being sustained by collateral axons in the brain.

Developmental study of the expression of B50/GAP-43 in rat retina

B50/GAP-43 has been implicated in neural plasticity, development, and regeneration. Several studies of axonally transported proteins in the optic nerve have shown that this protein is synthesized by developing and regenerating retinal ganglion cells in mammals, amphibians, and fish. However, previous studies using immunohistochemistry to localize B50/GAP-43 in retina have shown that this protein is found in the inner plexiform layer in adults. Since the inner plexiform layer contains the processes of amacrine cells, ganglion cells, and bipolar cells to determine which cells in the retina express B50/GAP-43, we have now used in situ hybridization to localize the mRNA that codes for this protein in the developing rat retina. We have found that B50/GAP-43 is expressed primarily by cells in the retinal ganglion cell layer as early as embryonic day 15, and until 3 weeks postnatal. Some cells in the inner nuclear layer, possibly a subclass of amacrine cells, also express B50/GAP-43 protein and mRNA; however, the other retinal neurons-bipolar cells, photoreceptors, and horizontal cells express little, if any, B50/GAP-43 at any stage in their development. Early in development, the protein appears in the somata and axons of ganglion cells, while later in development, B50/GAP-43 becomes concentrated in the inner plexiform layer, where it continues to be expressed in adult animals. These results are discussed in terms of previous proposals as to the functions of this molecule.

Developmental expression of trkB and low-affinity NGF receptor in the rat retina

Brain-derived neurotrophic factor (BDNF) promotes the survival of retinal ganglion cells, but these effects are dependent on the developmental stages, and a number of retinal ganglion cells are eliminated during pre- and neonatal stages. We have examined the expression of BDNF receptors, trkB and low-affinity nerve growth factor receptor (LNGFR), in the rat retina during these period using Northern blot analysis. The expression of trkB and LNGFR displayed two peaks during embryonic day 17 (E17) through postnatal day 1 (P1), and during P14-P17, indicating that it may play an important role in neuronal development and neuronal cell death.

Pathfinding at the mammalian optic chiasm

Axons of retinal ganglion cells in the eye form a system of retinal projections, which carry information about the world around us to targets in the brain for processing. Recent work combining video imaging technology, manipulations of mouse embryos in vivo, and molecular approaches have begun to shed light on how this major sensory pathway in the mammalian brain comes about during embryonic development.

The relative time course of axonal loss from the optic nerve of the developing guinea pig is consistent with that of other mammals

The relative timing of a number of events during the development of the visual system has recently been suggested to be consistent across a number of mammalian species (Dreher & Robinson, 1988). Some conflicting reports, however, had suggested that the precocial guinea pig might represent an exception to the generalized scheme. A quantitative study was thus carried out on the development of the optic nerve and retina of the guinea pig. Consistent with the prediction of a stable relative time course of mammalian visual development, axons and growth cones were found in the optic stalk from the 24th postconceptional day (40% of the period from conception to eye opening--the cecal period), the peak number of axons was observed on the 32nd postconceptional day (56% of the cecal period), and the phase of rapid axonal loss extended to the 39th and 42nd postconceptional days (68-74% of the cecal period). The number of axons in the adult optic nerve (117,000) represented about 37% of the peak number of axons. Additional observations indicated that during development of the optic nerve the mean axonal diameter increased approximately threefold from 0.31 microns to 1.06 microns. As in other mammals studied so far, myelination was first noted after the period of rapid axonal loss and continued until in the adult 97% of axons were found to be myelinated. In the retina, the presence of pyknotic profiles in the ganglion cell layers extends throughout the periods of loss of the optic nerve axons. Finally, the presence of pyknotic profiles in the amacrine sublayer suggest that in the guinea pig, as in other mammalian species, there is a loss of displaced amacrine cells as well as ganglion cells from the ganglion cell layer.

Taurine, GABA and GFAP immunoreactivity in the developing and adult rat optic nerve

The localizations of taurine, gamma-aminobutyric acid (GABA) and glial fibrillary acidic protein (GFAP) within the developing rat optic nerve were determined using immunocytochemical techniques on tissues from animals ranging in age from embryonic day 20 to postnatal 28 days. Mature nerves from 3-4-month-old adults were also examined. At the younger ages, taurine immunoreactivity was intense and localized specifically to the optic nerve axons, but by postnatal day 15 and thereafter its predominant localization was in macroglia. Some of these glia were astrocytes as indicated by the specific marker, GFAP. GABA immunoreactivity was present at the same time as taurine but was found only in macroglia. In mature nerves the patterns of taurine, GABA and GFAP distribution (within glia) were highly similar.

Responses of retinal axons in vivo and in vitro to position-encoding molecules in the embryonic superior colliculus

We show that rat retinal ganglion cell axons exhibit no topographic specificity in growth along the rostral-caudal axis of the embryonic superior colliculus (SC). Position-related, morphological differences are not found between temporal and nasal axon growth cones. However, embryonic retinal axons respond in vitro to a position-dependent molecular property of SC membranes. In vivo, regional specificity in side branching is the earliest indication that axons make topographic distinctions along the rostral-caudal SC axis. Our contrasting in vivo and in vitro results indicate that molecules encoding rostral-caudal position in the SC neither guide nor restrict retinal axon growth, but may promote the development of topographic connections by controlling specificity in the extension or stabilization of branches.

Time-course and extent of retinal ganglion cell death following ablation of the superior colliculus in neonatal rats

This study has examined the deleterious effect of superior colliculus (SC) ablation on the viability of identified retinotectally projecting ganglion cells in the neonatal rat retina. The time-course and extent of lesion-induced retinal ganglion cell (rgc) death has been determined and an estimate obtained for the rate of clearance of individual dying neurons. In order to demonstrate the projection of rgcs to the SC and the subsequent death of these same neurons after SC lesions, the fluorescent dye diamidino yellow (DY) was injected into the left SC of anesthetized 2 day old Wistar rats (P2: day of birth = P0). DY retrogradely labels the nuclei of tectally projecting rgcs; if these identified rgcs subsequently die, their DY-labelled nuclei become pyknotic and can be visualized in retinal wholemounts. At P4 the rats were again anesthetized and the injected area, seen as a yellow patch in the SC, was removed by aspiration. Rats were perfused 2 to 336 hours after the lesion and retinal wholemounts of the right eye were prepared. Control rats received only DY injections and were perfused at times corresponding to the lesioned animals. In three sham-operated rats; the injected SC was reexposed at P4 but the tectal tissue was not removed. In each of the 42 rats that were analyzed, about 10% of the retina containing retrogradely labelled rgcs was counted; the number of pyknotic versus normally labelled rgcs was determined and changes in normal cell density were also assessed. Pyknotic rates in control and sham-operated rats were similar (average 0.8%, n = 11). In SC-lesioned rats, the proportion of pyknotic DY-labelled rgcs increased to about 2.5% 4 to 8 hours postlesion (PL); the peak period of death occurred at 23 hours PL (8.0%). The amount of pyknosis decreased thereafter and most dying cells had been eliminated by 50 hours PL. Phagocytosis of dying cells was a common feature of retinae in SC lesioned rats. In the long-term (336 hours) rats, counts of normal DY-labelled rgcs in corresponding regions of control and lesioned rats revealed an average decrease in rgc density of 47.3% after P4 tectal ablation. Calculations suggest a clearance time of about 3 hours for dying neonatal rgcs.

Influence of position along the medial-lateral axis of the superior colliculus on the topographic targeting and survival of retinal axons

Topographic order in the rat retinocollicular projection emerges from an initially diffuse projection during an early postnatal remodeling period that is coincident with the period of naturally occurring ganglion cell death. Here, we examine the relationship between a retinal axon's position along the medial-lateral axis as it enters the superior colliculus (SC) and its ability to form an appropriately positioned arbor and survive the remodeling period. At E18-E19, prior to map remodeling, axons labeled with focal DiI injections in the periphery of temporal, nasal, superior or inferior retina are widespread along the medial-lateral SC axis. At P12, after remodeling, the distributions of axons remain widespread over the medial-lateral SC axis relative to the positioning of their terminal arborizations, and resemble the distributions labeled at E18-E19, with the exception that the small proportion of axons most widely mispositioned along the medial-lateral SC axis are less frequent. These data indicate that the most widely mispositioned retinal axons are preferentially eliminated, but that a high proportion of retinal axons mispositioned along the medial-lateral axis as they enter the SC can correct their position, form topographically appropriate arbors, and survive the remodeling period.

A morphological and morphometric analysis of the optic nerve in the hypothyroid rat

This investigation was designed to morphologically evaluate the effects of hypothyroidism on the development of myelin and axons in the rat optic nerve. Four pups from each group of normal and propylthiouracil-induced hypothyroid rats were sacrificed at 14, 21, 28, and 35 postnatal days. Optic nerves were studied by both light and electron microscopes. The hypothyroid animals had significantly reduced body and brain weights compared to those of their age-matched controls. In the hypothyroid animals, the cross-sectional area of the optic nerve, the fiber density, and fiber occupancy were significantly diminished compared to those of the controls. The mean individual fiber size was unaffected. However, the relationship between the total axonal area to myelin thickness was similar in the control and experimental groups, implying that the feedback mechanism between myelinating cells and axons was not affected by hypothyroidism. Thus, this study indicates that the principal insult of neonatal hypothyroidism results in a delay in myelin acquisition of myelinated fibers, resulting in diminished cross-sectional area of the optic nerve, fiber density, and fiber occupancy.

Effects of combined pre- and postnatal ethanol exposure (three trimester equivalency) on glial cell development in rat optic nerve

This study evaluated the effects of a combined gestational and 10 day postnatal alcohol exposure (human three trimester equivalency) on the development of glial cells in the rat optic nerve. Pregnant rats were exposed to alcohol via a liquid diet, then their pups were artificially reared and further exposed to alcohol for 10 postnatal days via a gastrostomy fed liquid diet. Control animals, born of pair fed dams, were artificially reared on pair fed isocaloric diets. Optic nerve tissues were prepared for light and electron microscopic studies from animals on gestational days (G) 15 and 20 and postnatal days (P) 5, 10, 15, 20 and 90. There were fewer glial cells per cross-section on day 15 and the cross-sectional areas of optic nerves were smaller on days G20, P15 and P90 in the ethanol exposed animals. There was an alcohol-induced delay in the appearance of immature cells within the oligodendroglia lineage and a decrease in the number of oligodendroglia present at 15 and 20 days, indicating a delay in the maturation of oligodendroglial cells. These effects were compensated for by 90 days. Maturation of the astrocytic cell lineage was generally unaffected by the alcohol although there was evidence of increased numbers of cells in the lineage. There was no consistent indication of alcohol-induced degeneration of glial cells or their organelles. Thus, alcohol exposure for all of gestation and 10 postnatal days in the rat causes a delay in oligodendrocyte maturation but appears to have no long-term effects on the glial cell population of the optic nerve. Such a delay, by contributing to delays in myelin development, could help to explain some of the neurological dysfunctions associated with developmental alcohol exposures.

Spatio-temporal patterns of retinal ganglion cell death during Xenopus development

During development of the retina in mammals and birds, most retinal ganglion cells (RGC) that are produced are eliminated later in development by cell death. In lower vertebrates, however, such massive cell death has not been observed; total ganglion cell number increases linearly during most of development. Using 3H-thymidine or 5-bromodeoxyuridine labeling of retinal cell nuclei, we have been able to identify postmitotic RGC populations in Xenopus central retina at different developmental stages and follow their fate during development to postmetamorphic stages. RGC populations that become postmitotic between embryonic stages 32 and 49, during the initial stages of retinal growth, lose 40-77% of their cells during metamorphosis (approximately 4,000-5,000 cells). Twenty percent of the RGC present at stage 54, which later disappear, represent the same population of dying RGC that were present at stage 49. This suggests that the ganglion cells that became postmitotic between stage 49 and 53/54 show no apparent decline in numbers during metamorphosis. Since thyroxine is known to stimulate an increase in RGC number as well as the extent of fiber projection on the tectum, we suggest that this reduction in RGC numbers is not due to thyroxine-induced neuronal cell death. After stage 54, however, binocular vision develops in Xenopus (Keating, '74) and ipsilateral fibers begin to grow into thalamic visual neuropils (Hoskins and Grobstein, '85). We suggest, therefore, that as in mammals, in which RGC elimination correlates with binocular segregation of contralateral and ipsilateral retinal axons in visual centers, a similar process may occur in the frog among those RGC projecting to thalamic visual neuropils.

Short- and long-term effects of combined pre- and postnatal ethanol exposure (three trimester equivalency) on the development of myelin and axons in rat optic nerve

This study evaluated the effects of a combined gestational and 10 day postnatal alcohol exposure (human three trimester equivalency) on the development of myelin and axons in rat optic nerve. Rats were exposed during gestation via liquid diet, then their artificially reared pups were further exposed for 10 postnatal days via an ethanol-containing diet fed by gastrostomy. Control animals from pair-fed dams were artificially reared for 10 days on pair-fed isocaloric diets. Anesthetized animals were perfused with fixative on gestational days (G) 15 and 20 and postnatal days (P) 5, 10, 15, 20, and 90, then optic nerve tissues prepared for electron microscopy. Optic nerve cross-sectional areas were generally less from G20 through P90 in ethanol exposed animals. Counts of the number of myelinated nerve fibers per unit area and of the numbers of fibers in different stages of myelin development revealed that alcohol exposure caused a delay in myelin acquisition at 10 and 15 days that was compensated for at 20 and 90 days. Myelin thickness as a function of axon diameter was decreased in the alcohol exposed animals from 10 through 90 days, indicating a permanent reduction in the relative thickness of myelin. These results show that alcohol exposure for all of gestation and 10 postnatal days in the rat (human three trimester equivalency) causes a permanent reduction in myelin thickness along with a delay in myelin acquisition in the optic nerve. Such alterations in developing and adult myelin could help to explain some of the neurological and visual dysfunctions associated with developmental alcohol exposures.

Numbers of axons innervating mystacial vibrissa follicles in newborn and adult rats

Electron-microscopic techniques were used to determine the numbers of axons in the deep vibrissal nerves innervating the C1 and C4 follicles in newborn and adult rats. All counts were made from thin sections taken after the nerve had entered the follicle capsule (FC). In newborn animals, the nerves supplying the C1 (n = 10) and C4 (n = 10) follicles contained an average (means +/- standard deviation) of 355.0 +/- 40.0 and 233.9 +/- 19.2 axons, respectively. In the adult animals (n = 10 for C1 and n = 9 for C4), the respective values were 314.4 +/- 26.2 and 233.3 +/- 34.4 axons. There were no significant differences between the values for the counts from the neonates and adults for either follicle (p greater than 0.01, independent t tests). In the vibrissal nerves of neonates, both degenerating axons and occasional growth cones were visible. Such profiles were not observed in the nerves taken from adults.

Cell death and interocular interactions among retinofugal axons: lack of binocularly matched specificity

Naturally occurring ganglion cell death has been attributed to competitive interactions among axons at their targets during development of the retinofugal pathways. The present study is concerned with the hypothesis that interocular interactions leading to ganglion cell death are restricted to binocularly conjugate terminals in the optic nuclei. We tested this hypothesis in newborn rats by making localized retinal lesions, which denervate a restricted portion of the contralateral optic targets. When these rats reached adulthood, the ipsilaterally projecting ganglion cells of the intact eye were then studied following retrograde labeling with horseradish peroxidase. Results were compared with those from a normal, control group and from rats that had one eye removed on the day of birth. In those retinal loci binocularly conjugate to the lesion in the opposite eye, no localized cell rescue could be found among the ipsilaterally projecting ganglion cells. The same retinal loci, however, showed clear cell rescue after contralateral enucleation. Independent, anterograde, studies of the ipsilateral retino-collicular projection verified that lesions of equivalent size to those used in the retrograde study reliably create aberrant expanded uncrossed terminal fields. The present data suggest that the interocular interactions involved in the diminished ganglion cell loss which follows monocular enucleation are not dependent on topographically specific binocular matching. The phenomena of naturally occurring cell loss and of retinotopically specific interocular interactions may therefore be independent during normal development.

Retinal ganglion cell death during regeneration of the frog optic nerve is not accompanied by appreciable cell loss from the inner nuclear layer

We estimated cell numbers in the ganglion cell and inner nuclear layers of adult frog (Hyla moorei) retinae, examining normal animals and those with regenerated optic nerves. Analysis of sections stained with cresyl violet showed that cell numbers in a nasotemporal strip, which included the area centralis and visual streak, were comparable between sides for both these cellular layers in normal animals. In line with our previous observations, after optic nerve regeneration cell numbers in the ganglion cell layer had fallen by 35-43% compared to the unoperated sides. By contrast cell numbers remained similar for the inner nuclear layers on the two sides. We conclude that retrograde transneuronal degeneration had not taken place in the inner nuclear layer in response to ganglion cell death.

Differential expression of class I and class II major histocompatibility complex antigen in early postnatal rats

Cells expressing major histocompatibility complex (MHC) antigens are rarely found in normal mature brains, but cells resembling microglia can be induced to express these antigens following the onset of neural degeneration. In young rats, these cells show spontaneous expression of class I MHC antigens, which is further enhanced in the superior colliculus by the degeneration resulting from eye removal. By contrast, class II MHC antigen expression does not occur spontaneously and can only be induced by eye removal when the lesion is performed after the first postnatal week, when the optic tract begins to myelinate. We suggest that different signals are responsible for induction of class I and of class II MHC antigen expression.

VIP-mediated increase in cAMP prevents tetrodotoxin-induced retinal ganglion cell death in vitro

Afferent influences on natural cell death were modeled in retinal cultures derived from neonatal rats. Tetrodotoxin (TTX) blockade of electrical activity produced a significant reduction in surviving retinal ganglion cell (RGC) neurons during a critical period of development, similar in magnitude to the reduction observed during natural cell death in the intact retina at a similar developmental stage. The addition of vasoactive intestinal peptide (VIP) protected the RGCs from the lethal action of TTX. This effect was specific, since the related peptides PHI-27 and secretin produced no significant increase in RGC survival. Radioimmunoassay of cyclic nucleotides showed that TTX decreased culture levels of cAMP and that this trend was reversed by VIP. Decreases in RGC survival associated with TTX electrical blockade were prevented by 8-bromo:cAMP or forskolin. Furthermore, VIP10-28, the C-terminal fragment that inhibits VIP stimulation of adenylate cyclase, reduced the number of surviving RGCs. Thus, our results suggest that VIP, acting by increasing cAMP, has a neurotrophic effect on electrically blocked RGCs and may be an endogenous factor modulating normal cell death in the retina.

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.

Topographic organization of the retinocollicular projection in the neonatal rat

The topographic order of the retinocollicular projection in the rat was examined from birth until maturity. Small, localized deposits of rhodamine-filled latex microspheres were placed into the superior colliculus at different locations. To minimize labeling fibers of passage deposit sites were typically, although not exclusively, placed into the caudal-lateral pole of the colliculus. Examination of the area and density of labeled cells in the retinae of these animals led to the following conclusions: (1) At each age examined, the location of the majority of labeled cells was observed to be in appropriate topographic register with the deposit site in the superior colliculus. (2) Confirming the work of previous investigators, errors in topographic projection were observed. These were present in both the contralateral and ipsilateral retinae and decreased with increasing postnatal age. The mature pattern was present by P10. (3) Quantitatively, the number of retinal ganglion cells terminating nontopographically within the colliculus constituted a relatively minor proportion of the total number of labeled cells in both retinae. It is concluded that the majority of the retinal ganglion cells make topographically appropriate terminations within the superior colliculus during development.

A comparison of the initial retinal ganglion cell projection to the contralateral superior colliculus in albino and pigmented rats

Newborn albino and pigmented rats received localised Fast blue (FB) injections into the most caudal part of the contralateral superior colliculus (SC). A proportion of retinal ganglion cells (RGC's) from the temporal retina which in the adult projects exclusively to the rostral half of the colliculus are labelled by injections to the caudal colliculus in neonatal animals. The majority of these cells die during the period of naturally occurring cell death in the retina, which occurs during the first 10 postnatal days. The object of this experiment was to see whether albino rats, which have well documented abnormalities in axon pathfinding in their visual system, had a larger number of cells in temporal retina which initially project to caudal colliculus than pigmented animals. On postnatal day 2 (P2), the ratio of temporal to nasal RGCs projecting to the caudal SC is greater in albino than pigmented rats (6.2% vs 2.65%). After the wave of naturally occurring cell death, at P14, when many of the neonatal errors have been eliminated, the ratio of temporal to nasal RGC's is reduced to 1.71% for albinos versus 1.53% for pigmented rats.

Maturation of the corpus callosum of the rat: II. Influence of thyroid hormones on the number and maturation of axons

Quantitative electron microscopy has been used to study the number of callosal axons in the corpus callosum of normal and hypothyroid rats during postnatal development. At birth, the normal corpus callosum contains 4.4 x 10(6) axons. This number increases to 11.4 x 10(6) by 5 days of age (P5) and then, in contrast to cats and primates, remains constant until at least P60, the oldest age examined. The number of axons in the corpus callosum of hypothyroid animals is not significantly different from the values observed in normal rats at all ages studied, although the callosal axons of hypothyroid rats remain structurally immature. As extensive elimination of callosal axons has been shown to occur in normal rats past P5, we conclude that new callosal processes grow through the corpus callosum past this age that compensate numerically for the loss. Moreover, as the number of callosally projecting neurons seems to be higher in hypothyroid rats than in normal controls, it seems that the constant axon number derives from more parent neurons, and thus that there are more axon collaterals per callosal neuron in a normal animal than in a hypothyroid one. Taken together, these data indicate that although hypothyroidism does not alter the total number of callosally projecting axons, it interferes with the normal processes that define or sculpt the projection fields, thereby leading to a numerically normal projection with abnormal topography.

Limited topographic specificity in the targeting and branching of mammalian retinal axons

We have studied in rats the topographic targeting of retinocollicular axons anterogradely labeled by focal retinal injections of the axon tracer DiI. We find that developing retinal axons widely mistarget along both the medial-lateral and the rostral-caudal axes of the superior colliculus (SC). In neonatal rats, labeled axons originating from injection sites in the temporal periphery covering less than 1% of the retina grow over most of the contralateral SC, suggesting that the growth cones of many axons initially fail to recognize their appropriate target region at the rostral SC border. Some of these axons correct their targeting errors and are retained; most do not and are eliminated. In neonates, peripheral nasal axons transiently develop branches throughout the SC. Branches formed by nasal axons are later restricted to a discrete terminal zone at the topographically appropriate, caudal SC border. At the neonatal stage, injections in temporal or nasal retina do result in a zone of increased labeling in the topographically correct region of the SC, but this zone is considerably larger than that labeled by a similar injection at a later stage. Thus, although the early projection is very diffuse, there is some bias for the correct region of the SC. Our findings indicate that in rats, developing retinal axons show only a limited specificity in their topographic targeting and branching. We conclude that mechanisms in addition to directed axon growth are required to establish the order characteristic of mature mammalian retinal projections.

The microvascular system of the optic nerve in control and enucleated rats

The microvascular system of the optic nerve of the rat was examined morphometrically to determine the effect of enucleation of one eye at birth on the microvascular development in the contralateral optic nerve. For this purpose, two groups of rats were used: three were unilaterally enucleated on the day of birth and studied on postnatal Day 28; three littermates were used as controls. Using plastic embedded semithin sections, we analyzed various parameters and compared the results statistically. The average diameter of microvessels up to 7.5 microns was found to be 4.7 +/- 0.2 micron in controls and 5.3 +/- 0.5 micron in experimental rats. The density of microvessels expressed as the mean number of sectioned capillaries per tissue area was 137 +/- 25/mm2 in the control group and 169 +/- 32/mm2 in the experimental group. The intravascular volume fraction percentage (Vv), which represents the volume fraction of the capillary network per unit of optic nerve volume (mm3/mm3%), was 0.06% in the controls and 0.10% in the enucleated rats. Total length of capillaries per unit of volume (Lv) averaged 1050 +/- 112 and 2235 +/- 195 mm/mm3 in control and experimental groups, respectively. The internal capillary surface area available for metabolic exchange expressed per volume unit (Sv) was 15.5 +/- 2.1 and 37.2 +/- 2.8 mm2/mm3 in control and experimental groups, respectively. These results, together with the lack of ultrastructural modifications in the vascular walls of microvessels, suggest that these rearrangements of the capillary system in the enucleated group could be triggered by an increase in the optic nerve metabolism resulting from monocular vision.

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.

Duration of retinogenesis: its relationship to retinal organization in two cricetine rodents

The Mongolian gerbil (Meriones unguiculatus) has a prolonged period of development relative to other muroid rodents. We have explored the consequences of this relatively long period of maturation on retinal cell number and topography by comparing the duration and topography of neurogenesis in the gerbil retina with that of a closely related species which develops rapidly, the Syrian hamster (Mesocricetus auratus) (Sengelaub et al.: J. Comp. Neurol. 246:527-543, 1986). An analysis of thymidine-labeled retinas indicate that cells destined for the gerbil retinal ganglion cell layer are generated for at least 12 embryonic days, twice the duration in the hamster. The period of cell loss in the gerbil retinal ganglion cell layer extends for at least 14 postnatal days, more than twice as long as in the hamster. The gerbil retina is generated in a center-to-periphery gradient for both retinal ganglion cells and displaced amacrine cells, while no such gradients are evident in the hamster retina. We conclude that the longer developmental period of the gerbil is associated with 1) a longer period of neurogenesis resulting in greater retinal cell number, 2) the expression of spatial gradients in neurogenesis, and 3) a larger eye at maturity. The last two factors, in part, may be related to the development of a highly differentiated area centralis and visual streak in the retina of this rodent. Unrelated to duration of growth, early differences in retinal shape between these two species contributes to the development of retinal topography. The gerbil, but not the hamster retina, is initially asymmetric, longer in its nasotemporal than its dorsoventral dimension. The gerbil retina then grows asymmetrically, producing a spherical retina, and coincident in time, a nasotemporally extended visual streak.

Neonatal enucleations reduce number, size, and acetylcholinesterase histochemical staining of neurons in the dorsal lateral geniculate nucleus of developing rats

Previous studies have demonstrated that transient patterns of acetylcholinesterase (AChE) activity are characteristic of geniculo-recipient regions of rat cortical area 17 during the second and third postnatal weeks of life. Neonatal enucleation results in a marked reduction of this transiently expressed cortical AChE. Parallel studies have demonstrated that the dorsal lateral geniculate nucleus (dLGN) also expresses AChE transiently during development. The present study examines neuronal number and size as well as AChE histochemical staining in the dLGN of normal and neonatally enucleated rat pups to determine whether changes in dLGN neurons could account for the decreased visual cortical AChE staining that results from neonatal enucleation. Changes in 4 parameters in dLGN were noted after neonatal enucleation. First, a 26-37% shrinkage in the volume of dLGN occurred contralateral to enucleation. Second, enucleation resulted in a loss of 16-30% of AChE-stained neuronal somata. Third, remaining AChE-positive neuronal somata appeared shrunken by approximately 40%. Fourth, intensity of AChE histochemical staining of individual dLGN neurons was reduced by approximately 24% following neonatal enucleation. These data suggest that loss of transient AChE activity in cortical area 17 consequent to neonatal enucleation is secondary to enucleation-induced alterations in the dLGN; these alterations include loss of neurons, shrinkage of neurons, and an apparent decrease in the ability of neurons to synthesize AChE. These data support the hypothesis that geniculocortical projection neurons express AChE transiently during development of geniculocortical connectivity and indicate that normal afferent connections and/or activity are important for the transient expression of AChE by these neurons.

Fetal brain grafts rescue adult retinal ganglion cells from axotomy-induced cell death

After intraorbital transection of the optic nerve of adult rats, 90% of the retinal ganglion cells die within 30 days. Since fetal brain extracts and cocultured fetal target regions support the survival of retinal ganglion cells in vitro (Nurcombe and Bennett: Exp. Brain Res. 44: 249-258, '81; McCaffery et al.: Exp. Brain Res. 48: 377-386, '82; Armson and Bennett: Neurosci. Lett. 38: 181-186, '83) we investigated whether cell death in the adult retina could be prevented by transplanting fetal (E16) thalamus and tectum to the proximal stump of the optic nerve of adult rats that was completely transected 2-3 mm behind the optic disc. Unoperated eyes contained 119,973 (+/- 939, SEM) retinal ganglion cells, estimated from axon counts of the intact optic nerve. Of these, 11,601 (+/- 1,857) remained in control operated eyes at 30 days postoperation while in the eyes of grafted rats, 35,086 (+/- 2,278) retinal ganglion cells were counted. Thus, 23,485 (= 22% of those normally dying after transection of the optic nerve) ganglion cells were rescued by the fetal grafts from cell death normally following axotomy. These results indicate that fetal target regions of retinal ganglion cells contain and/or produce neurotrophic molecules that promote the survival of adult axotomized retinal ganglion cells.

Nonuniform retinal expansion during the formation of the rabbit's visual streak: implications for the ontogeny of mammalian retinal topography

We have studied the distribution of retinal ganglion cells (RGCs) which have been retrogradely labeled from massive bilateral injections of the enzyme horseradish peroxidase into the retino-recipient nuclei of foetal and postnatal albino rabbits aged from the 24th postconceptional day (24PCD) to adulthood. The number of labeled RGCs increases from about 447,000 on the 24PCD to a peak of about 525,000 on the 27PCD. From the 29PCD to birth (31/32PCD), the number of RGCs rapidly declines to about 375,000. During the next 20 d, the number of RGCs stabilizes at about 335,000. After the 51PCD, the number of RGCs gradually declines to the adult value of about 280,000. Retinal area steadily increases from about 40 mm2 on the 24PCD to about 500 mm2 in the adult, while RGC density decreases. However, the reduction in RGC density is nonuniform: RGC density in the visual streak drops from 18,600 RGCs mm2 on the 24PCD to 4700 RGCs/mm2 in the adult, whereas RGC densities at the superior and inferior edges of the retina decreases proportionally much more (from 9300 to 105 RGCs/mm2 and from 12,000 to 170 RGCs/mm2, respectively). As a result of this differential reduction in RGC density, the streak/inferior edge ratio changes from 1.6:1 to about 28:1. In the periods from the 24PCD to the 29PCD and from the 32PCD to adulthood, the proportional increases in the streak/superior edge and streak/inferior edge RGC density ratios are linearly related to the proportional increases in retinal area. However, between the 29PCD and 32PCD, the RGC density ratios increase at a greater rate than retinal area. We conclude that (1) the centro-peripheral difference in RGC density that is already present on the 24PCD might be attributable to differential RGC generation; (2) the redistribution of RGCs between the 24PCD and adulthood is mainly due to nonuniform expansion of the retina, with minimal expansion of the visual streak and maximal expansion at the superior and inferior retinal edges; and (3) a small component of the increase in the centro-peripheral RGC density ratio, which becomes apparent between the 29PCD and 32PCD, is probably due to differential RGC loss. We discuss the pattern of retinal expansion in the rabbit and the factors which might contribute to it.