Selective inhibition of ventral temporal but not dorsal nasal neurites from mouse retinal explants during contact with chondroitin sulphate.

We have determined whether chondroitin sulphate (CS) glycosaminoglycans are sufficient to direct a selective inhibition of neurite growth from ventral temporal (VT) but not from dorsal nasal (DN) retina in mouse embryos; this may underlie the formation of axon divergence in the optic chiasm. Explants from the retinal region of embryonic day-14 mouse were grown on a laminin-polylysine substrate near to a circular spot coated with CS. In control cultures, in which no CS was added to the spot, both VT and DN retinal neurites grew extensively into the coated territory. When presented with spots coated with 10 mg/ml CS, neurite growth from the VT retina into the CS territory was dramatically reduced but that from the DN retina was not significantly affected. The selective inhibition to VT neurites was completely abolished by treatment with chondroitinase ABC, indicating a specific contribution of CS glycosaminoglycan in this regionally specific behaviour. This differential behaviour was not observed in explants presented with a lower or higher concentration of CS or in explants grown on substrate coated with a different laminin concentration. Thus, a critical ratio of CS to laminin seems to be essential to induce this differential behaviour in retinal neurites towards contact with CS. Furthermore, this behavior was not observed in explants cultured directly on a CS-rich substrate, suggesting that contact with growth-promoting molecules is necessary for the selective responses of retinal neurites during subsequent contact with CS. We concluded that CS glycosaminoglycan is sufficient to drive selective inhibition of VT but not DN neurites and that, together with a critical combination of growth-promoting factors, it may control the axon divergence process at the mouse optic chiasm.

Expression of phosphacan and neurocan during early development of mouse retinofugal pathway.

We have investigated whether the two major brain chondroitin sulfate (CS) proteoglycans (PGs), phosphacan and neurocan, are expressed in patterns that correlate to the axon order changes in the mouse retinofugal pathway. Expression of these proteoglycans was examined by polyclonal antibodies against phosphacan and N- and C-terminal fragments of neurocan. In E13-E15 mouse embryos, when most optic axons grow in the chiasm and the optic tract, phosphacan and neurocan were observed in the inner regions of the retina. In the chiasm and the tract, phosphacan but not neurocan was expressed prominently at the midline and in the deep parts of the tract. Both proteoglycans were observed on the chiasmatic neurons, which have been shown to regulate axon divergence at the chiasmatic midline and the chronotopic fiber ordering in the tract, but phosphacan appeared to be the predominant form that persists to later developmental stages. Intense staining of both proteoglycans was also observed in a strip of glial-like elements in lateral regions of the chiasm, partitioning axons in the stalk from those in the tract. We conclude that phosphacan but not neurocan is likely the major carrier of the CS glycosaminoglycans that play crucial functions in axon divergence and age-related axon ordering in the mouse optic pathway. Furthermore, localization of these carrier proteins in the optic pathway raises a possibility that these two proteoglycans regulate axon growth and patterning not only through the sulfated sugars but also by interactions of the protein parts with guidance molecules on the optic axons.

Sodium butyrate induces apoptosis in MSN neuroblastoma cells in a calcium independent pathway.

Sodium butyrate (NaBt), a histone deacetylase inhibitor, can cause apoptosis in a number of cancer cells. However, the mechanism of this action is poorly understood. Increased intracellular [Ca(2+)] level has been suggested as a likely mechanism, but there is little corroborating data. In this report we provide evidence that NaBt-treated MSN neuroblastoma cells undergo massive apoptosis in the presence of serum and regardless of external or internal [Ca(2+)] levels. Presented data suggest that apoptotic effect of NaBt is both time- and dose-dependent (LD50 1 mM); and that, presence of serum or cAMP, a second messenger molecule that modulates the apoptotic program in a wide variety of cells could not circumvent the apoptotic effect of NaBt. Our findings suggest that NaBt-induced apoptosis in MSN neuroblastoma cells occurs via a pathway that is independent of Ca(2+) flux, intracellular [Ca(2+)] or cAMP levels. Further, we also present data that exclude a role for PKC or histones acetylation.

Differential responses of temporal and nasal retinal neurites to regional-specific cues in the mouse retinofugal pathway.

Retinal explants of mouse embryos were cultured together with explants of different regions in the retinofugal pathway in order to investigate whether ventral temporal (VT) and dorsal nasal (DN) retinal neurites showed differential responses to regional-specific cues in the pathway. In the presence of the chiasm, biased outgrowth of retinal neurites was found in explants of both retinal regions, which was accompanied by a reduction in total neurite growth in the VT but not the DN retina. Such differential responses to the diffusible negative influence were also observed when explants of two retinal origins were cocultured with the ventral diencephalon, but were not found with the dorsal diencephalon that contains targets of the optic axons. Indeed, extensive neurite invasion was found in the dorsal diencephalic explants and this ingrowth was more prominent for VT than DN neurites, showing a difference in axons from a distinct position in the retina to contact-mediated stimulatory activity within the target nuclei. We conclude that neurites from different regions of the retina show differential responses to the regional-specific cues in the diencephalon. These cues exist in both diffusible and contact-mediated forms that may shape the characteristic course and organization of retinal axons in decision regions of the optic pathway and the visual targets.

Inhibition of caspase-3-like activity reduces glutamate induced cell death in adult rat retina.

Retinal cell death induced by over-stimulation of glutamate receptors is related to the programmed cell death or apoptosis. However, little is known about the intracellular events that lead to this cell death process in the retina. In this study, we asked if caspase-3 family cysteine proteases regulate cell death in an explant culture of adult rat retina after exposure to excessive glutamate. Cells with DNA fragmentation were first detected in the ganglion cell layer 3 h after a brief exposure to 20 mM glutamate; whilst those in the inner nuclear layer were first observed 6 h after the glutamate lesion. Caspase-3-like activity, as indicated by immunostaining of the fractin antibody that recognizes actin fragments generated by caspase-3 family proteases, was seen 40 min after glutamate treatment. Staining was first detected in the ganglion cell layer and then in the inner nuclear layer, preceding the appearance of cells with DNA fragmentation in these layers. Colocalization study showed that all cells with DNA breaks were fractin positive, indicating that caspase-3 family activity was involved in the glutamate-induced cell death in the adult rat retina. Furthermore, DEVD-CHO, a tetrapeptide inhibitor for caspase-3 family members, reduced dramatically the fractin staining and significantly alleviated glutamate-induced cell death and DNA fragmentation in the ganglion cell layer and inner nuclear layer. Inhibitor for caspase-1-like activity, YVAD-CHO, neither reduced the fractin staining nor showed comparable neuroprotective effects to the retina. We conclude that glutamate-induced apoptotic cell death in adult rat retina is mediated by a specific activation of cysteine proteases related to the caspase-3 family, and an intervention to the caspase-3 proteases provides effective protection to retinal neurons against glutamate excitotoxicity.

Heparan sulfate proteoglycan expression in the optic chiasm of mouse embryos.

Previous studies have demonstrated that heparan sulfate (HS) proteoglycans (PGs) regulate neurite outgrowth through binding to a variety of cell surface molecules, extracellular matrix proteins, and growth factors. The present study investigated the possible involvement of HS-PGs in retinal axon growth by examining its expression in the retinofugal pathway of mouse embryos by using a monoclonal antibody against the HS epitope. Immunoreactive HS was first detected in all regions of the retina at embryonic day (E) 11. The staining was gradually lost in the central regions and restricted to the retinal periphery at later developmental stages (E12--E16). Prominent staining for HS was consistently found in the retinal fiber layer and at the optic disk, indicating a possible supportive role of HS-PGs in axon growth in the retina. At the ventral diencephalon, immunostaining for HS was first detected at E12, before arrival of any retinal axons. The staining matched closely the neurons that are immunopositive for the stage-specific embryonic antigen 1 (SSEA-1). At E13 to E16, when axons are actively exploring their paths across the chiasm, immunoreactivity for HS was particularly intense at the midline. This characteristic expression pattern suggests a role for HS-PGs in defining the path of early axons in the chiasm and in regulating development of axon divergence at the midline. Furthermore, HS immunoreactivity is substantially reduced at regions flanking both sides of the midline, which coincides spatially to the position of actin-rich growth cones from subpial surface to the deep regions of the optic axon layer at the chiasm. Moreover, at the threshold of the optic tract, immunoreactive HS was localized to deep parts of the fiber layer. These findings indicate that changes in age-related fiber order in the optic chiasm and optic tract of mouse embryos are possibly regulated by a spatially restricted expression of HS-PGs.

N-methyl-D-aspartate-induced excitotoxicity in adult rat retina is antagonized by single systemic injection of MK-801.

Intravitreal injection of N-methyl-D-aspartate (NMDA) produced a substantial damage to the adult rat retina that was largely restricted to inner retinal layers, including the ganglion cell layer (GCL), inner nuclear layer (INL), inner, and outer plexiform layers. This retinal damage was significantly reduced by a systemic injection of a low dose of MK-801 (0.5 mg/kg), a potent NMDA-receptor antagonist. This neuroprotection was dose dependent and was most effective when the antagonist was given 1 h before NMDA insult. An intraperitoneal injection of 0.5 mg/kg MK-801 provided a virtually complete protection to the retina to the NMDA-induced toxicity, as indicated quantitatively by the number of DiI-filled retinal ganglion cells, the number of cells in the GCL and INL that undergo DNA fragmentation, and the edematous changes in retinal thickness. A post-lesion administration of MK-801 was still able to provide an effective neuroprotective effect to the retina, but this protection was lost when MK-801 was given 4 h after NMDA exposure. The current results indicate a therapeutic potential of systemic application of MK-801 in protecting the adult rat retina from neurologic disorders related to excessive activation of NMDA receptors.

Axon routing at the optic chiasm after enzymatic removal of chondroitin sulfate in mouse embryos.

The effects of removing chondroitin sulfate from chondroitin sulfate proteoglycan molecules on guidance of retinal ganglion cell axons at the optic chiasm were investigated in a brain slice preparation of mouse embryos of embryonic day 13 to 15. Slices were grown for 5 hours and growth of dye-labeled axons was traced through the chiasm. After continuous enzymatic digestion of the chondroitin sulfate proteoglycans with chondroitinase ABC, which removes the glycosaminoglycan chains, navigation of retinal axons was disrupted. At embryonic day 13, before the uncrossed projection forms in normal development, many axons deviated from their normal course, crossing the midline at aberrant positions and invading the ventral diencephalon. In slices from embryonic day 14 embryos, axons that would normally form the uncrossed projection at this stage failed to turn into the ipsilateral optic tract. In embryonic day 15 slices, enzyme treatment caused a reduction of the uncrossed projection that develops at this stage. Growth cones in enzyme-treated slices showed a significant increase in the size both before and after they crossed the midline. This indicates that responses of retinal axons to guidance signals at the chiasm have changed after removal of the chondroitin sulfate epitope. We concluded that the chondroitin sulfate moieties of the proteoglycans are involved in patterning the early phase of axonal growth across the midline and at a later stage controlling the axon divergence at the chiasm.

Expression of chondroitin sulfate proteoglycans in the chiasm of mouse embryos.

Chondroitin sulfate (CS) proteoglycans have been implicated as molecules that are involved in axon guidance in the developing neural pathways. The spatiotemporal expression of CS was investigated in the developing retinofugal pathway in mouse embryos by using the CS-56 antibody. Immunoreactive CS was detected in inner regions of the retina as early as embryonic day 11 (E11). Its expression in subsequent stages of development followed a centrifugal, receding gradient that appeared to correlate with the sequence of axogenesis in the retina. In the chiasm, immunoreactive CS was expressed at E12, before the arrival of retinal axons. When the retinal axons navigated in the chiasm at E13-E14, immunoreactive CS remained at a low level in the optic fiber layer of the chiasm but was observed prominently in the caudal parts of the ventral diencephalon. This pattern followed closely the array of stage-specific-embryonic-antigen-1-positive neurons in the ventral diencephalon, with a V-shaped configuration that bordered the posterior boundary of the retinal axons, and a rostral raphe extension that ran across the decussating axons in the chiasm. Thus, the CS epitope is implicated in patterning the course of early retinal axons and in regulating axon divergence in the chiasm. At the lateral region of the chiasm, where the retinal axons cross the midline and approach the optic tract, a CS-immunopositive region coincided with the region in which active sorting of dorsal retinal axons from ventral retinal axons occurs. Moreover, at the threshold of the optic tract, the immunoreactive CS was restricted only to the deep part of the optic fiber layer, suggesting an inhibitory role of the CS epitope in repelling newly arrived axons to superficial regions of the optic tract during the development of chronotopic order at this part of the retinofugal pathway.

The effects of early prenatal monocular enucleation on the routing of uncrossed retinofugal axons and the cellular environment at the chiasm of mouse embryos.

Whereas it has been shown that early monocular enucleations produce a reduction in the uncrossed pathway from the surviving eye in rats and ferrets, similar evidence for binocular interactions in the development of the uncrossed component in mice is currently open to question. Using retrograde tracing, we have investigated the time course of changes in the uncrossed retinofugal pathway immediately after the early prenatal monocular enucleation in mouse embryos. Removal of one eye from C57 pigmented mice at embryonic day (E) 13 does not cause a reduction of the earliest uncrossed component from the central retina examined 1 day later at E14. However, a substantial reduction of the uncrossed pathway is seen at E15, the time when the major uncrossed projection first arises from the ventral temporal retina. This reduction is greater in E16 one-eyed embryos, indicating that most retinal axons from the ventral temporal retina rely on a binocular interaction for their turning at the chiasm. Further, early removal of one eye at E13 does not produce any obvious changes in the cytoarchitecture of RC-2-immunopositive radial glia at the chiasm, nor of the stage-specific antigen-1 (SSEA-1) -expressing neurons. This lack of changes in the cellular organization at the chiasm indicates that the reduction of the uncrossed pathway is probably produced by an elimination of binocular fibre interactions at the chiasm, rather than through a degenerative change of cellular elements at the chiasm as a consequence of the eye removal procedure.

Changes in axon arrangement in the retinofugal [correction of retinofungal] pathway of mouse embryos: confocal microscopy study using single- and double-dye label.

The changes in quadrant-specific fiber order in the retinofugal pathway of the C57-pigmented mouse aged embryonic day 15 were investigated by using single- (1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate; DiI) and double- (N-4-4-didecylaminostyryl-N-methylpyridinium iodide; 4Di-10ASP in addition to DiI) labeling techniques. At this earliest stage of development, before any fibers arrive at their targets, retinal axons display a distinct quadrant-specific order at the optic stalk close to the eye. This order gradually disappears along the stalk and is virtually lost at the chiasm, as shown in single-label preparations. The double-label preparations, in which the population peaks of fibers from two retinal quadrants are shown simultaneously in an image, show a fiber arrangement at the chiasm that is different from the pattern seen in the single-label preparations. A distinct and consistent preferential distribution of fibers from different retinal quadrants is shown in the chiasm. Before the midline, the central part of the cross section of the chiasm is dominated by dorsal fibers, whereas the rostral and caudal parts of the chiasm are dominated by ventral nasal and ventral temporal fibers, respectively. Moreover, the double-label preparations demonstrate a major reshuffling of fiber position after the fibers cross the midline. Fibers from ventral retina are shifted gradually to a rostral position at the threshold of the optic tract, whereas fibers from dorsal retina are shifted caudally. These changes in fiber position indicate a postmidline location in the chiasm, where fibers are re-sorted in accordance with their origins in the dorsal ventral axis of the retina, and suggest a change in axon response to guidance signals when the fibers cross the midline of the chiasm. These changes in fiber order may also be related to the re-sorting of fibers according to their ages at the postmidline chiasm.

Changes in morphology and behaviour of retinal growth cones before and after crossing the midline of the mouse chiasm - a confocal microscopy study.

The growth of retinal axons was investigated in different regions of the optic chiasm in C57 pigmented mouse embryos aged embryonic day 13 (E13) to E15. Individual retinal axons and their growth cones were labelled anterogradely by DiI and imaged using a confocal imaging system. In aldehyde-fixed embryos, retinal growth cones display a simple form in the optic nerve and become more complex in morphology in the chiasm. The complex form is particularly prominent in those axons that turn to the ipsilateral tract in the premidline region of chiasm. Moreover, complex growth cones are also commonly found in axons in the postmidline chiasm, which are markedly different in morphology from those axons in the premidline region, suggesting that the postmidline chiasm contains a novel environment for the pathfinding of retinal axons. In another experiment, the dynamic growth of retinal axons is studied in a brain slice preparation of the living retinofugal pathway. Retinal axons show an intermittent growth across the premidline and postmidline chiasm. Extensive remodelling of growth cone form followed by a shift in growth direction is commonly seen during the pause periods, indicating that signals that guide axon growth across the chiasm are not restricted to the midline, but are laid down throughout the chiasm. Moreover, dramatic changes in axon trajectory are noted first at the premidline chiasm where the uncrossed axons segregate from the crossed axons, and second at the postmidline chiasm where specific sorting of retinal axons according to their position in the dorsal ventral retinal axis and their ages are known to take place. These results show that there are two distinct environments, separated by the midline in the chiasm, where axons show different responses to local guidance cues and develop the distinct fibre orders.

Calcium ionophore-induced degradation of neurofilament and cell death in MSN neuroblastoma cells.

Extensive necrotic death of MSN neuroblastoma cells could be induced after incubation with the calcium ionophore, A23187. The reaction was concentration-dependent and time course-dependent. Levels of the 66 kd/alpha-internexin neurofilament protein (NF-66) and the cognate heat shock protein 70 (Hsc 70) decreased during the Ca2+-activated cell death. Addition of the calcium chelator, ethylene glycol-bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) restored the normal level of NF-66 and partially that of the Hsc 70. Use of either calpain I or calpain II inhibitor could alleviate the reduction of 66 kd protein during the ionophore treatment whereas only calpain I inhibitor treatment was effective in restoring the normal level of the Hsc 70. Neither of these calpain inhibitors could block the ionophore triggered cell death. EGTA was toxic to cells in a wide range of concentration suggesting a calcium-independent activation of cell death mechanism.

Temporal and spatial expression of glutamic acid decarboxylases in human fetal brain.

The expression of two isoforms of glutamic acid decarboxylase, GAD67 and GAD65, was analyzed in central nervous system (CNS) tissues obtained from normal second trimester human fetuses after elective termination of pregnancy. After RT-PCR amplification of sequences contained in total RNA extracts, Southern blotting indicated that GAD67 and GAD65 mRNAs can be detected in frontal pole tissue as early as the 12th week of gestation (12 GW). GAD67 message is strongly expressed during early second trimester and decreases slightly thereafter but remains abundant. In contrast, GAD65 message decreases rapidly and becomes undetectable by the 19 GW. However, GAD67 and GAD65 are similar in their spatial expression in the CNS at 22 GW. GAD67 and GAD65 messages are highly expressed in the cerebellum but expressed in low levels, if at all, in the spinal cord during this gestational period. These results suggest that GAD67 may have a greater role in neuron differentiation than GAD65 during human brain development.

Heterogeneous expression of neurofilament proteins in forebrain and cerebellum during development: clinical implications for spinocerebellar ataxia.

Using quantitative immunoblotting, we have measured the level of two mammalian neurofilament proteins, the 68-kDa NF-L and the 66-kDa NF-66 (alpha-internexin), in the rat CNS during development. NF-66 is localized in neurons and neuronal processes in both embryonic and postnatal brain. Importantly, NF-66 is more abundant than NF-L in both forebrain and cerebellum during development. The prevalence of NF-66 over NF-L is most pronounced in brain gray matter. The expression of both NF-66 and NF-L increases continuously during the first month after birth. In situ hybridization demonstrated that NF-66, but not NF-L is, expressed in the cerebellar granule cells. Our findings suggest that the neurofilaments are heterogeneous in developmental expression, among neuronal subtypes and in composition. Human NF-66 neurofilament has recently been mapped to chromosome 10q24. Careful analysis of the human genome map indicates NF-66 gene lies within the critical region of infantile-onset spinocerebellar ataxia (IOSCA). The characteristic developmental expression and spatial localization of the NF-66 gene suggests it as a candidate gene for the disease.

The 66-kDa neurofilament protein (NF-66): sequence analysis and evolution.

A 2.5 kb cDNA clone encoding the mouse 66 kd neurofilament protein (NF-66) was isolated and sequenced. The deduced protein sequence contains 501 amino acid residues. Comparison of the mouse, rat and human NF-66 indicated > 90% homology in protein sequence and 85% homology in coding nucleotide sequence. A high degree of homology was observed between NF-66 and other intermediate filament proteins especially in the alpha-helical domain. Zooblot analyses suggested that the putative ancestral gene for vimentin and NF-66 was detectable in the avian. By comparison, the ancestral sequence for GFAP appeared after that for vimentin.

Expression of neurofilament proteins during retinoic acid-induced differentiation of P19 embryonal carcinoma cells.

Retinoic acid (RA) induces P19 embryonal carcinoma cells to differentiate into neurons with the extension of neuritic processes. We used the P19 cell as a model system to elucidate the regulation of neurofilament (NF) expression. Four mammalian NF proteins, NF-66 (alpha-internexin), peripherin, NF-L and NF-M, and the neural-specific, growth-associated gene, GAP-43, were studied during the RA treatment of P19 cells in vitro. As controls, untreated P19 cells were maintained in parallel. Indirect immunofluorescent staining showed that in RA-treated, morphologically differentiated P19 cells NF-66 was expressed in neuron-like cells characterized by phase bright cell bodies and long neuritic processes. At various times P19 cells were harvested for protein analysis by immunoblotting with antibodies to individual NF proteins or for total RNA extraction and Northern blotting with cDNA probes for NF-66, -L, -M, peripherin and GAP-43. During induction, both NF-66 and NF-L were expressed but in distinct patterns. NF-66 mRNA and protein were detected after 6 days of induction. In contrast, NF-L mRNA, but not protein, was expressed in both induced and control cells. Neither NF-M nor peripherin were expressed during induction. During differentiation of P19 cells, NF-66 mRNA levels rose markedly by the 1st day, reached a plateau between the 3rd-5th days and declined by the 7th day. NF-66 protein accumulation lagged slightly, reaching maximum abundance about the 5th day. The kinetics of NF-66 expression were similar to that of GAP-43. However, the pattern of NF-L expression was distinct from that of NF-66. NF-L mRNA, and some protein, was expressed in both RA-treated and control cells within 6 h after plating, but was down-regulated to baseline level thereafter in both populations. Neither NF-M or peripherin expression was detected during the differentiation. In summary, NF-66 was up-regulated most robustly among the four NF proteins during differentiation in P19 cells and was the major NF protein correlated with neurite extension.

Cloning and developmental expression of human 66 kd neurofilament protein.

The complete human 66 kd neurofilament cDNA was isolated and its sequence was determined. Both the DNA and the predicted amino acid sequences showed a high degree of homology to the rat NF-66. This was substantiated by cross hybridization between the human NF-66 probe and rat NF-66 mRNA. Single gene copy was suggested from Southern blot analysis. RNase protection assay indicated that NF-66 was expressed in human fetal brain as early as the 16th gestational week. On the other hand, NF-L message was not detected until the 20th gestational week. NF-M message was not detectable up to the 24th gestational week.

Changes in fiber order in the optic nerve and tract of rat embryos.

In order to define the extent to which retinotopic order in the optic pathways may contribute to fiber segregation at the chiasm or to the formation of central maps, the arrangement of fibers in the optic nerve and tract of rat embryos, on embryonic days 16.5 and 18.5, has been studied by placing a small granule of DiI in one of the four quadrants of the retina and tracing the filled fibers through transverse sections of the retinofugal pathway with confocal microscopy. There is a distinct quadrant-specific order in the optic stalk immediately behind the eye, with fibers from the ventral nasal, dorsal nasal, dorsal temporal, and ventral temporal retina arranged sequentially across the rostrocaudal axis of the cross section of the stalk. However, this distinct order is not maintained very far. There is a gradual increase in the degree of overlap between fibers from the different quadrants as the fibers pass towards the chiasm. The dorsal groups of fibers intermingle extensively along almost the entire length of the stalk, but the fibers from ventral sectors remain separate until they reach the prechiasmatic region, where the ventral temporal and the ventral nasal fibers spread throughout the rostrocaudal extent of the stalk and the chiasm. The initial quadrant-specific order is completely lost at the chiasm. However, beyond the optic chiasm, the fibers are reorganized into another distinct order. In the optic tract, there is a segregation of dorsal from ventral fibers, but the nasal and temporal groups remain intermingled. The results of this study indicate that the earliest fibers in the developing optic tract are arranged according to topographical rules that differ from those obtaining behind the eye. Since all topographical order is lost between these two levels, there must be an active sorting mechanism in the region where the chiasm joins the tract. Possibly this mechanism is related to the development of the dorsoventral axis of the topographic maps in the central visual targets.

Developmental changes produced in the retinofugal pathways of rats and ferrets by early monocular enucleations: the effects of age and the differences between normal and albino animals.

Early monocular enucleations were done in rats, either at embryonic day 16 (E16) or on the day of birth, and the surviving uncrossed pathway was studied either at birth for some of the animals enucleated prenatally, or in the adult for all of the other animals. The uncrossed pathways were studied by using HRP as a retrograde tracer. The neonatal enucleations showed the increase of the surviving uncrossed component previously documented by others. In contrast to this, a prenatal enucleation produced a significant reduction in the surviving uncrossed pathway at birth. If these animals survived to be adults, then the surviving uncrossed pathway was slightly increased relative to normal. We conclude that two quite distinct mechanisms have been exposed by these experiments, one acting prenatally and producing a reduction in the uncrossed pathway, and the other acting postnatally and producing an increase. The postnatal effect, which is due to a decrease of the normally occurring ganglion cell death, thus neutralizes the prenatal effect, so that the most effective demonstration of the prenatal effect is to be seen before the period of cell death (early postnatal in rats and ferrets). The same methods were applied to prenatal ferrets at E26-E28 and, in order to see the maximum prenatal effects, the uncrossed pathways were studied at birth in all of these animals. There was a severe reduction of the uncrossed pathway throughout, and this was greatest in the animals with the earliest enucleations. Since the uncrossed pathway in normally reared albino animals is abnormally small, the effects of an early prenatal enucleation in albino rats and ferrets were compared with the effects in normally pigmented animals in order to determine whether the early enucleation was producing an abnormality comparable to the albino abnormality. Prenatal enucleations reduced the uncrossed pathway not only in normally pigmented but also in albino neonatal rats and ferrets. Further, the characteristic position of the nasal border of the temporal retina, which is abnormal in albino animals, was unaffected by the enucleation in either the albino or the pigmented animals, except where, in ferrets, enucleations produced a complete loss of the temporal concentration of ipsilaterally projecting ganglion cells (the temporal crescent). The earlier enucleations showed a greater tendency to produce such a complete loss of the temporal crescent. We conclude that the developmental mechanisms affected by the early enucleations are distinct from those that act to produce the albino abnormality even though both produce an abnormally small uncrossed pathway.(ABSTRACT TRUNCATED AT 400 WORDS)