Adult Rat Superior Colliculus Research Articles

Limited Spatial Spread Explains the Dependence of Visual Response Adaptation on Stimulus Size in Rat Superior Colliculus Neurons

Although adaptation to light occurs in the eye and mainly preserves the full dynamic range of neuronal responses during changing background illumination, it affects the entire visual system and helps to optimize visual information processing. We have shown recently that in rat superior colliculus (SC) neurons adaptation to light acts as a local low-pass filter because, in contrast to the primate SC, in rat collicular neurons adaptation to small stimuli is largely limited to the vicinity of the adaptor stimulus. However, it was unclear whether large visual stimuli would induce the same spatially limited adaptation. We addressed this question by evaluating the effects of 1.8°, 6.2° and 20.8° wide adaptor stimuli on test stimuli of variable size. Single unit recordings in the adult rat SC were employed to estimate the response amplitude. Small, 1.8° and 6.2° adaptors habituated visual responses only to stimuli smaller than the adaptive stimuli. However, the 20.8° adaptor dramatically reduced responses even to test stimuli >3 times wider than the adaptor (up to 70° wide). The latter result may be explained by a nearly complete occlusion by a large adaptor of the neuron’s receptive field (RF). All these results are consistent with the idea of a limited spatial spread of adaptation in rat SC neurons that is the consequence of high convergence of retinal inputs, in which small RFs limit the spatial spread of adaptation. It is concluded that, in this limited spatial spread of adaptation, rodent SC resembles higher visual system areas in primates and indicates potential differences in visual information processing between rodents and primates.

Graft outcomes influenced by co-expression of Pax7 in graft and host tissue.

Cell replacement therapies offer promise in the treatment of neurotrauma and neurodegenerative disorders and have concentrated on the use of primary fetal brain tissue. However, there is a growing promise of using neural stem cells, in which case other factors may be important in their successful engraftment. We therefore investigated whether the co-expression of the major developmental transcription factor (Pax7 in this study) of donor tissue to graft site influences transplant survival and differentiation in the rat midbrain. Neural progenitor cells were prepared from either the Pax7-expressing dorsal (DM) or non-Pax7-expressing ventral mesencephalon (VM) of embryonic EGFP(+/+) rats. Cells were dissociated and grafted into the adult rat superior colliculus (SC) lesioned with quinolinic acid 3 days previously, a time shown to be associated with the up-regulation of Pax7. Grafts were then examined 4 weeks later. Our results suggest the origin of the graft tissue did not alter graft survival in the SC; however, dorsal grafts appear to have a higher incidence of neuronal survival, whereas ventral grafts have a higher incidence of astrocytic survivors.

Pax7 expression in the adult rat superior colliculus following optic nerve injury

The transcription factor Pax7 has been implicated in the normal development of the superior colliculus and continues to be expressed in the adult superior colliculus, where it is concentrated in the retino-recipient laminae. Here we assessed, immunohistochemically, Pax7 expression in the adult rat superior colliculus after unilateral intraorbital optic nerve transection. We show that after optic nerve transection, the number of Pax7-expressing cells increased to re-establish the developmental rostral-caudal gradient and that these Pax7-expressing superior colliculus cells were neurons. These findings may have implications for the design of therapeutic interventions to restore functional connections in the adult visual pathway.

Induction of the candidate-plasticity NGFI-A protein in the adult rat superior colliculus after visual stimulation

In this work, we studied the visually driven expression of the plasticity-related transcription factor NFGI-A in the superficial layers of the rat superior colliculus (sSC) using immunohistochemistry. After dark adaptation, NGFI-A expression was completely down-regulated, indicating this protein is not constitutively expressed in the sSC. Light stimulation for 10 min after dark adaptation was insufficient to induce detectable levels of this protein. But after 30 min of light stimulation, few NGFI-A + cells were observed in the superficial layers, indicating that the minimal time of stimulation that is sufficient to induce this protein is sometime between 10 and 30 min. The number of NGFI-A + cells increased progressively, reaching a peak after 90 min. This peak is not reached if animals are returned to darkness after 30 min of stimulation, when a presumable peak in NGFI-A mRNA is reached. Light stimulation of animals in which the retinocollicular or corticocollicular projections were removed revealed that NGFI-A expression is mainly driven by retinal contralateral projections. Removal of corticocollicular projections did not cause any change in the NGFI-A expression in the ipsilateral sSC, in relation to the contralateral (control) sSC, suggesting that this pathway has a minor influence. Our results showed that NGFI-A protein expression in the sSC is entirely dependent on visual stimulation and suggests that the sSC visual circuitry is an interesting model for studies about the involvement of this transcription factor in synaptic plasticity.

A quantitative study of the neuronal nitric oxide synthase expression in the superficial layers of the adult rat superior colliculus after perinatal enucleation

The level of expression of the neuronal nitric oxide synthase (nNOS) in the retinorecipient layers of rat superior colliculus (SC) was investigated in adult rats after neonatal enucleation using two biochemical methods: (1) measurement of the in vitro specific-activity of NOS by the conversion of [3H]-arginine to [3H]-citrulline and (2) immunochemical analysis by western blotting and densitometry of immunoreactive bands using antibodies that recognise the three prominent isoforms of nNOS, α, β and γ. A total of 20 Lister rats were used in this study. We have shown that the deprivation of the retinocollicular projections at early postnatal ages induces no significant change in the specific-activity of nNOS. We also have shown that the deafferentation does not significantly influence either the total amount of nNOS in the SC superficial layers or the relative contribution ratio of nNOS isoforms. In conclusion, the expression and activity of nNOS in the SC retinorecipient layers was shown not to be dependent on the presence of retinal afferents during development.

Expression of messenger RNAs for glutamic acid decarboxylase, preprotachykinin, cholecystokinin, somatostatin, proenkephalin and neuropeptide Y in the adult rat superior colliculus

The mammalian superior colliculus is an important subcortical integrator of sensorimotor behaviours. It is multi-layered, each layer containing specific neuronal types and possessing distinct input/output relationships. Here we use in situ hybridisation methods to map the distribution of seven neurotransmitters/neuromodulator systems in adult rat superior colliculus. Coronal sections were probed for preprotachykinin, cholecystokinin, somatostatin, proenkephalin, neuropeptide Y and the enzymes glutamic acid decarboxylase and choline acetyltransferase, markers for GABA and acetylcholine respectively. Cells expressing glutamic acid decarboxylase messenger RNA were the most abundant, the highest density being found in the superficial layers. Many cells containing proprotachykinin messenger RNA were found in stratum zonale and the upper two-thirds of stratum griseum superficiale; cells were also located in deeper tectal laminae, particularly caudomedially. Most cholecystokinin messenger RNA expressing cells were located in the superficial layers with a prominent band in the middle third of stratum griseum superficiale. Cells expressing moderate to high levels of somatostatin messenger RNA formed a dense band in the lower third of stratum griseum superficiale/upper stratum opticum; two less distinct tiers of labelling were seen in deeper layers. These in situ hybridisation data reveal three distinct sub-laminae in rat stratum griseum superficiale. Cells expressing moderate to low levels of proenkephalin messenger RNA were located in lower stratum griseum superficiale/upper stratum opticum and intermediate laminae. A cluster of enkephalinergic cells was located medially in the deep tectal laminae. Expression of neuropeptide Y messenger RNA was relatively low and mostly confined to cells in stratum griseum superficiale and stratum opticum. No choline acetyltransferase messenger RNA was detected. This in situ analysis of seven different neurotransmitters/neuromodulator systems sheds new light on the neurochemical organisation of the rat superior colliculus. The data are related to what is known anatomically and physiologically about intrinsic and extrinsic tectal circuitry, and the potential involvement of different neuropeptides in these circuits is discussed. The work forms the basis for future developmental studies examining the effects of transplantation and visual deprivation/deafferentation on tectal neurochemistry and function.

Growth characteristics of ganglion cell axons in the developing and regenerating retino-tectal projection of the rat.

Retinal ganglion cell (RGC) axons from the temporal retina project to the anterior superior colliculus (SC), whereas nasal retinal axons project to the posterior SC. The stripe assay has shown that temporal retinal axons avoid growing on membrane stripes from the posterior SC but nasal retinal axons show no growth preference. Several putative guidance molecules have been identified in target tissues for these axons during development in vertebrates. Regenerating axons from adult rat retinae also possess the capacity to recognize appropriate target cells and to form functional connections in vivo and in vitro. However, the expression of information for axonal guidance and target recognition in mammals and birds seems to be limited to the period when central nervous system projections develop during embryogenesis. Nevertheless, the deafferented adult rat SC re-establishes target information recognizable by embryonic rat retinae, although the re-expression of guidance factors after deafferentation does not seem to be a mere recapitulation of their normal developmental expression pattern. The roles of astrocytes and microglia in axon guidance are also discussed.

Retinal Ganglion Cell Axons Recognize Specific Guidance Cues Present in the Deafferented Adult Rat Superior Colliculus

During development, retinal ganglion cell axons establish a topographically ordered projection from the retina to the superior colliculus (SC). The putative guidance activities for retinal axons that operate during embryonic development are not detectable in the normal adult SC. However, these cues reappear upon transection of the optic nerve of adult rats. In the present study, we used a modified version of the "stripe assay," in which membranes from either anterior or posterior SC alternated with laminin stripes. Temporal embryonic retinal axons consistently avoid membranes from embryonic posterior SC, but only rarely from adult deafferented SC. However, they are attracted to membranes from both embryonic and adult deafferented anterior SC. Nasal retinal axons only show a significant preference for membranes from posterior SC after deafferentation. When retinal axons were offered a choice to grow on membranes either from their embryonic or their deafferented target regions, they showed a preference for the deafferented SC. On carpets consisting of laminin and membranes from normal SC (not deafferented) or nontarget regions (inferior colliculus), temporal and nasal axons grow either in a random fashion or show preferences for the laminin stripes. Our modified version of the classic stripe assay shows specific growth preferences of embryonic retinal axons for membrane lanes from their appropriate embryonic or deafferented adult target regions. These findings suggest that the deafferentation of the SC in adult rats triggers the reexpression of specific guidance activities for retinal axons. Those "attractive" guidance cues appear to be differentially expressed in the developing and deafferented SC.

Antibody that neutralizes myelin-associated inhibitors of axon growth does not interfere with recognition of target-specific guidance information by rat retinal axons.

During development, many CNS projection neurons establish topographically ordered maps in their target regions. Myelin-associated inhibitors of neurite growth contribute to the confinement of fiber tracts during development and limit plastic changes after CNS projections have been formed. Neutralization of myelin-associated growth inhibitors leads to an expansion of the retinal innervation of the superior colliculus (SC). In the lesioned adult mammalian CNS, these long projection neurons are usually unable to regrow axons over long distances after lesion due to myelin-associated inhibitors, which interfere with axonal growth in vivo and in vitro. Application of a specific antibody directed against myelin-inhibitors (IN-1) promotes regrowth of corticospinal tract or retinal ganglion cell axons. In the present study, we asked whether application of an antibody to myelin-associated growth inhibitors would lead to disturbances of target-specific axon guidance. To examine this issue, we used an in vitro model, the "stripe assay", to examine the behavior of rat retinal ganglion cell axons on membranes from embryonic and deafferented adult rat SC. On membrane preparations from embryonic rat SC, retinal fibers avoid posterior tectal membranes, possibly due to the presence of a repulsive factor. Nasal retinal axons show a random growth pattern. On membranes prepared from the deafferented adult rat SC, temporal and nasal axons prefer to grow on membranes prepared from their specific target region, which suggests the involvement of target-derived attractive guidance components. The results of the present study show that retinal axons grow significantly faster in the presence of IN-1 antibody that neutralizes myelin-associated growth inhibitors present in the membrane preparations from the adult rat SC. IN-1 antibody, however, does not interfere with specific axonal guidance. This suggests that axonal guidance and specific target finding are independently regulated in retinal axons.

Developmental changes of nitric oxide synthase in the rat superior colliculus.

We used the activity of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) to detect the presence of nitric oxide synthase (NOS) in the developing rat superior colliculus. Our results showed that NOS is present in cells and neuropil in the developing and adult rat superior colliculus. The first NOS-positive cells were detected at postnatal day 7 and were weakly stained. During the following days the number of stained cells increased markedly, reaching a peak by postnatal day 15, coinciding with the time of eye opening in the rat. By the end of the third postnatal week, both the number and intensity of stained cells showed an adult-like pattern. We conclude that NOS expression lags behind the initial period of reorganization of the retinotectal projection. However, NOS activity could be involved in the subsequent synaptic remodeling and plasticity of the retinocollicular projection.

Reactive changes in the adult rat superior colliculus after deafferentation

We have recently shown that optic nerve transection is followed by a delayed appearance (7-14 days after lesion) of putative guidance activities for retinal axons in the superior colliculus (SC) of adult rats. In the present study we analysed with immunohistological methods whether there is a correlation between localized reactive changes in the cellular compartment of the SC after deafferentation and the reappearance of putative guidance activities. The optic nerve of adult female Sprague-Dawley rats was transected. After 1 day to 1 year the contralateral SC was sectioned sagitally and examined with antibodies directed against Vimentin and glial fibrillary acidic protein (GFAP) to label astrocytes, myelin basic protein (MBP) to detect oligodendrocytes, OX-42 to stain microglia and SMI-31 to determine degeneration of afferent retinal ganglion cell axons. These stainings were performed in order to detect differences in the cellular composition of macro- and microglial cells between the anterior and posterior regions of the SC. All cell populations we examined in the SC showed reactive changes after optic nerve transection. Microglia activation occurred as soon as 2 days after optic nerve transection in the anterior part of the SC. After 4 days, a homogeneous distribution of activated microglia was seen in the whole SC. GFAP upregulation occurred without a specific regional distribution. Only Vimentin-expressing astrocytes appeared with a delay of 1-2 weeks after deafferentation and were strongly restricted to the afferent optic tract and the very anterior region of the SC. In conclusion, a temporo-spatial correlation between the re-expression of guidance activities and changes in the cellular compartment of the SC exists in the delayed re-expression of Vimentin in astrocytes and their strong spatial restriction. Activated microglia might be involved in triggering regional changes in an anterior-posterior graded fashion in the deafferented SC.

Neuromodulatory effects of angiotensin II in the visual layers of the rat superior colliculus.

Recent autoradiographic studies have revealed the presence of both AT1 and AT2 angiotensin II (AngII) receptor subtypes in the superficial layers of the rat superior colliculus (SC). We have investigated the effects of activating these receptors on visually evoked potentials (VEP) in the SC of adult rats. A recording injecting microelectrode filled with AngII was lowered into the superficial layers of the SC. AngII was injected at concentrations varying from 10(-4) to 10(-10) M. Injection of the peptide yielded a reduction in the amplitude of the VEP. This reduction usually occurred within 2-3 min following AngII injection with a 50% recovery of most of the signal 20-30 min thereafter. AngII did not modify the signal when injected in collicular layers ventral to the stratum opticum. Furthermore, concomitant injection of AngII with the specific AT, receptor antagonist Losartan failed to reduce the evoked response suggesting that the effects of AngII in the SC are likely mediated by AT1 receptors.

Expression of Low Affinity NGF (p75) Receptors in Rat Superior Colliculus: Studies in Vivo, in Vitro, and in Fetal Tectal Grafts

The distribution and level of expression of the low affinity nerve growth factor receptor (LNGFR) in the rat visual system has been investigated under a number of experimental conditions. The aim was to determine the cellular location of the receptor and to study the factors which influence its expression. The monoclonal antibody 192-Ig and immunohistochemical techniques were used to examine LNGFR expression in (i) developing and adult rat superior colliculus (SC), (ii) fetal collicular tissue transplanted to the midbrain of newborn host rats, (iii) the SC of rats which had been unilaterally enucleated at birth, and (iv) mixed glial cell cultures from the neonatal SC. The effect of eye removal on LNGFR immunoreactivity in other normally retino-recipient areas was also assessed. Postnatal maturation of the rat SC was associated with an increase in LNGFR immunoreactivity. At birth, weak staining was seen ventral to the superficial gray layer. Staining gradually became located more dorsally until in the adult there was a dense band of immunoreactivity that extended 50-100 μm from the surface. Immunoreactive processes and cellular-like profiles were seen. Compared to adult host SC there was considerably more LNGFR immunoreactivity in transplanted tectal tissue, irrespective of whether the grafts were connected to the host. LNGFR expression in transplants was patchy and usually contiguous with the graft surface. Staining was not obviously related to the distribution of astrocytes or microglia and very few cells were LNGFR positive in tectal glial cell cultures. In SC and in tectal grafts it is probable that LNGFR immunoreactivity was primarily associated with intrinsic neurons. In support of this, LNGFR expression in the superficial SC was unaffected by neonatal eye removal; however, LNGFR staining in the pretectum and diencephalon was reduced or absent on the side contralateral to visual deafferentation. These conflicting sets of data suggest that (i) LNGFRs in central visual pathways are sometimes, but by no means always, associated with retinal innervation and (ii) LNGFR expression in visual target areas originates from diverse sources and is influenced and regulated by a variety of factors.

Retinal afferent ingrowth to neocortical transplants in the adult rat superior colliculus is due to the regeneration of damaged axons.

Retinal afferent ingrowth to embryonic neural transplants in the adult rat superior colliculus may represent either sprouting of intact axons or the regeneration of transected axons. If ingrowth represents regeneration of damaged retinofugai axons, then lesions that axotomize more retinofugal axons at the transplantation site should induce greater retinal afferent ingrowth. Alternately, if ingrowth represents terminal or collateral sprouting of intact retinofugal axons at or near the transplant/host optic layer interface, then the magnitude of retinal afferent ingrowth should be directly related to the total area of this interface. To test between these two hypotheses surgical knife wounds were made either parallel (in the sagittal plane) or perpendicular (in the transverse plane) to the course of axons in the stratum opticum, embryonic neocortical tissue was transplanted at the coordinates of these tectal slits, and retinal afferent ingrowth visualized 1-90 days after surgery using anterogradely transported HRP.A zone of traumatic reaction (ztr) in the optic layers was seen in every case, characterized by hypertrophied axons and swollen terminal clubs at 1 day. Between 30 and 90 days the damaged retinofugal axons in the zone formed dense fascicles and neuroma-like tangles. Retinal afferent ingrowth occurred only across transplant interface regions with the ztr. The magnitude of ingrowth was directly related to the area of the ztr interface and not the total optic layer interface area. Retinal afferent ingrowth appears to reflect the intrinsic regenerative capacity of adult mammalian retinal ganglion cells and not sprouting of undamaged axons.

Appearance of target-specific guidance information for regenerating axons after CNS lesions

During development of the vertebrate visual system, an orderly projection of ganglion cells from the retina onto the superior colliculus (SC) is established. Mechanisms that might govern this process include the coordinated action of guidance and corresponding receptor molecules that are specifically distributed on the axons and their targets. In birds and mammals, information for axonal guidance and targeting appears to be confined to the time when the retinocollicular projection is being formed. Here we show that putative guidance activities for temporal and nasal retinal axons, which are not detectable in the normal adult SC, appear after optic nerve transection in adult rats. Both embryonic and adult retinal axons are able to respond to these guiding cues, although the guidance activities detectable in the deafferented adult rat SC might be different from those found during development. These findings imply that it might be possible to reestablish an ordered projection after lesions in the adult mammalian visual system.

Retinal afferents innervate functionally tectal but not neocortical grafts placed in lesioned superior colliculus of adult rats

Solid pieces of tectum or occipital neocortex derived from 17-day rat fetuses were placed over the lesioned right superior colliculus (SC) in adult rats as sheets retaining the internal structure of the embryonal tissue. The upper laminae of the recipient's SC (approximately up to stratum opticum) were removed by aspiration after the neocortex overlying the SC was aspirated out. Two to 5 months after the operation a microelectrode study of the neuronal electrical activity in the grafts was performed. Recordings from the tectal transplants revealed normal patterns of the spontaneous neuronal activity in all grafts wad clear neuronal reactions to visual stimuli in a large portion of them (6 out of 11). Visual reactions in these grafts were recorded from the majority of studied neurons( 185/226). The properties of the receptive fields as well as the range of latencies of the reactions corresponded to those characteristic of the normal SC. Topographic representation of the visual field upon the transplants was found. Recordings from the cortical grafts showed an abnormal character of the spontaneous neuronal activity and the absence of reactions to any sensory stimulation of the recipients. The data obtained suggest that regenerating optic axons in adult hosts retain specificity in functional innervation of only appropriate target neurons and can re-establish the topographic representation of the retina upon tectal grafts. Retinal afferents innervate functionally tectal but not neocortical grafts placed in lesioned superior colliculus of adult rats.

Different metabolic changes in the lateral geniculate nucleus and the superior colliculus of adult rats after simultaneous or delayed double enucleation

By means of the [ 14C]deoxyglucose method the local cerebral glucose utilization (LCGU) was measured in the lateral geniculate nucleus and the superior colliculus of rats with a simultaneous double enucleation, animals with an enucleation of the right eye between neonatal and adult stages followed by an enucleation of the left eye some months later and adult control rats. The control animals show LCGU values within the limits published by other observers. The LCGU values of the simultaneously double-enucleated rats are reduced on both sides to the same extent by about 20%. The dorsal lateral geniculate nucleus shows the largest decrease. The LCGU values of the rats with the delayed double enucleation have decreased even more (up to 37%). The right sided visual brain regions have a significantly lower LCGU than the corresponding regions on the left side. These findings indicate that unilateral enucleation from neonatal to adult stages leads to adaptive changes resulting in a higher metabolic vulnerability, which is revealed by the second enucleation.

Three morphologically distinct types of interface develop between adult host and fetal brain transplants: implications for scar formation in the adult central nervous system.

The development of the host/graft interface of cerebellar and cerebral transplants was studied 1-60 days after operation. Grafts from fetal Wistar rats were transplanted to a cavity over the superior colliculus of adult rats by removing parts of the overlying cortex and hippocampus according to the Björklund/Stenevi technique. In sham-operated control rats, in which a cavity was made in the brain but no graft was implanted, the parenchyma bordering the entire cavity developed a complete glial-meningeal scar within 2 weeks after operation consisting of multilayered glial processes, a basal lamina, and fibroblasts (meningeal cells). A similar interface also developed between graft and host in the most superficial parts of the transplantation cavity. In the basal parts of the transplantation cavity, the host/graft interface consisted either of an incomplete sheet of astrocyte processes aligned in parallel to each other but without a covering basal lamina or of completely fused neuropil without any morphological signs of separation between host and transplant. It is concluded that these three zones of host/graft interface are established by differential interaction between the growing transplant and the host cicatrix. At the basal host/graft parenchymatous interface the fetal transplant interferes with the normal adult cicatrization process of the host, possibly by either releasing inhibitory factors or by preventing contact between the astroglia of the host and fibroblasts (meningeal cells). In white matter regions of the transplantation cavity, voluminous cysts developed, both in sham-operated controls and in graft recipients, which were invaded by transplanted neurons.

Biochemical plasticity in the superior colliculus of adult rats after chronic visual cortex ablation

We have measured the changes of several neurochemical parameters in the adult rat superior colliculus (SC) 1.5–4 months after unilateral visual cortex ablation. High-affinity uptake of d-[ 3H]Asp was increased by 22.5% and glutamate decar☐ylase by 10% in the ipsilateral SC. This may be largely attributed to reactive synaptogenesis of intrinsic glutamatergic and GABAergic neurons, respectively. The noradrenaline content was increased by about 60 and 25% in the ipsi- and contralateral SCs, respectively. The latter probably reflects sprouting og noradrenergic fibres from the locus coeruleus.

Development of fetal retina, tectum, and cortex transplanted to the superior colliculus of adult rats

Earlier studies showed that embryonic retina, cortex, or tectum transplanted adjacent to the superior colliculus of newborn host rats differentiated many of the histological features appropriate for the donor region and developed interconnections with the host nervous system. In the study presented here, the same regions were transplanted to the brain of adult host rats and the development of these transplants was compared to those into newborn hosts. Retina, rostral tectum, or occipital cortex was dissected from donor rat embryos on gestational day 14 or 15. A portion of cortex was aspirated in 2-month-old host rats to expose the right superior colliculus, and one of the donor tissues was placed adjacent to the colliculus in each host. Two to 4 months after transplantation, transplant histology and neuronal interconnections between the transplant and host nervous system were studied by using Nissl and neurofibrillar stains and 3H-proline and HRP tract tracing techniques. Four main points can be drawn from these results. First, 80% of the transplants survived in adult hosts - a percentage comparable to that found in newborn hosts. Second, each of the types of tissues transplanted differentiated histological characteristics appropriate for its site or origin, although the degree of differentiation was always much less than in transplants to newborns. Third, the transplants developed only relatively local projections into the host cortex and superior colliculus. This contrasts with the extensive projections found from the transplants into the brain of newborn hosts. Fourth, no definitive projections from the host retina or brain were identified to any of the transplants into adults, whereas both cortical and tectal transplants into newborns received projections from the host.