Neural differentiation and synaptogenesis in retinal development

To investigate the pattern of neural differentiation and synaptogenesis in the mouse retina, immunolabeling, Brd U assay and transmission electron microscopy were used. We show that the neuroblastic cell layer is the germinal zone for neural differentiation and retinal lamination. Ganglion cells differentiated initially at embryonic day 13(E13), and at E18 horizontal cells appeared in the neuroblastic cell layer. Neural stem cells in the outer neuroblastic cell layer differentiated into photoreceptor cells as early as postnatal day 0(P0), and neural stem cells in the inner neuroblastic cell layer differentiated into bipolar cells at P7. Synapses in the retina were mainly located in the outer and inner plexiform layers. At P7, synaptophysin immunostaining appeared in presynaptic terminals in the outer and inner plexiform layers with button-like structures. After P14, presynaptic buttons were concentrated in outer and inner plexiform layers with strong staining. These data indicate that neural differentiation and synaptogenesis in the retina play important roles in the formation of retinal neural circuitry. Our study showed that the period before P14, especially between P0 and P14, represents a critical period during retinal development. Mouse eye opening occurs during that period, suggesting that cell differentiation and synaptic formation lead to the attainment of visual function.     

Horizontal cells of the mouse retina contain glutamic acid decarboxylase-like immunoreactivity during early developmental stages

We have used an antiserum to L-glutamic acid decarboxylase ((GAD), a synthesizing enzyme for gamma-aminobutyric acid (GABA)) to localize putative GABAergic neurons in the developing C57BL/6J mouse retina. At early developmental stages (embryonic day 17 to postnatal day 3), strong GAD-like immunoreactivity is detectable in cell bodies located within the neuroblastic layer. These cells have relatively large cell bodies and extend several sturdy processes which are oriented radially at these early stages. We have identified these cells as horizontal cells. In addition, cell bodies adjacent to the inner plexiform layer and both diffuse and punctate structures within the inner plexiform layer proper have weak GAD-like immunoreactivity at this time. By postnatal day 6, GAD-positive horizontal cell processes begin to form a horizontal network in the newly formed outer plexiform layer. Immunolabeling of amacrine cell bodies and of punctate structures in the inner plexiform layer becomes much stronger at this time, reaching a maximum staining intensity during the second postnatal week. After postnatal day 12, GAD-like immunoreactivity of the horizontal cells begins to decline; in 4-week-old mice the horizontal cells are no longer detectably labeled by this GAD antiserum. At the same time, the GAD-like-immunoreactive material in the inner plexiform layer becomes stratified, forming distinct layers. Amacrine cells and the inner plexiform layer remain GAD positive into adulthood.     

Developmental expression of somatostatin receptors in the rat retina.

The ontogeny of somatostatin receptor binding was studied in developing rat retina using the iodinated derivative of the somatostatin analog, SMS 204-090. Specific binding of the ligand was seen as early embryonic day (E) 15 in the region of the inner neuroblastic layer. At E19 binding was localized to the ganglion cell and developing inner plexiform layers. At postnatal day (P) 2, there was diminished binding on autoradiography in this region. At P11, binding was more intense in the inner plexiform layer, and there was discernible binding in the outer plexiform layer. In the adult retina, the binding was seen clearly in two distinct bands corresponding to the inner plexiform layer and the outer plexiform layer. There was a single saturable binding site with the dissociation constant (Kd) of 0.25 +/- 0.04 nM. Binding sites were fairly constant throughout development except for a significant decline during the first postnatal week (Bmax = 1.8). These results demonstrate the early appearance of somatostatin receptors in the rat retina with high levels present embryologically followed by a brief decline in the early postnatal period with a return to high levels by synapse formation (P11). These receptor data parallel previous reports of the appearance of the somatostatin mRNA and peptide in rat retina.     

Differential expression of phospholipase D1 in the developing retina

Expression patterns of phospholipase D1 (PLD1) in the developing rat retina were investigated using immunocytochemistry and Western blot analysis and compared with the expression patterns of glutamine synthetase. PLD1 immunoreactivity appeared first in a few neuroblasts in the middle of the mantle zone of the primitive retina by embryonic (E) day 13. PLD1-immunoreactive primitive ganglion cells were characterized in the ganglion cell layer by E17. Faint immunoreactivity at E17 profiled radially orientated cells and this pattern appeared up to postnatal (P) day 7. In the ganglion cell layer at P3, displaced amacrine cells and ganglion cells were classified. At P5, presumptive horizontal cells and amacrine cells were identified. By P7, a thin outermost layer of newly formed segments of the photoreceptor cells was also PLD1 immunoreactive. PLD1 immunoreactivity at P8 was limited to radial Müller cells and the outer segment layer of the photoreceptor cells, and the expression pattern was conserved to adulthood. Western blot analysis showed relatively high amounts of PLD1 protein at E17 and P3, a decrease at P7, and moderate amounts from P8 onward. Co-expression of PLD1 with glutamine synthetase in the retina appeared first after birth in differentiating neurons and in Müller cells by P8; thereafter the pattern was maintained. The expression pattern of the PLD1 during development of the retina suggests that PLD1 plays important roles in glutamate-associated differentiation of both specific neurons and radial glial cells, and in glutamate-mediated cellular signalling in Müller cells.     

Embryonic and postnatal development of microglial cells in the mouse retina

Macrophage/microglial cells in the mouse retina during embryonic and postnatal development were studied by immunocytochemistry with Iba1, F4/80, anti-CD45, and anti-CD68 antibodies and by tomato lectin histochemistry. These cells were already present in the retina of embryos aged 11.5 days (E11.5) in association with cell death. At E12.5 some macrophage/microglial cells also appeared in peripheral regions of the retina with no apparent relationship with cell death. Immediately before birth microglial cells were present in the neuroblastic, inner plexiform (IPL), and ganglion cell (GCL) layers, and their distribution suggested that they entered the retina from the ciliary margin and the vitreous. The density of retinal microglial cells strongly decreased at birth, increased during the first postnatal week as a consequence of the entry of microglial precursors into the retina from the vitreous, and subsequently decreased owing to the cessation of microglial entry and the increase in retina size. The mature topographical distribution pattern of microglia emerged during postnatal development of the retina, apparently by radial migration of microglial cells from the vitreal surface in a vitreal-to-scleral direction. Whereas microglial cells were only seen in the GCL and IPL at birth, they progressively appeared in more scleral layers at increasing postnatal ages. Thus, microglial cells were present within all layers of the retina except the outer nuclear layer at the beginning of the second postnatal week. Once microglial cells reached their definitive location, they progressively ramified.     

Postnatal development of dopamine D1 receptor immunoreactivity in the rat retina.

Dopamine, an important neuromodulator in the retina, controls the balance of rod cone photoreceptor activity and influences the activity of several interneurons. The postnatal development of dopaminergic neurons, visualized immunocytochemically, was compared to the development of dopamine D1 receptor immunoreactivity. Expression of D1 receptors was monitored throughout the postnatal development of the rat retina using a subtype-specific monoclonal antibody. D1 receptors are expressed in the inner plexiform layer beginning at birth. Labeling of the inner plexiform layer changed from a diffuse pattern, staining the entire layer, to the typical adult punctate staining, that was organized in layered bands and occurred in the second postnatal week. The staining did not co-localize with dopaminergic cells; instead, it colocalized with cells in the inner nuclear layer or the ganglion cell layer. Within these cells, D1 receptors were most heavily expressed in processes stratifying in the inner plexiform layer. Staining in the outer plexiform layer and in horizontal cells was found beginning in the second postnatal week. Clustering of the D1 receptor within plexiform layers, a process typical for the well-described function of dopamine modulation in the adult, occurred late in postnatal development. A possible function of D1 receptors in neuronal development is discussed.     

PSA-NCAM immunocytochemistry in the cerebral cortex and other telencephalic areas of the lizard Podarcis hispanica: differential expression during medial cortex neuronal regeneration

The lizard medial cortex, a region homologous to the mammalian dentate gyrus, shows postnatal neurogenesis and the surprising ability to replace its neurons after being lesioned specifically with the neurotoxin 3-acetylpyridine. As the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) is expressed during neuronal migration and differentiation, we have studied its distribution in adult lizards and also during the lesion-regeneration process. In the medial cortex of control animals, many labeled fusiform somata, presumably corresponding to migratory neuroblasts, appeared in the inner plexiform layer. There were also scattered immunoreactive granule neurons in the cell layer. Double immunocytochemistry with 5'-bromodeoxyuridine revealed that some of the PSA-NCAM-expressing cells in the inner plexiform and cell layers were generated recently. PSA-NCAM immunoreactivity was also present in the dorsomedial, dorsal, and lateral cortices, as well as in the dorsal ventricular ridge, the nucleus accumbens, and the nucleus sphericus. Twelve hours after the injection of 3-acetylpyridine, some medial cortex granule neurons appeared degenerated, although some of them still expressed PSA-NCAM. One to 2 days after the injection, most granule neurons appeared degenerated and no PSA-NCAM immunoreactivity was detected in the medial cortex cell layer. Four to 7 days after treatment, abundant labeled fusiform cells populated the inner plexiform layer and some immunoreactive somata were seen in the cell layer. Fifteen to 30 days after the neurotoxin injection, the number of PSA-NCAM expressing granule neurons augmented considerably and the level was still above control levels in lizards that survived 42 days. Our results show for the first time the expression of PSA-NCAM in a reptile brain, where it appears to participate in the migration and differentiation of granule neurons during adult neurogenesis and regeneration.     

Immunochemical localization of calretinin in the retina of the turbot (Psetta maxima) during development

The expression of the calcium-binding protein calretinin was analysed by immunohistochemistry techniques in the retina of turbot (Psetta maxima) from embryonic to juvenile stages. Calretinin immunoreactivity was first detected in retinae from newly hatched larvae, in which the anlage of the inner plexiform layer and a subset of amacrine and ganglion cells displayed a faint immunolabelling. First appearance of photoreceptors during larval life coincided with an increase in the intensity of the labelling. During subsequent larval development, the expression of calretinin affected distinctive retinal components. The inner plexiform layer, optic fiber layer, and a population of amacrine and ganglion cells were invariably labelled. Occasional bipolar cells were labelled at the end of the larval period. By metamorphosis, calretinin is sequentially expressed in horizontal cells, and bipolar immunoreactive cells become numerous. The pattern of calretinin immunoreactivity of the inner plexiform layer changes from the larval to juvenile period. In all cases, calretinin immunoreactivity exhibited variations between the peripheral retina, which contains the most recently differentiated retinal components, and the remainder of the differentiated retina. Our results suggest that the progressive expression of calretinin in the turbot retina appears associated with some degree of neuronal differentiation. Once the definitive pattern of calretinin immunoreactivity is established in the turbot retina, both similarities and differences with the calretinin location in the retina of other vertebrates can be demonstrated.     

Localization and developmental expression patterns of the neuronal K-Cl cotransporter (KCC2) in the rat retina.

The processing of signals by integrative neurons in the retina and CNS relies strongly on inhibitory synaptic inputs, principally from GABAergic and glycinergic neurons that serve primarily to hyperpolarize postsynaptic neurons. Recent evidence indicates that the neuron-specific K-Cl cotransporter 2 (KCC2) is the major chloride extrusion system permitting hyperpolarizing inhibitory responses. It has been hypothesized that depolarizing GABA responses observed in immature neurons are converted to hyperpolarizing responses in large part by the expression of KCC2 during the second week of postnatal development. The cell-specific localization and developmental expression of KCC2 protein have been examined in relatively few neural tissues and have never been studied in retina, of which much is known physiologically and morphologically about inhibitory synaptic circuits. We examined the localization of KCC2 in adult rat retina with immunohistochemical techniques and determined the time course of its postnatal expression. KCC2 expression was localized in horizontal cells, bipolar cells, amacrine cells, and, most likely, ganglion cells, all of which are known to express GABA receptor subtypes. Developmentally, KCC2 expression in the retina increased gradually from postnatal day 1 (P1) until P14 in the inner retina, whereas expression was delayed in the outer plexiform layer until P7 but reached its adult level by P14. These data support the hypothesis that the function of KCC2 is intimately involved in GABAergic synaptic processing. Furthermore, the delayed temporal expression of KCC2 in the outer plexiform layer indicates that GABAergic function may be differentially regulated in retina during postnatal development and that GABA may produce depolarizing responses in the outer plexiform layer at times when it generates hyperpolarizing responses in the inner plexiform layer.     

Ontogenetic expression of the vanilloid receptors TRPV1 and TRPV2 in the rat retina

The present study aimed to analyze the gene and protein expression and the pattern of distribution of the vanilloid receptors TRPV1 and TRPV2 in the developing rat retina. During the early phases of development, TRPV1 was found mainly in the neuroblastic layer of the retina and in the pigmented epithelium. In the adult, TRPV1 was found in microglial cells, blood vessels, astrocytes and in neuronal structures, namely synaptic boutons of both retinal plexiform layers, as well as in cell bodies of the inner nuclear layer and the ganglion cell layer. The pattern of distribution of TRPV1 was mainly punctate, and there was higher TRPV1 labeling in the peripheral retina than in central regions. TRPV2 expression was quite distinct. Its expression was virtually undetectable by immunoblotting before P1, and that receptor was found by immunohistochemistry only by postnatal day 15 (P15). RNA and protein analysis showed that the adult levels are only reached by P60, which includes small processes in the retinal plexiform layers, and labeled cellular bodies in the inner nuclear layer and the ganglion cell layer. There was no overlapping between the signal observed for both receptors. In conclusion, our results showed that the patterns of distribution of TRPV1 and TRPV2 are different during the development of the rat retina, suggesting that they have specific roles in both visual processing and in providing specific cues to neural development.     

On the development of the stratification of the inner plexiform layer in the chick retina

This study investigated the development of the subdivision of the chick inner plexiform layer (IPL). The approach included an immunohistological analysis of the temporal and spatial expressions of choline acetyltransferase, of the neural-glial-related and neural-glial cell adhesion molecules (NrCAM and NgCAM, respectively) and axonin-1, and of inwardly rectifying potassium (Kir) channels in 5- to 19-day-old (E5-E19) embryos. Ultrastructural investigations evaluated whether synaptogenesis accompanies the onset of differentiation of the IPL. We found that the differentiation of the IPL started at E9. Distinct cholinergic strata appeared, NrCAM immunoreactivity showed a poorly defined stratification, and Kir3.2 was expressed in the IPL and in the inner nuclear layer. From E10 until late E14, NgCAM- and axonin-1-immunoreactive strata emerged in an alternating sequence from the outer to the inner IPL. During this period, the NrCAM pattern sharpened, and eventually five bands of weaker and stronger immunoreactivity were found. Conventional synapses formed at the beginning of E9, and stratification of the IPL also began on the same day at the same location. Synaptogenesis and stratification followed a gradient from the central to the peripheral retina. The topographic course of differentiation of the IPL generally corresponded to the course of maturation of ganglion and amacrine cells. Synaptogenesis and the expression of G-protein-gated Kir3.2 channels accompanied the onset of stratification. These events coincide with the occurrence of robust and rhythmic spontaneous neuronal activity. The subsequent differentiation of the IPL seemed to be orchestrated by several mechanisms.     

Developmental expression of cannabinoid receptors in the chick retinotectal system

The cannabinoid system has been suggested to participate in processes such as antinociception, cognition, motor control, and, more recently, development of the nervous system. This study describes the expression of the CB1 cannabinoid receptor in the developing chick retina and optic tectum by means of conventional immunoperoxidase protocols. CB1 immunoreactivity was initially detected around the embryonic day 4 (E4) in both the retina and tectum. In the retina, CB1 immunoreactivity was first observed in presumptive ganglion cells and, subsequently, in the inner plexiform layer and two populations of neurons of the inner nuclear layer. The post-hatched chick exhibited a pattern of staining that included four sublayers of the inner plexiform layer, a few stained cells in the ganglion cell layer, and labeled neurons both in the inner and central parts of the inner nuclear layer. The latter two types of neurons appear to be amacrine and bipolar cells, respectively. In the tectum, CB1 first appeared in its most superficial zone and later in several tectal laminae, including a white matter layer (stratum album centrale; Cajal's layer 14). There was a remarkable and transient increase of labeling at E10, followed by a continuous reduction of staining until E18. In the post-hatched chick, tectal staining was mostly confined to layers 2-3 and 5-6. Stained perikarya were seldom observed in the tectum at any stage. These data are in agreement with a possible developmental function of CB1, as it is expressed several days before synaptogenesis ensues and exhibits transient expression in the optic tectum.     

Neurotrophin receptors (Trk A, Trk B, and Trk C) in the developing and adult human retina

In this study, the ontogeny and distribution patterns of three neurotrophin receptors (Trk A, Trk B, and Trk C) were examined in the human retinas. Immunohistochemistry was performed on sections of retina and optic nerve from fetuses (11-24 weeks of gestation, wg), one infant (4-month-old) and two adult (35- and 65-years-old) subjects. At 11 wg, Trk A was expressed in the nerve fiber and inner plexiform layers, while Trk B and Trk C were expressed in many neuroblastic cells. By 16-17 wg, the photoreceptors showed immunoreactivity for all three receptors. The ganglion cell layer and amacrine cells were conspicuously immunoreactive for Trk A and Trk C, but labeled diffusely for Trk B. The horizontal cells were labeled for Trk A and Trk B. The pattern was same in the retinas at midgestation (20-21 wg). Shortly after this period, there was an apparent decrease in receptor immunoreactivity in the fetal retinas. In the infant retina, Trk A immunoreactivity was absent from horizontal cells. The photoreceptors were immunopositive for Trk B and Trk C, in infant and adult retinas. In the adults, few cells of the ganglion cell layer and inner nuclear layer were clearly labeled for Trk A and Trk C, and diffusely for Trk B. The glial cells of the retina and optic nerve immunoreacted for Trk A only, right from fetal 16 wg. The early expression of Trk B and Trk C on neuroblastic cells suggests that both play a role in cell proliferation. The developmental distribution pattern of Trk A, on the other hand, provides evidence for its involvement in differentiation of the inner plexiform layer, horizontal cells and neuroglia. The results strongly suggest that photoreceptor development is mediated by Trk receptors. The novel localization of Trk B and Trk C on adult photoreceptors points to a possible therapeutic potential for BDNF and NT-3, respectively, in photoreceptor diseases.     

Plasmalemmal and vesicular gamma-aminobutyric acid transporter expression in the developing mouse retina

Plasmalemmal and vesicular gamma-aminobutyric acid (GABA) transporters influence neurotransmission by regulating high-affinity GABA uptake and GABA release into the synaptic cleft and extracellular space. Postnatal expression of the plasmalemmal GABA transporter-1 (GAT-1), GAT-3, and the vesicular GABA/glycine transporter (VGAT) were evaluated in the developing mouse retina by using immunohistochemistry with affinity-purified antibodies. Weak transporter immunoreactivity was observed in the inner retina at postnatal day 0 (P0). GAT-1 immunostaining at P0 and at older ages was in amacrine and displaced amacrine cells in the inner nuclear layer (INL) and ganglion cell layer (GCL), respectively, and in their processes in the inner plexiform layer (IPL). At P10, weak GAT-1 immunostaining was in Müller cell processes. GAT-3 immunostaining at P0 and older ages was in amacrine cells and their processes, as well as in Müller cells and their processes that extended radially across the retina. At P10, Müller cell somata were observed in the middle of the INL. VGAT immunostaining was present at P0 and older ages in amacrine cells in the INL as well as processes in the IPL. At P5, weak VGAT immunostaining was also observed in horizontal cell somata and processes. By P15, the GAT and VGAT immunostaining patterns appear similar to the adult immunostaining patterns; they reached adult levels by about P20. These findings demonstrate that GABA uptake and release are initially established in the inner retina during the first postnatal week and that these systems subsequently mature in the outer retina during the second postnatal week.     

Choline acetyltransferase-immunoreactive neurons in the developing rat retina

The development of cholinergic cells in the rat retina has been examined with immunocytochemistry by using antisera against choline acetyltransferase (ChAT). ChAT-immunoreactive (IR) cells were first detected at embryonic day 17 (E17) in the transitional zone between the neuroblastic layer (NBL) and ganglion cell layer (GCL). At E20, ChAT-IR cells are located exclusively in the GCL. At postnatal day 0 (P0), ChAT immunoreactivity appeared for the first time in cells at the distal margin of the NBL. Two prominent bands of labeled processes were first visible at P3, and by P15, these two bands resembled those of the adult retina. In addition, ChAT immunoreactivity appeared transiently in horizontal cells from P5 to P10. The number of ChAT-IR cells increased steadily up to P15. This resulted in a 93.8-fold increase between E17 and P15 (680-63,800 cells). However, after P15, the number declined by 19% from 63,800 cells at P15 to 51,800 in the adult. At all ages, the spatial density of each ChAT-IR cell population in the central retina was higher than in the periphery. In both central and peripheral regions, the peak density of ChAT-IR cells in the GCL was attained at E20. However, in the INL, the peak densities occurred at P3 in the central region and at P5 in the peripheral region. Up to P15, the soma diameter of ChAT-IR cells in the INL and GCL in each region increased continuously, reaching peak values at P15. Our results demonstrate that ChAT immunoreactivity is expressed in early developmental stages in the rat retina, as in other mammals, and that acetylcholine released from ChAT-IR cells may have neurotrophic functions in retinal maturation.     

The distribution of neuronal calcium sensor-1 protein in the developing and adult rat retina

We have examined the distribution of the neuronal calcium-binding protein, neuronal calcium sensor 1 (NCS-1) in the developing and adult rat retina using subcellular fractionation of the rat retina and immunohistochemistry. NCS-1 immunoreactivity was situated primarily in the ganglion cells, a class of amacrine cells, and in the inner plexiform layer (IPL). During development, NCS-1 protein expression closely followed that of the synaptic vesicle protein, synaptophysin, increasing dramatically in the IPL at postnatal day 3, the time when conventional synapses are formed in the retina. These findings suggest that NCS-1 plays a role in synaptogenesis in the retina and in synaptic transmission at conventional synapses but not ribbon synapses in the adult rat retina.     

Development of cholinergic amacrine cells is visual activity-dependent in the postnatal mouse retina

In the present study, we used immunocytochemistry to study the temporal and spatial arrangement of mouse cholinergic amacrine cells during postnatal retinal development under normal light/dark cycles and during visual deprivation. Choline acetyltransferase (ChAT)-immunolabeled cells were detected in the neuroblastic layer (NBL) and in the ganglion cell layer (GCL) at postnatal day 0 (P0). Between P3-5, two characteristic cholinergic bands were clearly identified in the inner plexiform layer (IPL). The signal intensity of somas and processes progressively increased over the first 2 postnatal weeks. Around eye opening at P12, cholinergic neurons were mature-like. This early developmental process was not altered by visual deprivation. After eye opening, the space between the two cholinergic bands increased continuously and the spatial regularity index changed constantly, indicating that the cholinergic neurons possibly underwent refinement during later postnatal development. The changes occurring following eye opening were retarded by visual deprivation. The morphologies of photoreceptors, horizontal cells, recoverin-positive OFF-cone bipolar cells, rod bipolar cells, dopaminergic amacrine cells, and Müller cells appeared normal. Their stratification in the outer plexiform layer (OPL) and the IPL was not affected by visual deprivation. However, glial cells grew vertically across the entire thickness of dark-reared retinas. Our results suggest that the development of cholinergic neurons before eye opening is independent of the lighting conditions. Their development after eye opening is greatly impeded by visual deprivation. This visual activity-dependent phase of development may be a critical period for the maturation and synaptic wiring of cholinergic amacrine cells in the mammalian retina.     

BMP4 expression in the developing rat retina

We investigated the expression of bone morphogenetic protein-4 (BMP4) in the developing retina. At E19, we found very intense BMP4 immunoreactivity (IR) in the nerve fiber layer. At P1, the inner plexiform layer exhibited very strong BMP4-IR. Thereafter, abundant BMP4 expression was kept to the adult period. These results suggest that BMP4 plays pivotal roles in the retina not only in the early embryonic period but also in the late embryonic and postnatal periods, and even in the adult.     

Localization of synapse-associated proteins during postnatal development of the rat retina.

The expression of synapse-associated proteins (SAPs) was monitored throughout postnatal development of the rat retina using specific antibodies and immunocytochemistry. The distribution of chapsin-110/postsynaptic density protein (PSD)-93, SAP90/PSD-95, SAP97 and SAP102 immunoreactivity was characterized. All SAPs were found to be expressed in the inner plexiform layer (IPL) from birth on or soon after birth. With the exception of SAP97, the IPL labelling changed from a diffuse pattern staining the whole developing IPL to the typical adult punctate synaptic staining in the second postnatal week. Staining in the outer retina was first observed at postnatal day 5 (P5) for all proteins at the onset of outer plexiform layer (OPL) development. All SAPs showed a differential cellular and temporal distribution being either exclusively pre- or postsynaptically localized. Except for SAP90/PSD-95, immunoreactivity was also detected in the nerve fibre layer throughout postnatal development. Possible functions of the early expression of SAPs well before differentiation and maturation of glutamatergic ribbon synapses are discussed.     

Transplantation of human embryonic retina to adult rat retina

Human embryonic retinas (postconceptional age 3-10 weeks) with or without retinal pigment epithelium were grafted to the retina of immuno-suppressed adult rat hosts. The development of the xenografts was followed up to 37 weeks of total age by histology and by immunohistochemistry for S-antigen. The donor tissue became rearranged in folded sheets with rosettes. The grafts developed approximately according to their intrinsic timetable, but with a developmental delay in the later stages. Occasionally, the grafts were well fused with the host retina. At 13 weeks of total age, the grafts contained areas of inner plexiform layer with presumptive ganglion cells, one neuroblastic layer, and cone precursor cells around rosettes. At 19 weeks, an outer plexiform layer and inner segments of the cones started to form. At 20 weeks, the first immunoreactivity for S-antigen was observed in photoreceptor precursors. Cone inner segments were clearly distinguishable at 28 weeks, and more S-antigen-positive rods were seen. At 31 weeks, rods were more differentiated, showing S-antigen-positive inner and outer segments. An inner limiting membrane with an apparent ganglion cell layer was only seen in one cograft of retina and retinal pigment epithelium at 37 weeks, indicating an important role of retinal pigment epithelium for graft differentiation. This study shows that human embryonic retina can be grafted to immuno-suppressed adult rat retina with long-term survival. A high degree of maturation can be obtained in the grafted tissue comparable to the layering of newborn human retina. It appears that most cell types develop. This model opens up possibilities for studying human retinal development with the goal of reaching a treatment for human degenerative retinal disorders.