The development and the topographic organization of the retinal ganglion cell layer in Bufo marinus

Summary The number and distribution of neurons in the retinal ganglion cell layer were studied from the metamorphic climax to adulthood in the toad Bufo marinus. Retinal wholemounts stained with cresyl violet showed that total neuron numbers increased from 55,000 at metamorphic climax to about 950,000 in adult animals. During the same time the entire retinal area increased 46-fold from an average 3.4 mm² to 157 mm². The morphological character of the neurons and their density across the retina changed during development. In metamorphosing animals, the neurons of the ganglion cell layer had a uniform appearance and their density increased slightly from the centre to the dorsal ciliary margin. After metamorphosis a high neuron density area, the visual streak, evolved in the retinal centre, resulting in the formation of a 6 to 1 density gradient from the visual streak out to the dorsal and ventral retinal poles in adult animals. Optic fibre numbers in juvenile and adult optic nerves were estimated to be 330,000 and 745,000, respectively, corresponding to similar ganglion cell numbers. One optic nerve was sectioned in a few animals and 4 weeks later the number of intact neurons — assumed to be displaced amacrine cells (DA) — was estimated. They amounted to 80,000 in juvenile and 189,000 in adult animals or about 20% of the total neuron population of the retinal ganglion cell layer, the remaining 80% being GC. A 1.7 to 1 density gradient of DA from the visual streak out to the dorsal and ventral retinal periphery was established. These results show that the visual streak evolves after metamorphosis from an originally uniform neuron distribution of the retinal ganglion cell layer. The possible mechanisms of the formation of the visual streak are discussed.     

Displaced retinal ganglion cells in normal frogs and those with regenerated optic nerves

Summary We have analysed the number and spatial distribution of displaced retinal ganglion cells in the frog Litoria (Hyla) moorei. A series of normal animals was compared with one in which the optic nerve was crushed and allowed to regenerate. Ganglion cells were labelled with horseradish peroxidase (HRP) applied to the optic nerve, and retinae were examined as sections or whole mounts. We analysed separately ganglion cells with somata displaced to the inner nuclear (Dogiel cells, DGCs) and to the inner plexiform layer (IPLGCs). These findings were related to data for the orthotopic ganglion cells (OGCs). The mean number of DGCs in the normal series was 2,550 (±281) and fell to 1,630 (±321) after regeneration, representing a mean loss of 36%. This reduction was not significantly different from the mean loss of 43% from the OGC population in which mean values fell from 474,700 (±47,136) to 268,700 (±54,395). In both the normal and the regenerate series, DGCs were estimated to represent means of only 0.6% of the OGC population. Densities of DGCs were highest in the nasoventral and temporo-dorsal peripheries; densities of both DGCs and OGCs were lower after optic nerve regeneration. We conclude that the factors which affect ganglion cell death during optic nerve regeneration, do so to similar extents amongst the DGC and the OGC populations. The IPLGCs were very rare in normal animals with a mean of 420 (±95). However, their numbers increased after regeneration to a mean of 3,350 (±690), estimated to be 1.2% of the OGC population. These cells normally favoured peripheral retina but became pan-retinal after regeneration. The primary dendrites of the majority of IPLGCs were oriented in the same direction as those of OGCs. We conclude that most IPLGCs were OGCs which had relocated their somata to the inner plexiform layer.     

Ultramicroscopical immunolocalization of PAX6 in the adult chicken retina

Cell type-specific PAX6 protein expression was examined in all retinal layers of the normal chicken retina. The most intense PAX6 immunostaining was found in the ganglion cell and inner nuclear layers, and in lower amounts in the optic nerve fiber, the inner plexiform and the photoreceptor layers. PAX6 immunostaining was variable in terms of its subcellular localization, even within one cell. PAX6 immunostaining was mainly localized in nuclear heterochromatin of the ganglion cell and inner nuclear layers whereas in the outer nuclear layer, PAX6 immunostaining was only observed in the intercellular space and the cytoplasm. In photoreceptors, the myoid portion of the inner segment showed PAX6 immunostaining, but the ellipsoid portion and the outer segment did not. The ultrastructural distribution pattern of PAX6 in the adult chicken retina suggests that normal expression of PAX6 is variable even in subcellular structures in the same cell type.     

Ocular toxoplasmosis: the role of retinal pigment epithelium migration in infection

Our aim was to study the migration of retinal pigmented epithelium (RPE) into the retinal layer during infection of C57BL/6 mice with Toxoplasma gondii. Eyes from infected and non-infected animals were analyzed on the 60th day of infection by light and transmission electron microscopy. Non-infected eyes showed a normal morphology. In contrast, we observed free parasites in the retinal vasculature, the presence of mononuclear inflammatory infiltrate (MNII) and parasites in the vasculature of choroids in infected eyes. No inflammatory infiltrate was observed; RPE cells were identified near the MNII in nuclear and plexiforme layers. RPE cells were also found on the ganglion cell layer and in the outer segments of the photoreceptor. The morphology showed that RPE cells caused a discontinuity in the nuclear and plexiforme layers. Clusters of parasites were found surrounded by RPE cells and MNII in the inner plexiforme layers. Ultrastructural analysis showed that RPE cells migrated through the epithelium into the inner retinal layers. We did not observe Toxoplasma cysts in many eyes in which pathological changes were detected. Only 8.3% of the animals had Toxoplasma cysts in the inner nuclear layer in the absence of inflammatory cells. The migration of RPE cells can be triggered by a disruption of the RPE monolayer or injury to the neural retina, as in the case of toxoplasmosis.     

The pearl mutation accelerates the schedule of natural cell death in the early postnatal retina

Summary The time of maximal occurrence of pyknotic nuclei in the retinal ganglion cell layer of postnatal pearl mutant mice is earlier than that in normal mice (Linden and Pinto 1985). Both ganglion and displaced amacrine cells and glia populate the ganglion cell layer. Thus, in order to show that ganglion cells themselves are affected, we counted the numbers of surviving axons in the optic nerve of postnatal day (PND) 0, 4, 12 and adult mice. On PND 0, pearl mutant mice had 139000 ± 2800 (SEM) optic axons, about 8% more than wild-type mice (128000 ± 1,700; p = 0.031) but on PND 4, pearl mutants had 24% fewer axons than wild-type mice (96000 ± 3700 and 119000 ± 4600, respectively; p = 0.008). Thus, pearl mutants lose nearly five times as many retinal ganglion cells as wild-type mice in the interval from PND 0 to 4. The number of axons present in adult mice was nearly equal (56700 ± 3200 for wild-type and 52500 ± 2700 for pearl mutants p = 0.37). We searched for evidence for changes in the schedule of cell death among other neurons of the retina by counting the number of pyknotic nuclei in the various retinal layers. On PND 4, pearl mutant mice had more pyknotic nuclei in the neuroblastic layer than wild-type mice (5000 ± 400 and 3900 ± 300, respectively; p < 0.05). The time-course of the appearance of pyknotic nuclei in the outer nuclear layer differed for the two genotypes (ANOVA, F=12.5, p < 0.001). The most striking difference was a greater number of pyknotic nuclei on PND 20 for the pearl mutants (1300) than for wild-type (480; p = 0.002). However, the total number of photoreceptors in adults did not differ between the two genotypes (3.6 × 10⁶ ± 2.4 × 10⁵ for wildtype and 3.7 × 10⁶ ± 3.3 × 10⁵ for pearl; p > 0.8). These results, taken together, show that natural cell death occurs at an earlier time for retinal ganglion cells of pearl mutants, but that the total number of retinal neurons surviving to adulthood is not affected appreciably by the mutation.     

Is Thy-1 expressed only by ganglion cells and their axons in the retina and optic nerve?

Summary The distribution of Thy-1 in the retina and optic nerve has been examined immunohistochemically, and compared to that of the astrocytic marker glial fibrillary acidic protein. The axons and cell bodies of ganglion cells were found to be Thy-1 positive as were processes within the inner plexiform layer. Transection of the optic nerve in the neonatal rat results in the rapid degeneration of the ganglion cells but some Thy-1 staining remains in the inner plexiform layer. We have estimated using an immunoassay of normal and optic nerve transected retinae that about 70% of the Thy-1 in the retina is on ganglion cells and their axons and the remainder is on cells which contribute processes to the inner plexiform layer, presumably amacrine, bipolar or Müller cells.In the optic nerve the Thy-1 was found to be limited to the fascicles of optic nerve fibres and the intrafascicular spaces, containing astrocytes and their processes, were not stained. Axotomy of the adult nerve, which produced axonal degeneration and astrocytic proliferation, led to a loss of over 95% of the Thy-1 from the nerve. We found no evidence that the astrocytes of the retina or optic nerve were Thy-1 positive in normal animals or during degeneration.     

Generation of retinal cells in the wallaby, Setonix brachyurus (quokka)

We have examined the generation of retinal cells in the wallaby, Setonix brachyurus (quokka). Animals received a single injection of tritiated thymidine between postnatal days 1-85 and retinae were examined at postnatal day 100. Retinae were sectioned, processed for autoradiography and stained with Cresyl Violet. Ganglion cells were labelled by injection of horseradish peroxidase into the optic tracts and primary visual centres. Other cells were classified according to their morphology and location. Retinal cell generation takes place in two phases. During the first phase, which concludes by postnatal day 30, cells destined to lie in all three cellular layers of the retina are produced. In the second phase, which starts by postnatal day 50, cell generation is almost entirely restricted to the inner and outer nuclear layers. Cells produced in the first phase are orthotopic and displaced ganglion cells, displaced and orthotopic amacrine cells, horizontal cells and cones. Glia in the ganglion cell layer, orthotopic amacrine cells, bipolar and horizontal cells. Muller glia, and rods are generated in the second phase. Cells became heavily labelled with tritiated thymidine in the central retina before postnatal day 7, over the entire retina (panretinal) by postnatal day 7 and from postnatal day 18, only in the periphery. The second phase of cell generation is initiated at P50, in a region extending from the optic nerve head to mid-temporal retina. Subsequently, cells are generated in annuli, centred on mid-temporal retina, which are seen at progressively more peripheral locations. Therefore, cell addition to the inner and outer nuclear layers continues for longer in peripheral than in mid-temporal retina. We suggest that such later differential cell addition to the inner and outer nuclear layers contributes to an asymmetric increase in retinal area. This non-uniform growth presumably results in more expansion of the ganglion cell layer peripherally than in mid-temporal retina and may play a role in establishing density gradients of ganglion cells.     

大鼠视神经切断后视网膜双极细胞PKC-α和recoverin的表达

为了探讨视神经切断后视网膜内部是否存在突触可塑性改变,本实验采用大鼠视神经切断模型,通过免疫组织化学方法检测视神经切断后视网膜双极细胞PKC-α和recoverin的表达变化。结果显示:正常视网膜中,PKC-α和recoverin阳性产物主要见于视网膜内核层、内网层及节细胞层,另外外核层也可见少量recoverin阳性细胞。视神经切断后3d,大鼠视网膜内网层高倍镜下可见PKC-α和recoverin免疫阳性终末的数量开始增加,14d时增至最高,21d、28d呈现逐渐减少的趋势。本研究结果提示视神经切断后视网膜双极细胞与节细胞之间的突触可能存在早期增生,后期溃变的可塑性变化。     

Chronotopographical distribution patterns of cell death and of lectin‐positive macrophages/microglial cells during the visual system ontogeny of the small‐spotted catshark Scyliorhinus canicula

The patterns of distribution of TUNEL‐positive bodies and of lectin‐positive phagocytes were investigated in the developing visual system of the small‐spotted catshark Scyliorhinus canicula, from the optic vesicle stage to adulthood. During early stages of development, TUNEL‐staining was mainly found in the protruding dorsal part of the optic cup and in the presumptive optic chiasm. Furthermore, TUNEL‐positive bodies were also detected during detachment of the embryonic lens. Coinciding with the developmental period during which ganglion cells began to differentiate, an area of programmed cell death occurred in the distal optic stalk and in the retinal pigment epithelium that surrounds the optic nerve head. The topographical distribution of TUNEL‐positive bodies in the differentiating retina recapitulated the sequence of maturation of the various layers and cell types following a vitreal‐to‐scleral gradient. Lectin‐positive cells apparently entered the retina by the optic nerve head when the retinal layering was almost complete. As development proceeded, these labelled cells migrated parallel to the axon fascicles of the optic fiber layer and then reached more external layers by radial migration. In the mature retina, lectin‐positive cells were confined to the optic fiber layer, ganglion cell layer and inner plexiform layer. No evident correlation was found between the chronotopographical pattern of distribution of TUNEL‐positive bodies and the pattern of distribution of lectin‐labelled macrophages/microglial cells during the shark′s visual system ontogeny.     

Spatial and temporal patterns of apoptosis during differentiation of the retina in the turtle

We investigated patterns of cell death in the turtle retina that could potentially be associated with the innervation of the optic tectum, and looked for mechanisms of retinal development that might be common to reptilian and homeotherm vertebrates. We used retinas of turtle embryos between the 23rd day of incubation (E23) (before the first optic fibres reach the optic tectum) and hatching (when all the optic fibres have established synaptic connections). Dying retinal neurons were identified in paraffin sections by the TUNEL technique, which specifically labels fragmented DNA. Apoptotic cells were found in the ganglion cell layer (GCL), the inner nuclear layer (INL), and the outer nuclear layer (ONL). Cell death in the GCL was intense between E29 and E47, and had disappeared by the day of hatching. In the INL, dead and dying cells were most abundant between E31 and E34, and progressively disappeared. The temporal pattern in the ONL was similar to the INL although the density was very low. In all the nuclear layers cell death spread from the dorso-temporal area of the central retina to the periphery. Additional dorsal to ventral and temporal to nasal gradients were distinguishable in a quantitative TUNEL analysis. The patterns of cell death observed in the developing turtle retina were thus similar to those found in birds and mammals. This process could be under the control of differentiation gradients in all the vertebrate classes.     

Regenerated synapses persist in the superior colliculus after the regrowth of retinal ganglion cell axons

Summary Synapse formation by retinal ganglion cell axons was sought in the superior colliculus of four adult rats 16–18 months after the optic nerve was transected and replaced by a peripheral nerve graft that guided regenerating RGC axons from the eye to the superior colliculus. The terminals of retinal ganglion cell axons were labelled by intravitreal injections of tritiated amino acids and studied by light and electron microscopic autoradiography. We found that (i) retinal ganglion cell axons had extended from the tips of the peripheral nerve grafts into the superior colliculus for approximately 350 ,m; (ii) within the superior colliculus, some regenerated retinal ganglion cell axons became ensheathed by CNS myelin; (iii) retinal ganglion cell terminals formed asymmetric synapses with dendrites of neurons in the superficial layers of the superior colliculus, mainly the stratum griseum superficialis.Regenerated (n=418) and normal retinal ganglion cell terminals (n=1775) in the superior colliculus were compared in terms of their size (area, perimeter, and maximum diameter), contacts per terminal, contacts per 10 m terminal perimeter, and post-synaptic structure contacted (dendritic spine, shaft, or soma). No statistically significant differences in the ultrastructural characteristics of the pre-synaptic profiles were apparent between the two groups. The post-synaptic structures contacted by axon terminals were similar in regenerated and control animals, although there were quantitative differences in the distributions of these contacts among dendritic spines and shafts.These results suggest that the regeneration of retinal ganglion cell axons in adult rats can lead to the formation of ultrastructurally normal synapses in the appropriate layers of the superior colliculus. The re-formed connections appear to persist for the life-span of these animals.A short account of this work was presented inSociety for Neurosdence Abstracts 14, 654 (1988).     

Somatostatin-like immunoreactivity in amacrine cells of the chicken retina

Somatostatin-like immunoreactivity was detected in chicken retina by radioimmunoassay. The levels of somatostatin-like immunoreactivity decreased after intra-ocular injection of kainic acid, but were not affected by destruction of the ganglion cells. By immunohistochemistry, somatostatinimmunoreactive amacrine cells were found in the inner nuclear layer. These cells were destroyed by kainic acid. At least some of the cells projected to all three sub-layers of the inner plexiform layer in which there were diffuse bands of fluorescence. Specific immunofluorescence was also detected at the level of the outer limiting membrane and the optic nerve fibre layer, but the outer nuclear and plexiform layers, horizontal, bipolar and ganglion cells did not show specific immunofluorescence.It is suggested that other amacrine cell sub-classes, defined in terms of their putative transmitter, may show specific patterns of cell body location and size, and terminal arborisation.     

Discrete distributions of putative cholinergic and somatostatinergic amacrine cell dendrites in chicken retina

The distributions of putative cholinergic and somatostatinergic amacrine cells of the chicken retina were compared. Acetylcholinesterase-positive amacrine cell bodies were concentrated at the border between the inner nuclear and plexiform layers. Similar amacrine cell bodies were detected in a displaced position in the ganglion cell layer. Both populations had dendrites joining the 4 bands of acetylcholinesterase activity in the inner plexiform layer. The cell bodies of somatostatin-immunoreactive amacrine cells were distinct from the intensely acetylcholinesterase-positive cell bodies. The immunoreactive terminal bands did not overlap the acetylcholinesterase-positive bands, except in the inner parts of the inner plexiform layer.     

Functional lamination in the ganglion cell layer of the macaque's retina

Close to the fovea of the primate retina the ganglion cell layer is at its maximal thickness and several layers of cells deep. In whole-mount preparations in which the ganglion cells had been retrogradely labelled to reveal the dendritic trees we have studied the distribution of the different ganglion cell types across the depth of the ganglion cell layer. The ganglion cells which project to the parvocellular layers (P ganglion cells) are found more vitread than those which project to the magnocellular layers (M ganglion cells). The cells which project to the midbrain lie in the outer part of the ganglion cell layer among the M cells and adjacent to the inner plexiform layer. Within the P and M classes of ganglion cell the On-centre cells lie more vitread than the Off-centre cells. These results are discussed with relation to the proportions of different cell types sampled with intraocular recordings from ganglion cells and the possible significance for the development of different types of ganglion cell.     

Evidence for an amacrine cell system in the ganglion cell layer of the rat retina

In the ganglion cell layer of the rat retina approx 50% of the cells with the Nissl morphology of neurons survive optic nerve section in infant and adult rats and cannot be retrogradely labelled with horseradish peroxidase. The number of neurons which can be retrogradely labelled with horseradish peroxidase from subcortical visual centres is similar to the number of axons in the optic nerve, and it is concluded that the small neurons do not send an axon into the optic nerve. The dendritic tree of the cells which have axons was demonstrated by filling the cells with horseradish peroxidase from the optic nerve. The dendritic structure of the cells which survive optic nerve section was shown by injecting horseradish peroxidase into the retina or impregnating with the Golgi method the cells which survive optic nerve section. A variety of amacrine cells were found in the ganglion cell layer which form branches in the lower part of the inner plexiform layer.It can be concluded that amacrine cells form a substantial number of the neurons in the ganglion cell layer.     

Developmental genetics of the retina: evidence that the pearl mutation in the mouse affects the time course of natural cell death in the ganglion cell layer

Summary The time course of natural cell death was studied postnatally in the ganglion cell and inner plexiform layers of the retina in the developing mouse. We examined congenic wild-type, albino and pearl mutants from birth to 12 days of age. In both wild-type and albino mice, natural cell death proceeded with an increasing rate from birth to a peak 6 days after birth, and with a decreasing rate there-after. In contrast, cell death in pearl mutants proceeded with essentially a decreasing rate postnatally. The populations of neurones and glial cells in the ganglion cell and inner plexiform layers of the retina were also determined in adult mice. It was shown that pearl mutants had a slightly smaller number of cells in those layers than both wildtype and albino mice, and that the difference was probably due entirely to the numbers of neurones. We conclude that the pearl mutation in the mouse affects the timing of developmental cell death, but the effect is not directly related to the amount of pigment in the eye.     

Expression of the 40 kDa catecholamine regulated protein in the normal and injured rat retina

Catecholamine regulated protein 40 (CRP40) has been shown to be expressed in the central nervous system (CNS) of several mammalian species where it may function in a similar manner to members of the heat shock protein (HSP) family. Immunohistochemical and immunoblotting techniques were utilized to investigate whether CRP40 is expressed in normal rat retinas. In addition, changes in CRP40 expression were studied following optic nerve transection. The immunohistochemical results showed that CRP40 is expressed in the normal rat retina. The protein was found to be highly expressed in the ganglion cell layer (GCL), the inner nuclear layer (INL) and the outer plexiform layer (OPL). In addition, a low level of CRP40 was found in the inner plexiform layer (IPL), and in the inner segment layer (ISL). No expression was found in the outer nuclear layer (ONL) of normal rat retina. The immunoblotting results show that CRP40 expression decreased in a time-dependent fashion after the optic nerve transection. This decrease indicates that the expression of CRP40 is dependent on the neuron's normal physiological state and that it plays an important function in physiological and pathological conditions in the retina.     

Ganglion Cell Distribution and Retinal Resolution in the Florida Manatee,Trichechus Manatus Latirostris

The topographic organization of retinal ganglion cells was examined in the Florida manatee (Trichechus manatus latirostris) to assess ganglion cell size and distribution and to estimate retinal resolution. The ganglion cell layer of the manatee's retina was comprised primarily of large neurons with broad intercellular spaces. Cell sizes varied from 10 to 60 μm in diameter (mean 24.3 μm). The retinal wholemounts from adult animals measured 446–501 mm² in area with total ganglion cell counts of 62,000–81,800 (mean 70,200). The cell density changed across the retina, with the maximum in the area below the optic disc and decreasing toward the retinal edges and in the immediate vicinity of the optic disc. The maximum cell density ranged from 235 to 337 cells per millimeter square in the adult retinae. Two wholemounts obtained from juvenile animals were 271 and 282 mm² in area with total cell numbers of 70,900 and 68,700, respectively (mean 69,800), that is, nearly equivalent to those of adults, but juvenile retinae consequently had maximum cell densities that were higher than those of adults: 478 and 491 cells per millimeter square. Calculations indicate a retinal resolution of ～19′ (1.6 cycles per degree) in both adult and juvenile retinae. Anat Rec, 2012. © 2011 Wiley Periodicals, Inc.     

Identification of amacrine and ganglion cells in the carp retina.

1. Amacrine and ganglion cells in the carp retina were identified from such criteria as photoresponses, intracellular dye staining, responses to optic nerve stimulation and behaviour to a synapse blocking agent. 2. Responses of ganglion cells were accompanied by spike discharges., either facilitated or suppressed by photic stimulation. The cells were also invaded by antidromic impulses, which survived after chemical synapses had been blocked by application of atomized CoCl2 solution. In subsequent histology of the Procion-stained neurones, the cell bodies were found in the ganglion cell layer and the axons were often traced. 3. Amacrine cells were subdivided into two types. The first type gave rise to transient depolarizations at both on- and offsets of spot and annulus illuminations, usually being associated with spike discharges of which the amplitudes varied in different cells. In histology, the cell bodies of this type were situated in the inner nuclear layer and dendrites ramified in two or more discrete sublayers of the inner plexiform layer (the stratified amacrine cell of Cajal). 4. The second type of amacrine cells produced sustained responses during illumination, being associated with no spike but with small oscillatory wavelets. The cell bodies were situated in the inner nuclear layer and the dendrites ramified in a single sublayer of the inner plexiform layer (the monolayered amacrine cell). 5. An attempt was made to see the effect of activation of centrifugal fibres on amacrine cells, but almost all of about 200 cells examined did not respond to optic nerve stimulation. Only two cells produced, with long latency, a small post-synaptic depolarization which disappeared after chemical synapses in the retina had been blocked. It is considered that the physiological role of the centrifugal system is insignificant in the carp retina.     

Synapsin I expression in the rat retina during postnatal development

Summary The expression of the synapsin I gene was studied during postnatal development of the rat retina at the mRNA and protein levels. In situ hybridization histochemistry showed that synapsin I mRNA was expressed already in nerve cells in the ganglion cell layer of the neonatal retina, while it appeared in neurons of the inner nuclear layer from postnatal day 4 onward. Maximal expression of synapsin I mRNA was observed at P12 in ganglion cells and in neurons of the inner nuclear layer followed by moderate expression in the adult. At the protein level a shift of synapsin I appearance was observed from cytoplasmic to terminal localization during retinal development by immunohistochemistry. In early stages (P4 and P8), synapsin I was seen in neurons of the ganglion cell layer and in neurons of the developing inner nuclear layer as well as in the developing inner plexiform layer. In the developing outer plexiform layer synapsin I was localized only in horizontal cells and in their processes. Its early appearance at P4 indicated the early maturation of this cell type. A shift and strong increase of labelling to the plexiform layers at P12 indicated the localization of synapsin I in synaptic terminals. The inner plexiform layer exhibited a characteristic stratified pattern. Photoreceptor cells never exhibited synapsin I mRNA or synapsin I protein throughout development.Abbreviations GCL ganglion cell layer - INB inner neuroblast layer - INL inner nuclear layer - IPL inner plexiform layer - ONB outer neuroblast layer - ONL outer nuclear layer - OPL outer plexiform layer