Cysteine sulfinate carboxylase in the visual pathway of adult chicken.

Cysteine sulfinate (CSA) carboxylyase, the enzyme which synthesizes taurine through hypotaurine, shows a higher activity in the inner plexiform and nuclear layer of adult chick retina compared to the outer plexiform and nuclear layers whereas the outer segments of photoreceptors do not show any activity of this enzyme. These observations suggest an endogenous synthesis of taurine preferentially in certain layers of retina. Therefore, taurine fulfills one more criteria which is required by a substance to be accepted as a neurotransmitter in an organ. Studies on the distribution of CSA-carboxylyase in the visual pathway and other brain areas show a very high activity of this enzyme in optic tectum followed by cerebral cortex, cerebellum, retina, lateral geniculate body and optic nerve, taken with chiasma and tract in decreasing order. On the other hand, analysis of the free amino acid pool reveals a very high content of taurine in retina as compared to optic tectum. Cysteine sulfinate carboxylyase activity and the content of taurine therefore do not seem to bear a good correlation and other mechanisms of release, uptake and degradation might be involved in regulating the taurine content in these tissues.     

Neurochemical Characteristics of Myelin-like Structure in the Chick Retina

Abstract: Certain characteristics of myelin-like structures in the chick retina were examined morphologically and biochemically. Developmental changes of 2', 3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) in the chick retina and optic nerve were examined. The measurable activity in the retina was first detected at 16 days of incubation and thereafter, it increased rapidly until 4 weeks post-hatching. By contrast, CNPase activity in the optic nerve reached the maximum level at 4 days post-hatching and maintained a constant level thereafter. The purifed myelin fraction from the chick retina showed higher activity of CNPase, whereas its activity in the retinal homogenate was very low. Hence, it was considered that the myelin fraction from the chick retina is similar to that of CNS myelin with respect to CNPase. Protein profiles of the purified myelin fractions isolated from the chick optic tectum, optic nerve, retina and sciatic nerve were analysed by SDS-polyacrylamide gel elec-trophoresis. Myelin fractions from the chick optic tectum and optic nerve contained basic protein (BP) and Folch-Lees proteolipid protein (PLP). Myelin fraction from the chick sciatic nerve contained BP, P2 and two glycoproteins (PO and 23K). In contrast, retinal myelin fraction contained only BP. PLP, PO, 23K and P2 proteins were definitely undetectable. Electron micrographs revealed that some axons in the optic nerve fiber layer of the chick retina were wrapped by a spiral-structured myelin-like sheath, which showed some differences from those of CNS and PNS myelin sheaths. It was suggested that the origin of the myelin-like structure in the chick retina is other than from oligodendroglia or Schwann cells.     

Cholinergic development in chick optic tectum and retina reaggregated cell cultures

The developmental profiles of acetylcholinesterase and choline acetyltransferase in chick optic tectum and retina cell aggregates, over a 30-day period, have been determined and compared with the corresponding developmental curves obtained in vivo. Both acetylcholinesterase and choline acetyltransferase activities in retina cell aggregates and the acetylcholinesterase activity in optic tectum cell aggregates usually lie between 40 and 90% of the values measured in vivo for the same cell (tissue) type and developmental age. However, the choline acetyltransferase activity in tectum aggregates increases only during the first 7 days of culture, and then decreases to reach a low value of 8% of that measured in vivo, by day 24. This fact, which is associated with widespread degeneration and cell death, could be attributed to the condition of natural deafferentiation occurring in a tectum cell aggregate. A parallel has been drawn between this behavior of a tectum cell aggregate and the effect of early embryonic eye removal on the development of the contralateral optic tectum in vivo. Thus, the tectum may have a biphasic pattern of development, with an autonomous period of growth of about 2 wk, followed by an afference-dependent phase, while the retina behaves, from a cholinergic point of view, as a relatively self-sufficient structure.Abbreviations AChE acetylcholinesterase - ChAT choline acetyltransferase - ACh acetylcholine - BW284 C51 dibromide 1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide     

THE DIRECT MEASUREMENT OF THE AXOPLASMIC TRANSPORT OF INDIVIDUAL RNA SPECIES: TRANSFER BUT NOT RIBOSOMAL RNA IS TRANSPORTED

Analysis of chick retinal and tectal RNA revealed that in addition to the major cytoplasmic RNAs (rRNA and tRNA), a number of the small mol wt nuclear RNAs (snRNAs) can also be detected. Subfractionation data indicated that one of these molecules, DD′, is of at least 95% nuclear location within the retina. Thus, very little, if any, of the retinal DD′ is available for axoplasmic transport from the retina into the optic nerve and tectum. Following intraocular injection of [³H]uridine, considerable incorporation of isotope into DD′ was observed within the optic tectum after 4, 8 and 16 days. This result indicates the presence of considerable local (i.e. tectal) synthesis. The specific activities of 29S, 18S and 5S rRNA and 4s tRNA relative to that of DD′ were measured in the optic tectum 8 and 16 days after the intraocular introduction of [³H]uridine. The same measurements were also made in intracranially injected animals. While the 29S/DD′, 18S/DD′ and 5S/DD′ specific activity ratios obtained were independent of the injection route, the 4S/DD′ ratio obtained from intraocularly injected animals was significantly greater (at least 2-fold) than that obtained from intracranially injected animals. Similar analysis was also performed with the optic nerve complex at 16 days post-injection with identical results. These results demonstrate that tRNA, but not rRNA, is transported from the retina into the optic nerve and tectum in the 2-day-old chicken.     

Up-regulation of protein kinase C in regenerating optic nerve fibers of goldfish: Immunohistochemistry and kinase activity assay

Protein kinase C (PKC) activation has been associated with synaptic plasticity in many projections, and manipulating PKC in the retinotectal projection strongly affects the activity-driven sharpening of the retinotopic map. This study examined levels of PKC in the regenerating retinotectal projection via immunostaining and assay of activity. A polyclonal antibody to the conserved C2 (Ca²⁺ binding) domain of classical PKC isozymes (anti-panPKC) recognized a single band at 79–80 kD on Western blots of goldfish brain. It stained one class of retinal bipolar cells and the ganglion cells in normal retina, as shown previously. Strong staining was not present in the optic fiber layer of retina or in optic nerve, optic tract, or terminal zone in tectum, with the exception of a single fascicle of optic nerve fibers that by their location and by L1 (E587) staining were identified as those arising from newly added ganglion cells at the retinal margin. Normal tectal sections showed dark staining of a subclass of type XIV neuron with somas at the top of the periventricular layer and an apical dendrite ascending to stratum opticum. In regenerating retina, swollen ganglion cells stained darkly and stained axons were seen in the optic fiber layer. In regenerating optic nerve (2–11 weeks postcrush), all fascicles of optic fibers stained darkly for both PKC and L1(E587). At 5 weeks postcrush, PKC staining could also be seen in the medial and lateral optic tracts and stratum opticum at the front half of the tectum and very lightly over the terminal zones. PKC activity was measured in homogenized tissues dissected from a series of fish with unilateral nerve crush from 1 to 5 weeks previously. Activity levels stimulated by phorbols and Ca²⁺ were measured by phosphorylation of a specific peptide and referred to levels measured in the opposite control side. Regeneration did not increase overall PKC activity in retina or tectum, but in optic nerve there was an 80% rise after the first week. The increased activity verifies that the increased staining in nerve represented an up-regulation of functional PKC during nerve regeneration. © 1998 John Wiley & Sons, Inc. J Neurobiol 36: 315–324, 1998     

Histochemical and cytochemical localization of acetylcholinesterase in retina and optic tectum of teleost fish.

The activity of cholinesterase and its cellular and subcellular localization were investigated in the retina and optic tectum of Eugerres plumieri and in the retina of Carassius carassius by means of radiometric, histochemical, and cytochemical procedures. In both fishes only the presence of acetylcholinesterase could be demonstrated. This study, besides confirming previous findings that acetylcholinesterase is located in the ganglion and amacrine cells of the retina as well as in the inner plexiform layer, in addition provides evidence that the enzyme is also present at the region of photoreceptor synapses between the cell bodies and apposing extensions of the horizontal cells of the same layer. The latter localization may indicate the involvement of a cholinergic mechanism at the functional contacts (transferapses) between the horizontal cells. In the optic tectum of Eugerres plumieri, histochemistry reveals fine distinguishable bands of acetylcholinesterase activity; two of the bands are quite sharply defined, whereas three others have rather a more diffuse appearance. The presence of these bands and their distribution may suggest a widespread distribution of cholinergic elements in the optic tectum.     

Collagen-tailed and globular forms of acetylcholinesterase in the developing chick visual system

We have carried out a comparative study of the developmental profiles of the enzyme acetylcholinesterase, and of its collagen-tailed and globular structural forms, solubilized in the presence of 1 M NaCl, 1% (w/v) sodium cholate and 2 mM EDTA, in the chick retina and optic lobes. The overall acetylcholinesterase activities, both per mg protein and per embryo or chick, are substantially higher in tectum than in retina, from embryonic day 16. The A₁₂ collagen-tailed form of the enzyme is present in similar amounts in the embryonic retina and optic tectum; however, while the A₁₂ activity increases significantly in retina after birth, both by percentage and in absolute terms, the tectal tailed enzyme follows a declining developmental profile, reaching a minimum after 6 months of life. On the other hand, the globular G₄ species shows developmental profiles, both in retina and tectum, rather similar to those obtained for the overall enzyme activity, while the G₂ and G₁ forms are present in comparable concentrations in both tissues. Besides, G₄ is the predominant globular form in the chick optic lobe after hatching, G₂ and G₁ being enriched in the embryonic tectum. In the case of retina, however, all the globular forms contribute more evenly to the total acetylcholinesterase activity, along the developmental period considered.The potential significance of some of the postnatal developmental profiles is discussed in terms of the progressive adjustment of retina and tectum to the requirements of visual function.     

Developmental Changes in the Biosynthesis of Sialoglycoprotein Substrates for Intrinsic Synaptic Sialidase

Abstract: N′-Acetyl-d -[6-³H]mannosamine was administered to 13- and 28-day-old rats by intraventricular injection. At various time intervals following the injection, synaptic membranes were prepared and the incorporation of radiolabel into sialic acid residues released from endogenous glycoproteins and gangliosides by intrinsic sialidase determined. Radiolabel was incorporated into synaptic membrane gangliosides and glycoproteins, and at all times tested, >90% of the label was associated with sialic acid. Sialic acid released from endogenous glycoproteins by intrinsic sialidase present in 28-day membranes incorporated only 20–25% as much radiolabel per nmole as sialic acid released by mild acid hydrolysis or by exogenous neuraminidase. In contrast, sialic acid released from glycoproteins present in 13-day-old membranes by intrinsic sialidase, mild acid hydrolysis, or exogenous neuraminidase all were similarly labelled. At both ages the specific radioactivity (cpm/nmol) of sialic acid released from gangliosides by the intrinsic enzyme was similar to the total ganglioside sialic acid released by mild acid hydrolysis. The results identify glycoprotein substrates for intrinsic synaptic membrane sialidase as a distinct metabolic class in the mature brain and suggest the occurrence of a developmentally related change in the metabolism of these glycoproteins.     

Composition and labeling by [3H]glycerol and [3H]serine of phospholipids of the chicken retina and optic tectum

The content and fatty acid composition of phospholipids and the in vivo labeling of lipids by [3H]glycerol and [3H]serine was studied in the retina and the optic tectum of young chickens. The tectum had a higher content of phospholipids and a significantly lower ratio of choline (CGP) to ethanolamine (EGP) glycerophospholipids than the retina. Lipids of the chicken optic system were characterized by a high proportion of polyenoic fatty acids of the n-6 series compared to other species. Intravitreally injected [3H]glycerol was incorporated into all glycerol-containing lipids of the retina, especially in CGP and EGP. Most of the label from [3H]serine was found in serine glycerophospholipids (SGP). The time-dependent distribution of both precursors among retinal lipids was consistent with de novo synthesis as well as metabolic interconversions of lipids. Thus, [3H] from serine also appeared in EGP and CGP, indicating the presence and activity of SGP decarboxylase and EGP-n-methyl transferase. Lipids labeled with both precursors in retina were subsequently found in the tectum, via axoplasmic transport. Even though different lipid classes were labelled by each precursor the proportion of lipids transported to the tectum was similar in both cases (about 1% of the label present in retina).     

Expression and involvement of gicerin, a cell adhesion molecule, in the development of chick optic tectum

Gicerin is a cell adhesion molecule belonging to the immunoglobulin superfamily. It has both a homophilic binding activity and a heterophilic binding activity to neurite outgrowth factor (NOF) a molecule belonging to the laminin family. We have reported many studies on the heterophilic activity of gicerin and NOF, but the function of its homophilic binding activity in vivo had been unclear. In the retina, gicerin is expressed in retinal ganglion cells only when they extend neurites to the optic tectum. In this report we have found that gicerin is also transiently expressed in the optic tectum during this time. First, cell aggregation assays were used to show that gicerin expressed in the optic tectum displays homophilic binding activity. Then, explant cultures of embryonic day 6 chick optic tectum on gicerin-Fc chimeric protein-coated dishes and NOF-coated dishes were carried out. It was found that gicerin-gicerin homophilic interactions promoted cell migration, whereas heterophilic interactions with NOF induced neurite formation. Furthermore, when anti-gicerin antibodies were injected in order to examine the effect of gicerin protein in the formation of the tectal layer in ovo, cell migration was strongly inhibited. These data suggest that homophilic interaction of gicerin participates in the migration of neural cells during the layer formation and plays a crucial role in the organization of the optic tectum.     

Compression and expansion without impulse activity in the retinotectal projection of goldfish

The capacity of regenerating optic fibers to undergo retinotopic compression and expansion in the absence of impulse activity was tested by eliminating activity with periodic intraocular injections of tetrodotoxin (TTX) during regeneration. To test for compression, the posterior half of tectum was removed and the optic nerve crushed. For expansion, the temporal half of retina was ablated and the nerve also crushed. The projection was then subsequently examined with electrophysiological mapping and autoradiographic tracing. Like electrically active fibers, silent fibers formed a retinotopically ordered projection that was compressed onto the anterior half tectum. Similarly, TTX-treated fibers from a nasal half retina formed a retinotopic projection that was expanded across the entire tectum. Except for some enlargement of receptive fields produced by the TTX, the topography was equivalent to that formed by active fibers. Thus, fibers can apparently maintain relative positions irrespective of absolute tectal position without the benefit of activity-dependent ordering. This implies the existence of an activity-independent mechanism for relative positioning that may operate across larger distances than the activity-dependent ordering responsible for fine topography and ocular dominance columns.     

Competitive interactions between retinal ganglion axons for tectal targets explain plasticity of retinotectal projection in the servomechanism model of retinotectal mapping

The mechanism of topographic mapping of retinal ganglion cells to the midbrain was previously elucidated by the servomechanism model, which is based on the fact that cells expressing Eph-receptors respond specifically to surface expressing membrane-bound ephrin-ligands at a critical level. The retina has increased nasal-to-temporal gradient of Eph receptor-density, and the optic tectum/superior colliculus has increased rostral-to-caudal gradient of membrane-bound ephrin-ligand. An axon from the retina has an identification tag of a certain level of Eph-receptor density depending on its retinal position, and adheres to the site on the tectum/superior colliculus expressing ephrin-ligands at a critical ligand-density level. The servomechanism model rigidly defines positions of axon terminals on the midbrain. However, optic nerve regeneration experiments combined with halved retina or tectum show a plastic or flexible mapping (expansion, compression and transposition of tectal projections). To reconcile the discrepancy between the rigid model and the plastic behavior, competition between retinal axon terminals for a target site was introduced to the servomechanism. The servomechanism/competition model succeeded in computer simulations of the plastic mapping of retinal axons on the tectum. Recent experiments of upregulated ligand-density on the tectum during nerve regeneration and the role of axonal competition are discussed.     

Distribution of an endogenous lectin in the developing chick optic tectum

We determined the cellular localization of an endogenous lectin at various times during the development of a well-characterized region of chick brain, the optic tectum. This lectin is a carbohydrate-binding protein that interacts with lactose and other saccharides, undergoes striking changes in specific activity with development, and has previously been purified by affinity chromatography from extracts of embryonic chick brain and muscle. Cellular localization in the tectum was done by indirect immunofluoresecent staining, using immunoglobulin G derived from an antiserum raised against pure lectin. No lectin was detectable in the optic tectum examined at 5 days of embryonic development. From approximately 7 days of development, neuronal cell bodies and fibers were labeled by the antibody; and extracts of tectum contained hemagglutination activity that could be inhibited by lactose or by the antiserum. Lectin remained present in many tectal neuronal layers after hatching; but in 2-month-old chicks it was sparse or absent in most of the tectum except for prominent labeling of fibers in the stratum album centrale. The initial appearance of lectin in the optic tectum was not dependent on innervation by optic nerve fibers since bilateral enucleation during embryogenesis did not affect it. Lectin was detectable on the surface of embryonic optic tectal neurons dissociated with a buffer containing EDTA.     

Optic nerve input‐dependent regulation of neural stem cell proliferation in the optic tectum of adult zebrafish

Adult neurogenesis attracts broad attention as a possible cure for neurological disorders. However, its regulatory mechanism is still unclear. Therefore, they have been studying the cell proliferation mechanisms of neural stem cells (NSCs) using zebrafish, which have high regenerative potential in the adult brain. The presence of neuroepithelial‐type NSCs in the optic tectum of adult zebrafish has been previously reported. In the present study, it was first confirmed that NSCs in the optic tectum decrease or increase in proportion to projection of the optic nerves from the retina. At 4 days after optic nerve crush (ONC), BrdU‐positive cells decreased in the optic tectum's operation side. In contrast, at 3 weeks after ONC, BrdU‐positive cells increased in the optic tectum's operation side. To study the regulatory mechanisms, they focused on the BDNF/TrkB system as a regulatory factor in the ONC model. It was found that bdnf was mainly expressed in the periventricular gray zone (PGZ) of the optic tectum by using in situ hybridization. Interestingly, expression level of bdnf significantly decreased in the optic tectum at 4 days after ONC, and its expression level tended to increase at 3 weeks after ONC. They conducted rescue experiments using a TrkB agonist and confirmed that decrease of NSC proliferation in the optic tectum by ONC was rescued by TrkB signal activation, suggesting stimuli‐dependent regulation of NSC proliferation in the optic tectum of adult zebrafish. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419–437, 2017     

Levels of choline intermediates in the visual system structures and in peripheral nerve of the rat: Comparison with neural tissues of a lower vertebrate (Mustelus canis) and an invertebrate (Loligo pealei)

The levels of choline intermediate endogenous pools in structures of the visual system (retina, optic nerve, lateral geniculate body, superior colliculus) and in sciatic nerve of adult (4-month-old) and young (30-day-old) rats were measured. The amounts were also obtained from retina, optic nerve, optic tectum and cranio-spinal nerves of a primitive elasmobranch, the smooth dogfish Mustelus canis, and from related nervous structures (retina, optic lobe, fin nerve, stellar nerve and stellate ganglia) of a marine invertebrate, the squid Loligo pealei. In all regions of rat nervous system, the pool size of CDP-choline was much smaller than that of free choline, whereas GroPCho was present in a relatively higher content. The pool sizes of choline intermediates in 30- and 120-day-old rats were nearly the same. In nervous system regions of the dogfish and squid, the values followed the same general trend as observed for rat. Squid nervous tissues had the lowest choline and GroPCho contents. The rat retina showed the lowest glycerophosphorylcholine phosphodiesterase activity. The chemical studies described here confirm the basic similarity in the pattern of choline intermediate pool sizes among animal species widely different in phylogenetic position. The data highly reinforce the idea that the precursor role of choline and catabolic pathways for the maintenance of the PtdCho membraneous pool seem highly conserved during evolution.     

金鱼再生的视神经在顶盖投射精确化的DiI顺行标记研究

本文用微量显微注射法,在金鱼视网膜的背侧用亲脂类荧光染料DiI标记少量神经节细胞,通过顺行标记研究了视神经再生过程中视网膜顶盖投射的精确化过程。在损伤视神经后的不同时期观察了再生视神经纤维在顶盖整装片上的分布。在再生早期它们以超出正常的途径由背腹两侧进入顶盖,广泛分布。但其中大部分仍分布于顶盖腹侧的靶区。在再生晚期通过精确化,重建如正常鱼一样精确的视网膜顶盖投射。这个精确化过程表现在以下三方面:(1)再生于顶盖错误区域的再生视神经纤维的消失;(2)再生早期视神经纤维主干上生长的侧部分支的消失;(3)到达靶区的再生视神经纤维形成重迭的终末分支。由以上结果推测,顶盖中可能存在两类不同的因子:一类是普通诱向因子,存在于整个顶盖中,它在再生早期引导再生的视神经纤维长入顶盖。另一类是神经营养因子,它具区域特异性,在再生晚期引导视神经纤维到达顶盖靶区,形成精确的视网膜顶盖投射。     

Taurine trophic modulation of goldfish retinal outgrowth and its interaction with the optic tectum

Summary. Goldfish retinal explant outgrowth in the presence of fetal calf serum is stimulated by taurine. In the absence of it, but with glucose in the medium, length of neurites is still elevated by the amino acid. Using the medium in the presence of glucose, but in the absence of fetal calf serum, we explored the effect of optic tectum medium from cultures of them coming from goldfish without crush of the optic nerve or 3, 5, 10, 14 and 20 days after crush. Retinal explants, intact or from goldfish with crush of the optic nerve 10 days prior to starting the culture, were employed in order to measure the possible effect of optic tectum media and the inter action with taurine. In other type of experiments the optic nerve was crushed 1, 2, 4, 7 and 10 days before dissection of the optic tectum, and then co-cultured with intact or 10 days post-crush retinal explants. Optic tectum media produced a time-dependent effect on outgrowth in lesioned retinas with a maximum effect around 5 days after the lesion for the corresponding optic tectum. Taurine, 4 mM, did not further affect the outgrowth in the presence of optic tectum media, but did significantly increase length of neurites either in intact or in post-lesion retinas. Co-culture of optic tectum at different days post-lesion and retinas at 10 days post-lesion increased the outgrowth around 4 days post-lesion, in a preparation resulting in mutual effects of both types of tissues. The addition of taurine in these conditions did not further increase outgrowth, rather inhibited it according to the time after lesion of optic nerve corresponding to the co-cultured optic tectum. The effect of taurine was concentration-dependent, since 0.2 mM was more effective than 2 or 4 mM in the presence of optic tectum with lesion of 2 days. These results demonstrate the time-course of the regeneration processes in the visual system of goldfish, indicating the crucial periods after crush in which the tectum could produce stimulation and later decrease or no effect on outgrowth from the retina. In addition, they are evidences of the interaction between taurine and optic tectum production of time-produced specific agents. The mechanisms underlying these effects are closely related to calcium, as it was demonstrated by the addition of extracellular or intracellular chelators to the medium, which inhibited the effects of the optic tectum and the trophic properties of taurine in this system. The inhibitor of taurine transport, guanidoethylsulfonate, also decreased the stimulatory effects of the optic tectum and of taurine, indicating an interaction of substances produced by the tectum with taurine, and an effect of taurine mediated through its entrance to the cells. Overall, retinal explants outgrowth in the absence of fetal calf serum, the interaction of agents of the optic tectum and taurine modulates outgrowth from the retina, and these effects are mediated by calcium levels and by the levels of intracellular taurine.     

Differential expression of calretinin in the developing and regenerating zebrafish visual system

Calretinin is a calcium-binding protein which participates in a variety of functions including calcium buffering and neuronal protection. It also serves as a developmental marker of retinal ganglion cells (RGCs). In order to study the role of calretinin in the development and regeneration of RGCs, we have studied its pattern of expression in the retina at different developmental stages, as well as during optic nerve regeneration by means of immunohistochemistry. During development, calretinin is found for the first time in RGCs when they connect with the optic tectum. Optic nerves from adult zebrafish were crushed and after different survival times, calretinin expression in the retina, optic nerve tract and optic tectum was studied. From the day of crushing to 10 days later, calretinin expression was found to be downregulated within RGCs and their axons, as was also observed during the early developmental stages of RGCs, when they are not committed to a definite cell phenotype. Moreover, 13 days after lesion, when the regenerating axons arrived at the optic tectum, a recovery of calretinin immunoreactivity within the RGCs was observed. These results indicate that calretinin may play an important role during optic nerve regeneration, Thus, the down-regulation of Calretinin during the growth of the RGC axons towards the target during development as well as during their regeneration after injury, indicates that an increase the availability of cytosolic calcium is integral to axon outgrowth thus recapitulating the pattern observed during development.     

Larval size constraints determine directional ontogenetic shifts in the visual system of teleosts1

We summarize characteristic sequences of morphological change in the teleost visual system from larvae to large adults at the level of the retina, the optic tract and the optic tectum. These shifts include sizes and ratios of cone and rod receptor cells, sizes and types of retinal ganglion cells and optic tract fibers as well as features of the optic tectum. Teleost larvae are the smallest vertebrates known. We suggest that the utilization of color contrasts as an adaptive benefit dictates the starting point of morophological development, which is a pure cone retina in most fish larvae. The direction of morphological and functional shifts in the teleost visual system during growth is determined by continuous retinal stretch, which allows for improving visual abilities. The larval visual system probably provides just adequate photopic (cone-)acuity for plankton feeding, but limited space in the retina hampers optimization of both, photopic resolving power and sensitivity Limited space also Irevents the simultaneous development of the scotopic (rod-)system. Over a wide range of body sizes, morphological parameters change, photopic and scotopic resolving power, acuity and sensitivity improve. Size constraints in the teleost visual system and lifefong shifts in sensory capacities are discussed with respect to ecology and the niche concept.