Distribution of [3H]RNA in goldfish optic tectum following intraocular or intracranial injection of [3H]uridine. Evidence of axonal migration of RNA in regenerating optic fibers.

The distribution of [³H]RNA in the goldfish optic tectum following eitherintra-ocular orintracranial injection of [³H]uridine during optic fiber regeneration has been studied by light (LMA) and electron (EMA) microscopic autoradiography.In one group of 4 fish both optic nerves were crushed, and 18 days later [³H]uridine was injected into the right eye. A second group of 5 fish, in which only one optic nerve had been crushed, received intracranial injections of [³H]uridine 18 or 22 days after the crush. All fish were sacrificed 24 days after crushing the optic nerves, a time when regenerating optic fibers have entered the tectum and are establishing functional reconnections. Tecta were fixed in situ with glutaraldehyde, dissected out, and samples were processed for LMA and EMA. Controls were carried out to ensure that [³H]RNA was the only radioactive component present in the tissue after fixation.The distribution of silver grains related to [³H]RNA in intraocularly injected goldfish was different from that following intracranial injection. Following intraocular injection virtually all the [³H]RNA was located in the layers of the left optic tectum (contralateral to the side of intraocular injection) where the regenerating optic fibers course and terminate, whereas virtually no radioactivity was present in the right optic tectum. EMA quantitative analysis of the labeled layers of the left optic tectum revealed that perikarya of cells, most of which are glial cells, had a density of grains related to [³H]RNA of 20–28 g/100 sq.μm; axonal growth cones had a density of 14–24 g/100 sq.μm. Grain densities over non-axonal cell structures were markedly lower, ranging between 3 and 6 g/sq.μm. Grains located over axons and growth cones accounted for 50–60% of all counted grains.Inintracranially injected goldfish, either 2 or 6 days after injection, silver grains were clustered over leptomeninges as well as vessels and parenchymal cells of the tectal strata containing the regenerating optic fibers. In the stratum opticum a high grain density was seen over glial cells, whereas virtually no grains were present over the fascicles of regenerating axons. EMA quantitative analysis revealed a grain density over glial and other parenchymal cells of the stratum opticum of 67 g/100 sq.μm, whereas densities over growth cones and regenerating axons were 1.3 g/100 sq.μm and 1.8 g/100 sq.μm respectively. Grains located over axons and growth cones accounted for 3.3% of all counted grains.On the basis of the present and previous findings it is suggested that followingintraocular injection of [³H]uridine the [³H]RNA present inside regenerating optic axons is transported from the ganglion cells of the retina; on the other hand, the [³H]RNA present in surrounding glial cells is the result of local utilization of [³H]RNA precursors which also migrate from the retina along with the [³H]RNA.It is also concluded that 2 and 6 days followingintracranial injection of [³H]uridine no substantial tranfer of [³H]RNA from glial cells to regenerating optic fibers occurs in the goldfish optic tectum.     

Amphibian-specific regulation of polysialic acid and the neural cell adhesion molecule in development and regeneration of the retinotectal system of the salamander Pleurodeles waltl

Antibodies specific to the neural cell adhesion molecule (NCAM-total), the 180 × 10³ My component of NCAM (NCAM-180) and polysialic acid (PSA) were used in immunohistochemistry and Western blots to detect the spatiotemporal dynamics of these molecules in development and regeneration of the retinotectal system of Pleurodeles waltl. NCAM-total and NCAM-180 are continuously expressed in the retina, optic nerve, and tectum of the developing and adult salamander. This is also found for the 140 × 10³ My component of NCAM in Western blots of the retina. In the larval retina, PSA is present in the inner plexiform layer (IPL) and a few cells in all nuclear layers. At metamorphosis, PSA expression in the retina strongly increases in the layer of cone photoreceptor somata. Several cells in the inner nuclear layer and Muller cell processes also begin to express PSA. This pattern persists into adulthood. The optic nerve and the tectum are strongly PSA-immunoreactive throughout development. In the adult optic nerve and optic fiber pathway in the brain, PSA expression is selectively downregulated. In the crush-lesioned adult optic nerve, regenerating fibers are NCAM-180-positive but PSA-negative. This demonstrates a molecular difference between growing nerve fibers of Pleurodeles in development and in regeneration. PSA regulation is closely correlated with metamorphosis, thus suggesting that PSA expression may be under hormonal control. Some aspects of PSA and NCAM isoform expression patterns in the retinotectal system of salamanders differ considerably from that of other vertebrates. The substained expression of NCAM isoforms in adult salamanders might be due to secondary simplification (paedomorphosis). © 1993 Wiley-Liss, Inc.     

Choline acetyltransferase immunoreactivity suggests that ganglion cells in the goldfish retina are not cholinergic

Published evidence that ganglion cells in the retinae of nonmammalian species are cholinergic is strong but indirect. In this paper we report results of attempts to demonstrate choline acetyltransferase immunoreactivity in ganglion cells of goldfish retina using two different antisera against choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme. We obtained ChAT-immunoreactive staining of amacrine and displaced amacrine cells in the retina and type XIV cells in the tectum, but we obtained no direct immunocytochemical evidence that ganglion cells in the goldfish retina are cholinergic. Thus, ganglion cells identified by retrograde transport of propidium iodide were never ChAT-immunoreactive. Intraocular injections of colchicine did not result in the appearance of a population of ChAT-immunoreactive neurons in the ganglion cell layer. ChAT-immunoreactive axons were not observed in intact, ligated, or transected optic nerves. And finally, the ChAT immunoreactivity of cells and fibers in the optic tectum was unaffected by deafferentation. These experiments provide no positive evidence that any ganglion cells in goldfish retina contain the acetylcholine-synthesizing enzyme, ChAT. While it is possible that our method is too insensitive to detect the enzyme in ganglion cell somata or too specific to recognize the form of ChAT present in these cells, the fact that we can stain putatively cholinergic retinal amacrine cells and tectal neurons makes these alternative explanations improbable. We conclude that it is unlikely that any of the ganglion cells in the retina are cholinergic and that alternative explanations should be sought for previously published results that suggest that they are.     

Observations on the afferent and efferent connections of the avian isthmo-optic nucleus

The efferent and afferent connections of the avian isthmo-optic nucleus (ION) were studied using light microscopic techniques. Injections of [³H]proline into the nucleus resulted in labeling of centrifugal endings in the retina at the junction of the inner plexiform layer and inner nuclear layer, but produced no other transported label to any thalamic or mesencephalic nucleus. The origin of the tectal afferents to the ION was demonstrated by means of injections of [³H]proline into the most superficial layers of the optic tectum and by stereotaxic injections of horseradish peroxidase into the ION. The tectal efferent cell bodies were located in lamina h of the optic tectum and at the junction of laminae h and i.     

The development of NADPH-diaphorase and nitric oxide synthase in the visual system of the cichlid fish, Tilapia mariae

The pattern of NADPH-diaphorase expression was studied in the retina and optic tectum of the cichlid fish Tilapia mariae during the first developmental stages. NADPH-diaphorase activity was seen early, at hatching. In the retina a few cell bodies of the retinal inner nuclear layer showed a faint labeling. Scattered labeled cells were found in the stratum periventriculare of the optic tectum, while the optic nerve was unlabeled. Two days after hatching, the number of labeled neurons increased in the inner nuclear layer and a few stained cell bodies were also scattered in the ganglion cell layer. Both the inner and outer plexiform layers showed a diffuse staining and the optic nerve was devoid of labeling. In the optic tectum several positive cells in the periventricular layer, with their dendritic trees extending in the superficial fibrous layer, were found. In 1-month-old Tilapia, NADPH-diaphorase staining and nitric oxide synthase immunoreactivity were found to overlap in both the retina and optic tectum. The density of NADPH-diaphorase labeled neurons in the inner nuclear layer of the retina and in the stratum periventriculare of the optic tectum was largely reduced in comparison with 2 days posthatching embryos. These findings indicated an early and transient production of nitric oxide in the retina and optic tectum of Tilapia, suggesting a functional role for nitric oxide in the development of visual structures in aquatic vertebrates.     

Multiple nicotinic acetylcholine receptor genes are expressed in goldfish retina and tectum

cDNAs encoding a novel nAChR structural subunit (GFn alpha-3) and a ligand-binding subunit (GF alpha-3) have been isolated from a goldfish retina cDNA library. The protein encoded by GFn alpha-3 shares 88% amino acid similarity with that encoded by GFn alpha-2, a structural subunit gene previously identified to be expressed in this system (Cauley et al., 1989). The ligand-binding subunit (GF alpha-3) is likely the goldfish homolog of the rat alpha-3 gene (Boulter et al., 1986). Northern blots and S1 protection experiments show that GFn alpha-3 and GF alpha-3 genes are expressed in retina and brain. GFn alpha-3 identifies multiple RNAs differing in their 3' untranslated regions. In situ hybridization analysis demonstrates GFn alpha-3, GFn alpha-2, and GF alpha-3 expression by cells of the retinal ganglion cell layer. Unlike GFn alpha-2 and GF alpha-3, GFn alpha-3 is expressed at highest levels by cells of the retina's inner nuclear layer. In the optic tectum, both GF alpha-3 and GFn alpha-3 genes are expressed by cells of the periventricular zone, as well as more superficial layers. These results suggest the presence of multiple nAChR systems in retina and tectum. In addition, they indicate that tectal nAChRs may arise from remote (ganglion cell) as well as local (tectal cell) synthesis.     

Glutamate-immunoreactivity in the retina and optic tectum of goldfish

Glutamate was immunohistochemically localized in the goldfish retina and tectum at the light and electron microscopic (E.M.) levels using double affinity purified antisera against glutaraldehyde conjugated L-glutamate. In retina, glutamate-immunoreactivity (Glu+) was observed in cone inner segments, cone pedicles, bipolar cells, a small number of amacrine cells and the majority of cells in the ganglion cell layer. The latter were shown to be ganglion cells by simultaneous retrograde labeling. Centrally, Glu+ was observed in axons in the optic nerve and tract, and in stratum opticum and stratum fibrosum et griseum superficialis (SFGS) of the tectum. The Glu+ in the optic pathway disappeared four days after optic denervation and was restored by regeneration without affecting the Glu+ of intrinsic tectal neurons. In tectum, Glu+ was also observed in torus longitudinalis granule cells, toral terminals in stratum marginale, some pyramidal neurons in the SFGS, multipolar and fusiform neurons in stratum griseum centrale, large multipolar and pyriform projection neurons in stratum album centrale, and many periventricular neurons. Glu+ was also localized within unidentified puncta throughout the tectum and within radially oriented dendrites of periventricular neurons. At the E.M. level, a variety of Glu+ terminals were observed. Glu+ toral terminals formed axospinous synapses with dendritic spines of pyramidal neurons. Ultrastructurally identifiable Glu+ putative optic terminals formed synapses with either Glu+ or Glu- dendritic profiles, and with Glu- vesicle-containing profiles, presumed to be GABAergic. These findings are consistent with the hypothesis that a number of intrinsic and projection neurons in the goldfish retinotectal system, including most ganglion cells, may use glutamate as a neurotransmitter.     

Centrifugal innervation of goldfish retina from ganglion cells of the nervus terminalis

Cobaltous-lysine was applied to one optic nerve of normal goldfish in order to determine the source of the centrifugal innervation of the retina. Cobalt-filled cells were not observed in the optic tectum, pretectum, thal-amus, or hypothalamus. However, filled cells were observed outside the central nervous system either interspersed between the olfactory nerve fibers orrostrally along the ventromedial aspect of the olfactory bulbs. These cells appear to correspond to the ganglion cells of the nervus terminalis. The cells were located bilaterally and had dendrites that branched in close proximity to the cell body and axons that coursed caudally through the medial olfactory tract. The axons traveled in the ventral forebrain and entered the optic tracts. The axons also gave off fine branches that appeared to terminate in the vicinity of the anterior commissure and in the preoptic region. Application of cobaltous-lysine to a cut olfactory tract resulted in cobalt-filled fibers in the optic tracts, retinal optic fiber layer, and retinal ganglion cell layer. However, the precise terminations of these fibers within the retina could not be readily established. The results are discussed with respect to the plethora of sources of retinopetal cells observed by others in fish and with respect to the innervation of the retina by luteinizing hormone-releasing hormone axons.     

Localization and Synaptic Release of N-acetylaspartylglutamate in the Chick Retina and Optic Tectum

The neuropeptide, N-acetylaspartylglutamate (NAAG), was identified in the chick retina (1.4 nmol/retina) by HPLC, radioimmunoassay and immunohistochemistry. This acidic dipeptide was found within retinal ganglion cell bodies and their neurites in the optic fibre layer of the retina. Substantial, but less intense, immunoreactivity was detected in many amacrine-like cells in the inner nuclear layer and in multiple bands within the inner plexiform layer. In addition, NAAG immunoreactivity was observed in the optic fibre layer and in the neuropil of the superficial layers of the optic tectum, as well as in many cell bodies in the tectum. Using a newly developed, specific and highly sensitive (3 fmol/50 microl) radioimmunoassay for NAAG, peptide release was detected in isolated retinas upon depolarization with 55 mM extracellular potassium. This assay also permitted detection of peptide release from the optic tectum following stimulation of action potentials in retinal ganglion cell axons of the optic tract. Both of these release processes required the presence of extracellular calcium. Electrically stimulated release from the tectum was reversibly blocked by extracellular cadmium. These findings suggest that NAAG serves an extracellular function following depolarization-induced release from retinal amacrine neurons and from ganglion cell axon endings in the chick optic tectum. These data support the hypothesis that NAAG functions in synaptic communication between neurons in the visual system.     

Retrograde transport of goldfish optic nerve proteins labeled by N-succinimidyl [H]propionate

Following injection of the acylating reagent N-succinimidyl [³H]propionate into the optic nerve of goldfish, labeled protein appeared in the ipsilateral retina and contralateral tectum in a time-dependent manner. Autoradiography indicated the presence of the labeled material in the neuroplasm of the retinal ganglion cells and their projections. While most of the recoverable injected radioactivity was confined to the injection site even after 1 week, labeled proteins arriving in the retina by retrograde flow or in the tectum by anterograde flow had distinctly different patterns, a result suggesting specific transport processes rather than diffusion. In contrast to reported studies with the labeling agent in other species, a prominent 68,000 molecular weight component was not seen. The results are dicussed in relation to the role of retrograde transport in regeneration.     

Distribution of substance P-like immunoreactive retinal ganglion cells and their pattern of termination in the optic tectum of chick (Gallus gallus)

Substance P-like immunoreactive (SP-LI) neurons were identified within the inner nuclear layer and ganglion cell layer of the chick retina. The SP-LI cells in the inner nuclear layer consisted of several subtypes of neurons, differing in soma size and dendritic arborization. In the ganglion cell layer a population of moderately labelled SP-LI neurons was also present. About 6-9 microns in diameter and spaced 50-80 microns apart, they formed a regular array across the entire retina, with a density of about 400 cells/mm2 in the superior temporal retina, declining to less than 100 cells/mm2 in the peripheral retina. The total number of SP-LI cells in the ganglion cell layer was approximately 75,000. Individual axons could be followed toward the optic nerve head. Lesions near the optic nerve head resulted in axotomy of ganglion cells within a limited portion of the retina. Two days of postaxotomy there were numerous SP-LI swellings in the proximal segments of axotomized axons. SP-LI neurons in the axotomized zone were larger, more numerous, and showed increased staining of their processes. Fourteen days following a retinal lesion, there was depletion of all SP-LI cells in the ganglion cell layer within the axotomized zone, but the SP-LI neurons in the inner nuclear layer were not noticeably affected. Following a localized injection of rhodamine-coupled latex beads into the optic tectum, a population of retinal ganglion cells (RGCs) in the contralateral retina was retrogradely labelled. Many of these cells also exhibited SP-like immunoreactivity. Examination of the optic tectum indicated the presence of SP-LI fibres in laminae 2-13 (nomenclature of Cajal: Histologie du Systeme Nerveux. Vol. 2. Paris: Maloine, '11), with immunoreactive terminal regions present mainly in laminae 2-4, 7, and 9-13. SP-LI cell bodies were found predominantly in laminae 10-12 and 13. Fourteen days following a retinal lesion, SP-LI processes and terminals were depleted from laminae 2 and 3. Immunoreactive cells and processes in the remaining laminae of the optic tectum were not noticeably altered. The present report confirms the existence of SP-LI retinal ganglion cells in the chick retina and demonstrates their contribution to lamina specific SP-LI arborization in the optic tectum.     

Retinal ganglion cells in two teleost species,Sebastiscus marmoratus and Navodon modestus

Distribution patterns of ganglion cells in the retina were examined in Nissl-stained retinal whole mounts of Sebastiscus and Navodon. The existence of area centralis in the temporal retina in both species suggests binocular vision. In Navodon, another high density area was found in the nasal retina, and a dense band of ganglion cells was observed along the horizontal axis between the two high-density areas. There is an obvious trend for the ganglion cell size to increase as the density decreases. The total number of ganglion cells was estimated to be about 45 × 10⁴ in Sebastiscus and 87 × 10⁴ in Navodon, whereas the total number of optic nerve fibers was about 35 × 10⁴ and 70 × 10⁴, respectively. The retinal ganglion cells labeled with HRP were classified into six types according to such morphological characteristics as size, shape, and location of the soma as well as dendritic arborization pattern. Type I cells have a small round or oval soma in the ganglion cell layer and a small dendritic field in the inner plexiform layer. Type II cells are similar to type I cells, but the dendrites arborize more closely to the ganglion cell layer in the innermost region of the inner plexiform layer. Type III cells have a medium-sized round soma in the ganglion cell layer, and the dendrites extend in an extremely wide area in the inner plexiform layer with few branches. Type IV cells have a large soma which is located in the ganglion cell layer. Dendrites emanate from the soma in all directions, branching out several times within a rather small region in the innermost part of the inner plexiform layer. Type V cells have large somata of various shapes, usually dislocated to the inner plexiform or granular layer. The dendrites extend in every direction and occupy an extremely large area in the inner plexiform layer. Type VI cells have the largest somata, which are also dislocated to the inner plexiform or granular layer. Type VI cells have a characteristic triangular or fan-shaped dendritic field. Soma size and the axon diameter are intimately linked, that is, small somata of type I and II cells give off thin axons, and large somata of type V and VI give off thick axons. Medium-sized somata of type III cells or large somata of type IV cells, which have rather small dendritic fields, give off medium-sized axons. The histograms of the soma areas in the whole retina are quite similar to the histograms of the diameters of the optic nerve fibers.     

GABA-like immunoreactive neurons in the retina of Bufo marinus: evidence for the presence of GABA-containing ganglion cells.

gamma-Aminobutyric acid (GABA)-like immunoreactive (IR) neurons in the retina of the cane toad Bufo marinus were revealed using immunohistochemistry on retinal wholemount preparation and sectioned material. GABA-IR neurons included horizontal, bipolar and amacrine cells in the inner nuclear layer and small to medium sized cells in the ganglion cell layer. A few IR axons were seen in the optic fiber layer of the retina. Following the injection of the carbocyanine dye, DiI into the optic tectum ganglion cells were retrogradely filled. A small population of DiI-filled ganglion cells (2.8%) was found to be GABA-IR. GABA-IR neurons in the ganglion cell layer without DiI label were considered to be displaced amacrine cells of which 45.3% were GABA positive. It is proposed that GABA-containing ganglion cells may form an inhibitory projection to visual centers of the anuran brain.     

A monoclonal antibody specific for the visual system of the goldfish

Although several monoclonal antibodies (MAbs) have been reported that recognize antigens present in ganglion cells of the mammalian retina, these MAbs do not cross-react in the goldfish. In the current study we have identified a MAb that recognized a 14.4 kDa antigen that is present on the ganglion cells in the retinae of goldfish but is absent from the retinae of all other species that were tested. No other cell type in the goldfish retina or optic nerve was labeled with this MAb. Furthermore, the axons in the optic nerve, optic tract and the terminal layers in the optic tectum were labeled.     

Dorsotemporal retinal ganglion cell axons of goldfish are located in the dorsal rather than ventral optic tract

Cobaltous-lysine was injected into the eyes of goldfish after a slit was made in the temporal retina. Cobalt-filled optic fibers were found in the dorsal optic tract and tracing them to their destinations revealed that they terminated rostrally in the peripheral edges of both the dorsal and ventral aspects of the optic tectum. Hence, axons from ganglion cells in the dorsotemporal retina are in the dorsal optic brachium rather than in the ventral optic brachium as was previously assumed.     

Regenerating optic fibers correct large-scale errors by random growth: evidence from in vivo imaging

Regenerating optic fibers in goldfish make large-scale errors when they invade tectum and subsequently correct these to generate a projection with moderate retinotopic order by 1 month. The behavior of fibers underlying these extensive rearrangements is not well understood. To clarify this, we have imaged optic fibers in living adult goldfish at 2-4 weeks of regeneration. A small number of neighboring retinal ganglion cells were labeled with microinjections of DiI and imaged in the dorsal tectum with a cooled CCD camera on a fluorescence microscope for 5 to 8 hours. Nearly all fibers were simple unbranched processes and had endings that were highly dynamic showing both growth and retraction. Fibers from dorsal retina that normally innervate ventral tectum were frequently observed in dorsal tectum. These ectopic fibers oscillated more frequently between growth and retraction and retracted more often than ventral optic fibers. Like retinotopic fibers, ectopic fibers exhibited net growth but they showed no apparent directional preference toward their retinotopic position. In contrast, large errors along the anterior-posterior axis corresponding to nasal-temporal retina were rare and there was no differential behavior that distinguished these fibers.     

The effect of light exposure following an intraocular injection of [H]N-acetylmannosamine on the labeling on gangliosides and glycoproteins of retina ganglion cells and optic tectum of singly caged chickens

Ten-day-old chickens that after a 2-day-period of adaptation to dark received an intraocular injection of [³H]N-acetylmannosamine ([³H]ManNAc) and were exposed, individually housed, to light, have more labeling in the gangliosides and glycoproteins of the ganglion cell layer of retina and in the contralateral optic tectum compared to their counterparts that remained in darkness. No differences were found in the labeling of the acid soluble fraction of the ganglion cell layer between the animals in dark and light at 0.5 and 5 h after the injection of [³H]ManNAc.No differences could be observed in the quality of storage of the gangliosides labeled in light with respect to those labeled in dark, but those labeled in light had a higher percent of labeling released by neuraminidase at 5 h after the intraocular injection of the labeled precursor.In animals exposed to intermittent light, the increased labeling with respect to dark was smaller than that found in animals exposed continously to light.     

The response of optic tract glia during regeneration of the goldfish visual system. II. Tectal factors stimulate optic tract glia

After transection, retinal ganglion cell axons of the goldfish will regenerate by growing into a primary target tissue, the optic tectum. To determine what role the target tissue may play in regulating glial cell growth, we measured biosynthetic activity of optic tract glia following excision of the optic tectum and compared it to activity of glia found in the regenerating visual system. Ablation of the tectum reduced glial incorporation of both [³H]thymidine and [³⁵S]methionine. Tectal ablation also led to nearly 80% reduction of amino acids incorporated by oligodendroglia as well as a decrease in the amount of newly synthetized protein found within multipotential glia and within cytoplasmic projections of astroglia. Since the tectal influence upon optic tract glia was detected at a time when tract and tectum are physically separated, we sought to determine if the optic tectum contained soluble glia-promoting factors. A soluble fraction recovered from tecta of the regenerating visual system increased amino acid incorporation within optic tract glia at 2–3-fold above preparations incubated with fractions from control, intact tecta. Comparisons of radiolabeled proteins separated by sodium dodecyl polyacrylamide gel electrophoresis from regenerating and factor-stimulated optic tract were similar and indicated that a soluble tectal fraction promoted biosynthesis of specific glial proteins. Our findings suggest that during regeneration of the goldfish visual system glia are influenced by humoral factor(s) released from the synaptic target site.     

Retinal origin of the fasciculus medialis tractus optici in adult goldfish

In teleosts, the fasciculus medialis tractus optici departs the optic tract and follows an aberrant path towards the midbrain tectum. To determine the retinal origin of the fascicle, horseradish peroxidase was injected into localized lesions of either the optic nerve or the retina. The fasciculus medialis axons of adult goldfish derive selectively from ganglion cells residing in the dorsotemporal portion of the central retina. The fasciculus medialis appears to be an integral part of the chronotopic ordering of fibers in the optic tract.     

Growth of the adult goldfish eye. II. Increase in retinal cell number

The retinas of adult goldfish, one to four years of age, 4–23 cm in length, were examined with standard paraffin histology to determine if new cells were being added with growth. Retinal cell nuclei were counted and the area of the retina was measured. An analysis of cell densities in various regions throughout the retina showed that the cells are distributed nearly homogeneously. The density (No./mm² of retinal surface) of ganglion cells, inner nuclear layer cells and cones decreases with growth, but the density of rods remains constant. Thus the rods account for a larger proportion of the cells in larger retinas. The total number of cells per retina increases: the ganglion cells from 60,000 to 350,000; the inner nuclear layer cells from 1,500,000 to 4,000,000; the cones from 250,000 to 1,400,000; the rods from 1,500,000 to 15,000,000. This increase in the number of retinal neurons implies the formation of even more new synapses, and suggests the adult goldfish retina as a model for both neuro- and synaptogenesis.