Quantitative studies of retinal ganglion cells in a turtle, Pseudemys scripta elegans: II. Size spectrum of ganglion cells and its regional variation

Regional specialisation of the ganglion cell density in the retina of the native duck (Anas platyrhynchos) of Bangladesh

In this study, retinal whole-mount specimens were prepared and stained with 0.1% cresyl violet for the ganglion cell study in the native duck (Anas platyrhynchos). The total number, distribution and size of these cells were determined in different retinal regions. The mean total number of ganglion cells was 1 598 501. The retinal area centralis had the highest ganglion cell density with 11 200 cells/mm(2) . Number of ganglion cell bodies was the highest in temporal area, followed by dorsal, nasal and ventral areas. Ganglion cell size ranged from 5.25 to 80 μm(2) . In the temporal and nasal region, most of the cells were ranged from 15 to 25 μm(2) , and in the dorsal and ventral region, most of the cells were ranged from 12 to 25 μm(2) . There was a marked trend for the retinal ganglion cell size to increase as the population density decrease towards the periphery. A population of small ganglion cells persisted into the central area just above the optic disc and the largest soma area was in the ventral zone of the retina. Thus, the specialisation of ganglion cell densities and their sizes support the notion that the conduction of visual information towards the brain from all regions of the retina is not uniform, and the central area is the fine quality area for vision in native duck.

Number, distribution and size of retinal ganglion cells in the jungle crow (Corvus macrorhynchos)

A retinal ganglion cell density map was generated using Nissl-stained retinal whole mounts from the jungle crow (Corvus macrorhynchos). The total number, distribution and size of these cells were determined in the area centralis, as well as in temporal, nasal, dorsal and ventral retinal regions. The mean total number of ganglion cells was estimated to be 3.6 x 10(6). The highest densities were found in the area centralis (25 600 /mm2) and the dorso-temporal part of the retina, suggesting the highest quality of vision. This density diminished nearly concentrically from the central area towards the retinal periphery. The number of ganglion cells was highest in the temporal retina followed, in order, by the nasal, dorsal and ventral retinal regions. Based on ganglion cell size, the retina seemed to consist of the following five regions: central, temporal, nasal, dorsal and ventral. Ganglion cell size ranged from 16 to 288 microm2, with smaller cells predominating in central regions above the optic disc and larger cells comprising more of the peripheral regions. The present study showed two highly populated areas of ganglion cells in the crow retina and it is expected that the crow retina provides well-developed monocular and binocular vision.

Receptive field structure-function correlates in developing turtle retinal ganglion cells

Mature retinal ganglion cells (RGCs) have distinct morphologies that often reflect specialized functional properties such as On and Off responses. But the structural correlates of many complex receptive field (RF) properties (e.g. responses to motion) remain to be deciphered. In this study, we have investigated whether motion anisotropies (non-homogeneities) characteristic of embryonic turtle RGCs arise from immature dendritic arborization in these cells. To test this hypothesis, we have looked at structure-function correlates of developing turtle RGCs from Stage 23 (S23) when light responses emerge, until 15 weeks post-hatching (PH). Using whole cell patch clamp recordings, RGCs were labelled with Lucifer Yellow (LY) while recording their responses to moving edges of light. Comparison of RF and dendritic arbor layouts revealed a weak correlation. To obtain a larger structural sample of developing RGCs, we have looked at dendritic morphology in RGCs retrogradely filled with the tracer horseradish peroxidase (HRP) from S22 (when RGCs become spontaneously active, shortly before they become sensitive to light) until two weeks PH. We found that there was intense dendritic growth from S22 onwards, reaching peak proliferation at S25 (a week before hatching), while RGCs are still exhibiting significant motion anisotropies. Based on these observations, we suggest that immature anisotropic RGC RFs must originate from sparse synaptic inputs onto RGCs rather than from the immaturity of their growing dendritic trees.

Developmental changes in the fibre population of the optic nerve follow an avian/mammalian-like pattern in the turtle Mauremys leprosa

The changes in the axon and growth cone numbers in the optic nerve of the freshwater turtle Mauremys leprosa were studied by electron microscopy from the embryonic day 14 (E14) to E80, when the animals normally hatch, and from the first postnatal day (P0) to adulthood (5 years on). At E16, the first axons appeared in the optic nerve and were added slowly until E21. From E21, the fibre number increased rapidly, peaking at E34 (570,000 fibres). Thereafter, the axon number decreased sharply, and from E47 declined steadily until reaching the mature number (about 330,000). These observations indicated that during development of the retina there was an overproduction and later elimination of retinal ganglion cells. Growth cones were first observed in the optic nerve at as early as E16. Their number increased rapidly until E21 and continued to be high through E23 and E26. After E26, the number declined steeply and by E40 the optic nerve was devoid of growth cones. These results indicated that differentiation of the retinal ganglion cells occurred during the first half of the embryonic life. To examine the correlation between the loss of the fibres from the optic nerve and loss of the parent retinal ganglion cells, retinal sections were processed with the TUNEL technique. Apoptotic nuclei were detected in the ganglion cell layer throughout the period of loss of the optic fibres. Our results showed that the time course of the numbers of the fibres in the developing turtle optic nerve was similar to those found in birds and mammals.

Ganglion cell types of the turtle retina that project to the optic tectum: Intracellular HRP injections of retrogradely, rhodamine-marked cell bodies

The turtle retina has been shown to have a variety of different morphological ganglion cell types as well as distinct physiological ganglion cell types. The major projection of the retina to the brain in nonmammalian vertebrates is to the optic tectum. In this study, we address the question of which retinal ganglion cell types project to the optic tectum in the turtle. Fluorescent rhodamine-labeled microspheres were used to trace the retinal ganglion cell projection to the superficial layers of the optic tectum. The fluorescent ganglion cell somata, retrogradely marked by transport from the contralateral optic tectum, were impaled with micropipettes containing rhodamine-horseradish peroxidase solution and this dye was iontophoresed into the cells under visual control. Most of the morphological ganglion cell types described in Golgi studies (Kolb, 1982; Kolb et al., 1988) were stained. Thus, the small cell types G1, G2, G3, G5, G6, and G7; the medium-sized types G10, G11, G12, G13, and G14; and the large-sized types G15, G16, G19, G20, and G21 project to the optic tectum in the turtle. We have added a new type, G2a, which proves to have some differences from the original G2 in branching pattern. We were unable to stain the small type G4, the medium-sized types G8 and G9, and the large cell types G17 and G18: this suggests that they might not project to the superficial layers of the dorsolateral optic tectum, at least, in the turtle.

Topography and morphology of retinal ganglion cells in Falconiforms: a study on predatory and carrion-eating birds

The topographic distribution of retinal ganglion cells and their cell body size have been studied in five Falconiform species, including predatory (chilean eagle Buteo fuscenses australis, and sparrow hawk Falco sparverius) and carrion-eating (chimango caracara Milvago chimango; condor Vultur gryphus, and black vulture Coragyps atratus) birds. All these species had a well defined nasal fovea and a horizontal streak. Instead of a temporal fovea as in eagles and hawks, an afoveate temporal area is present in chimango, condor, and vulture. The highest ganglion cell density was found in the nasal fovea of Falco and Buteo with 65,000 and 62,000 cells/mm2, respectively. A negative correlation between ganglion cell density and cell body size was found in all the species studied. The specializations of the temporal retina showed a rather homogenous population of medium sized neurons, while the nasal foveas showed a homogeneous population of smaller ganglion cells. Finally, the peripheral retina showed a heterogeneous population of large, medium, and small ganglion cells. Predatory behavior appears to be closely related to foveal specializations, and is best exemplified in the eagle and hawk and to a lesser extent in the chimango.

Psychophysically derived visual mechanisms in turtle. II--Spatial properties

Visual mechanisms isolated in Pseudemys by the two-color threshold technique of Stiles show peak wavelength sensitivities at 650 nm (red light) and 540 nm (green light). Ricco critical areas were measured for the two test wavelengths under three conditions: dark, moderate and intense backgrounds. As expected, critical spatial areas decreased with light adaptation. Under dark adaptation only rods and red-sensitive cones were operative, and one photon per 12 rods was sufficient for green-light threshold, as was one photon per four red-sensitive cones for red-light threshold. Rods apparently pool their information among the several receptors within the threshold area. Under light adaptation, rods were not functional and thresholds were determined by red-sensitive and green-sensitive cones alone. Cones did not share information over many receptors, requiring close to one photon per receptor to function at threshold.

Organization of retinogeniculate projections in turtles of the genera Pseudemys and Chrysemys

Organization of retinal projections to the dorsal lateral geniculate complex in turtles has been studied by means of light and electron microscopic axon tracing techniques. Orthograde degeneration studies with Fink-Heimer methods following restricted retinal lesions show the entire retina has a topologically organized projection to the contralateral dorsal lateral geniculate complex. The nasotemporal axis of the retina projects along the rostrocaudal axis of the geniculate complex; the dorsoventral axis of the retina projects along the dorsoventral axis of the geniculate complex. The projection to the ipsilateral dorsal lateral geniculate complex originates from the ventral, temporal and nasal edges of the retina. The nasotemporal axis of the ipsilateral retina projects along the rostrocaudal axis of the geniculate complex. It was not possible to determine the orientation of the dorsoventral axis of the ipsilateral retina on the geniculate complex. Light microscopic autoradiographic tracing experiments and electron microscopic degeneration experiments show the retinogeniculate projection has a laminar organization. Retinogeniculate terminals are found in both the neuropile and cell plate throughout all three subnuclei of the dorsal lateral geniculate complex but have a distinctive distribution in each subnucleus. In the subnucleus ovalis, they are frequent in both the neuropile and cell plate which forms the rostral pole of the complex. In the dorsal subnucleus, they are most prevalent in the outer part of the neuropile layer, less frequent in the inner part of the neuropile, and rare in the cell plate. In the ventral subnucleus, they are frequent in the outer part of the neuropile but are also common in the inner part of the neuropile and cell plate. These observations point to several principles of geniculate organization in turtles. First, the complex receives projections from the entire contralateral retina and a segment of the ipsilateral retina. It thus has monocular and binocular segments that together receive a topologically organized representation of the binocular visual space and the contralateral monocular visual space. Second, the three geniculate subnuclei receive information from different, specialized regions of the retina and visual space. Subnucleus ovalis receives information from the frontal binocular visual field. The ventral subnucleus receives information from the caudal binocular field. The dorsal subnucleus receives input from the contralateral monocular field. Third, there is a lamination of retinal inputs in the geniculate complex which differs in character within the three subnuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptide Y-immunoreactive amacrine cells in the retina of the turtle Pseudemys scripta elegans

Antiserum directed against neuropeptide Y selectively labeled certain amacrine cells in the turtle retina. The cell types, sizes, dendritic stratification, regional distribution, and degrees of immunolabeling were examined. The results indicated that three morphologically distinct cell types were labeled: types A, B, and C. Computer rotation of digitized data from camera lucida drawings was used to study dendritic stratification. The type A somata were large (11.5 micron in diameter), well-stained, and located in the third tier of the inner nuclear layer. Type A somata gave rise to well-stained processes which arborized within the inner plexiform layer in strata 1 and 3 and at the border between strata 4 and 5. Processes in stratum 1 were sparse and delicate with small boutons. Processes in stratum 3 were numerous and often coarse, with many large and small boutons. At the border between strata 4 and 5 the processes were frequently numerous but slender, with many small boutons. Occasional immunolabeled processes were found in the ganglion cell layer. The somata of the type B cells were smaller (9.0 micron in diameter) and gave rise to single labeled processes which descended into the inner plexiform layer and divided quickly into two secondary processes. These secondary processes gave rise to lightly labeled dendritic fields which arborized primarily in strata 2 and 4. The type C cells were usually observed at the periphery of the retina and had large somata (12.0 micron in diameter) with simple, but very elongated, dendritic arborizations in strata 1, 4, and 5. Observations also showed that type A and B cells were often found in close proximity to each other and suggested that dendrites of these cells made contact with each other. The labeled neurons were distributed relatively evenly throughout the retina except for the visual streak where they were sparse. This study indicates that neuropeptide Y-like immunoreactivity is found in more than one anatomically distinct class of amacrine cells in the turtle retina.

Retinal wholemounts: a simple method for precise mapping

A simple and inexpensive method to analyse retinal wholemounts is described. It allows the definition of regularly spaced sites to map regional specializations of the retina under light microscopy. After attaching a piece of translucent millimetre graph paper behind the slide holding the preparation, the two grid patterns of the graph paper and of the eye-piece grid are used in combination for the definition of sites for analysis and the return to specific areas as small as 50-100 microns2. The present method is simpler and offers several advantages when compared to others based on X-Y movements of the microscope stage or on photographic montage.

Distribution and morphology of retinal ganglion cells in the Japanese quail

A ganglion cell density map was produced from the Nissl-stained retinal whole mount of the Japanese quail. Ganglion cell density diminished nearly concentrically from the central area toward the retinal periphery. The mean soma area of ganglion cells in isodensity zones increased as the cell density decreased. The histograms of soma areas in each zone indicated that a population of small-sized ganglion cells persists into the peripheral retina. The total number of ganglion cells was estimated at about 2.0 million. Electron microscopic examination of the optic nerve revealed thin unmyelinated axons to comprise 69% of the total fiber count (about 2.0 million). Since there was no discrepancy between both the total numbers of neurons in the ganglion cell layer and optic nerve fibers, it is inferred that displaced amacrine cells are few, if any. The spectrum in optic nerve fiber diameter showed a unimodal skewed distribution quite similar to the histogram of soma areas of ganglion cells in the whole retina. This suggests a close correlation between soma areas and axon diameters. Retinal ganglion cells filled from the optic nerve with horseradish peroxidase were classified into 7 types according to such morphological characteristics as size, shape and location of the soma, as well as dendritic arborization pattern. Taking into account areal ranges of somata of each cell type, it can be assumed that most of the ganglion cells in the whole retinal ganglion cell layer are composed of type I, II and III cells, and that the population of uniformly small-sized ganglion cells corresponds to type I cells and is an origin of unmyelinated axons in the optic nerve.

Optic tectum of the eastern garter snake, Thamnophis sirtalis. IV. Morphology of afferents from the retina

The morphology of single retinal terminals in the optic tectum of the eastern garter snake was demonstrated by orthograde filling from extracellular injections of horseradish peroxidase (HRP) into the optic tract. HRP-filled terminals share a characteristic shape and structure. Their parent axons course caudally in the stratum opticum within fascicles of 200-300 fibers of varying diameters. Single axons exit a fascicle and course into either the stratum fibrosum et griseum superficiale, ventrally, or the stratum zonale, dorsally, where they bifurcate successively two or three times into preterminal branches. Each preterminal branch gives rise to many thin, terminal branchlets laden with boutons. The arbors are ellipsoidal with their long axes oriented mediolaterally and their short axes oriented rostrocaudally. Arbors vary in their overall size (from 45 to 150 micron), in the diameters of their parent axons (from less than 0.5 to 3.0 micron), and in the size of their terminal boutons (from 0.5 to 3.5 micron). Bouton size increased with increasing diameter of the parent axon. The great majority of arbors are confined to one of three retinorecipient sublayers in the superficial tectum. However, the full range of arbor sizes and axon diameters is present in each sublayer.

Organization of motor pools supplying the cervical musculature in a cryptodyran turtle, Pseudemys scripta elegans. I. Dorsal and ventral motor nuclei of the cervical spinal cord and muscles supplied by a single motor nucleus

In this and the accompanying paper (Yeow and Peterson, '86) we characterize motor nuclei of the cervical spinal cord in Pseudemys scripta and the motor pools of eight cervical muscles. We have identified three motor nuclei that supply the cervical musculature by using serial reconstructions of Nissl-stained spinal cords cut in three cardinal planes, and in experimental cases in which horseradish peroxidase (HRP) was applied to individual neck muscles. These nuclei are named according to their position as visualized in the transverse plane: dorsal, ventral, and medial. A fourth (ventrolateral) motor nucleus was never labelled following application of HRP to the cervical musculature and presumably innervates the forelimbs. The dorsal motor nucleus occupies the mid-dorsal to dorsolateral ventral horn of C1 and C2. It is composed of at least two morphological groups of motor neurons; one of these is a population of very large, fusiform profiles with transversely oriented dendrites that is found primarily in C1. The ventral motor nucleus occupies the tip of the ventral horn from C1 to C8. Its cells are significantly smaller and more numerous in rostral than in caudal cervical segments. In Nissl material, ventral nuclear profiles show little tendency to cluster into subgroups, but experimental cases suggest that there is some spatial dissociation of different motor pools within the ventral nucleus. The medial motor nucleus is described in the accompanying paper together with the motor pools of three cervical muscles that it supplies. Having identified the cervical motor nuclei we then used retrograde transport of HRP to characterize the motor pools of individual cervical muscles. Two superficial ventral muscles (mm. coracohyoideus and plastro-squamosus) are supplied by dorsal nuclear cells. M. coracohyoideus motor neurons are significantly larger than those of m. plastrosquamosus and the very large, fusiform cell type is relatively more numerous in the m. coracohyoideus motor pool. Dorsal and lateral muscles (mm. cervicocapitis, testocapitis, and transversalis cervicis) are supplied by ventral nuclear motor neurons. These cells are smaller, on average, than motor neurons supplying ventral musculature. The m. cervicocapitis motor pool lies dorsomedially in the tip of the ventral horn of C1 and C2; motor neurons supplying the more laterally placed mm. testocapitis and transversalis cervicis occur more ventrolaterally, in C2-C3 and C3-C5, respectively. Thus each of the five cervical muscles is supplied by a single motor nucleus, and their motor pools are organized into a musculotopic pattern.

Conduction velocity, size and distribution of optic nerve axons in the turtle, Pseudemys scripta elegans

Electrophysiological and morphological techniques have been used to characterize optic nerve axons in the red-eared turtle. Three distinct groups of axons are identified on the basis of conduction velocity and axon diameter. The first group (T1) is a small population of axons with large diameters (2.8-4.5 microns) and mean conduction velocities of 13 m/sec. The second group (T2) is a large population of axons with medium diameters (0.4-2.8 microns) and mean conduction velocities of 3 m/sec. The third group (T3) is a medium sized population of small diameter (0.2-0.6 micron), mostly unmyelinated axons with mean conduction velocities of 1 m/sec. There is a significant regional variation in the size, density and myelination of axons in the optic nerve. Large axons are found dorsally and ventrally, while smaller axons and the majority of unmyelinated fibers are found along a dorsotemporal to ventronasal axis through the nerve. Fink-Heimer techniques were used to trace the trajectories of axons of different sizes from the retina to the brain. Large diameter axons can be traced along the dorsal and ventral portions of the optic tract, with a dorsal group leaving the tract in the pretectum and a ventral group entering the basal optic tract. These observations suggest that the distribution of axons within the optic nerve reflects in part the distribution of ganglion cell somata in the retina. However, there is also some segregation of axons of different sizes according to their various central targets.

Tectoreticular pathways in the turtle, Pseudemys scripta. II. Morphology of tectoreticular cells

The morphology of tectoreticular neurons in turtles was examined with serial section reconstructions of neurons retrogradely filled with HRP. Six classes of tectal neurons project into the three tectobulbar pathways characterized in the preceding paper (Sereno, '85). (1) Large multipolar neurons with somata in the central gray layers, and with moderately branched dendrites sometimes spanning over a millimeter, project into the dorsal tectobulbar pathway, TBd. Their dendrites are covered with fine spicules and tend to arborize in the lower third of the superficial gray layers. (2) Medium-sized neurons with multiple radial dendrites and somata in the central white and upper periventricular layers probably project into the ipsilateral intermediate tectobulbar pathway, TBi. Their dendrites also bear fine spicules and usually reach the tectal surface. (3) Small radial cells in the periventricular layers, and (4) small bitufted radial cells in the superficial gray project into the small caliber component of the ipsilateral ventral tectobulbar pathway, TBv(sm). (5) Medium-sized central gray neurons with stratified dendrites, and (6) medium-sized central gray neurons with horizontal dendrites probably project into the medium caliber component of the ventral tectobulbar pathway, TBv(med). In contrast to TBd and TBi neurons, these last four classes emit a spray of long, filamentous dendritic appendages in the central gray and have dendritic arbors near the top of the superficial gray. The morphology of the neurons described in this and the preceding paper is briefly discussed in light of current ideas about tectally mediated sensorimotor transformations.