Location of retinal ganglion cells contributing to the early imprecision in the retinotopic order of the developing projection to the superior colliculus of the wallaby (Macropus eugenii)

Regenerating optic axons restore topography after incomplete optic nerve injury

Following complete optic nerve injury in a lizard, Ctenophorus ornatus, retinal ganglion cell (RGC) axons regenerate but fail to restore retinotectal topography unless animals are trained on a visual task (Beazley et al. [ 1997] J Comp Neurol 370:105-120, [2003] J Neurotrauma 20:1263-1270). Here we show that incomplete injury, which leaves some RGC axons intact, restores normal topography. Strict RGC axon topography allowed us to preserve RGC axons on one side of the nerve (projecting to medial tectum) while lesioning those on the other side (projecting to lateral tectum). Topography and response properties for both RGC axon populations were assessed electrophysiologically. The majority of intact RGC axons retained appropriate topography in medial tectum and had normal, consistently brisk, reliable responses. Regenerate RGC axons fell into two classes: those that projected topographically to lateral tectum with responses that tended to habituate and those that lacked topography, responded weakly, and habituated rapidly. Axon tracing by localized retinal application of carbocyanine dyes supported the electrophysiological data. RGC soma counts were normal in both intact and axotomized RGC populations, contrasting with the 30% RGC loss after complete injury. Unlike incomplete optic nerve injury in mammals, where RGC axon regeneration fails and secondary cell death removes many intact RGC somata, lizards experience a "win-win" situation: intact RGC axons favorably influence the functional outcome for regenerating ones and RGCs do not succumb to either primary or secondary cell death.

Analysis of EphB receptors and their ligands in the developing retinocollicular system of the wallaby reveals dynamic patterns of expression in the retina

The expression of EphB1 and B2 receptors and ephrins-B1, -B2 and -B3 in the retina and superior colliculus of the wallaby (Macropus eugenii) was examined during the development of the retinocollicular projection, using reverse transcription-polymerase chain reaction and immunohistochemistry. There was an early transient differential expression of EphB2 that was higher in ventral retina and restricted to the outer neuroblast layer, whereas a high ventral to low dorsal gradient of ephrin-B2 expression occurred there throughout the study period. However, there was no dorsoventral gradient of receptors or ligands in retinal ganglion cells or a mediolateral gradient of ephrins in the colliculus. These findings suggest a limited role for these molecules in topographic mapping across the mediolateral colliculus in the wallaby. Early in retinal development there is a complementary pattern of expression of ephrin-B1 and -B2 in the outer neuroblast layer that overlaps with expression of EphB2. Ganglion and amacrine cells also express EphB2. As development proceeds subpopulations of putative horizontal and bipolar cells, also expressing EphB2, come to reside in the inner nuclear layer and ephrin-B1 is expressed throughout the outer nuclear layer. At the same time cells expressing ephrin-B2, and subpopulations of horizontal and bipolar cells come to reside in the inner nuclear layer and there is a corresponding decrease in ephrin-B2 expression in the outer nuclear layer. This pattern of coexpression of receptors and ligands suggests a role for them in cell migration and maintenance of laminar boundaries.

Graded expression of EphA3 in the retina and ephrin-A2 in the superior colliculus during initial development of coarse topography in the wallaby retinocollicular projection

We describe the expression of EphA3 and EphA7 receptors and ephrin-A2 ligand in the retina and the superior colliculus during the development of the retinocollicular projection in the marsupial wallaby (Macropus eugenii), using immunoblotting and immunohistochemistry. EphA3 in the retina was in a striking, low central to high peripheral gradient, superimposed on which was a high temporal to low nasal level of expression. This distribution was evident from postnatal day 30, when axons are growing into the colliculus and forming a coarsely organized topographic projection, to postnatal day 65, when axons have arborized in their correct retinotopic positions. EphA7 showed a shallow centroperipheral gradient with no nasotemporal differences in expression. In the superior colliculus no rostrocaudal differences in distribution were detected for either of these receptors. Ephrin-A2 was distributed in a gradient increasing from the rostral to the caudal pole in the superficial layers of the superior colliculus only up to postnatal day 30. Ephrin-A2 was evenly distributed in the retina throughout development of the projection. Expression of EphA3 in the retina increased, while the expression of ephrin-A2 in the colliculus was downregulated over time. The graded expression of EphA3 and ephrin-A2 early in the development of the projection suggests that they play a role in establishment of coarse topography of retinal axons along the rostrocaudal axis of the superior colliculus. However, the gradients were not complementary, meaning that EphA3 alone cannot mediate the repulsive interactions with ephrin-A2 that have been postulated to underlie formation of the topographic map.

Marsupial retinocollicular system shows differential expression of messenger RNA encoding EphA receptors and their ligands during development

The protracted development of the wallaby (Macropus eugenii) has allowed study of messenger RNAs encoding Eph receptors EphA3 and EphA7 and ligands ephrin-A2 and -A5 in the retina and superior colliculus at intervals throughout the development of the retinocollicular projection: from birth, before retinal innervation, to postnatal day 95, when the projection is mature. Reverse transcription-polymerase chain reaction showed messenger RNAs for both receptors and ligands were expressed at all ages. EphA7 was expressed more highly in the rostral superior colliculus. Ephrin-A2 and -A5 were expressed more highly in the caudal colliculus. EphA3 was expressed in a complementary manner, more highly in temporal than in nasal retina. There are higher levels of expression of the ligands when the projection is only coarsely topographically organised. This suggests a role for them and their receptor EphA3 in this stage, by repulsive interactions which restrict temporal axons to rostral superior colliculus. This is the first account in a marsupial mammal of the appearance of this molecular family, substantiating its ubiquitous role in topographically organised neuronal connections. Nevertheless, expression is not the same as in the mouse, suggesting differences in the details of topographic coding between species.

Axon order in the visual pathway of the quokka wallaby

Axon order throughout the visual pathway of the quokka wallaby (Setonix brachyurus) was determined after localised retinal applications of the tracers DiI and/or DiASP. Postnatal days (P) 22-90 were studied to encompass the development and refinement of retinal projections. Order was essentially similar at all stages. Axons entered the optic nerve head true to their sector of retinal origin. In the optic nerve, nasal and temporal axons continued to reflect their retinal origin, dominating, respectively, the medial and lateral halves. By contrast, dorsal and ventral axons exchanged locations between the retrobulbar level and one-third the distance along the nerve; thus, the inversion of the dorsoventral retinal axis, imposed by the lens, was corrected. Decussating axons maintained their relative locations through the chiasm. At the base of the optic tract, nasal and temporal axons underwent an axial rotation to lie on the medial and lateral sides, respectively; thus nasal overlapped with ventral axons and temporal with dorsal axons. Axons maintained their alignments throughout the tract, and as a result, nasal and ventral axons invaded the superior colliculus medially, whereas temporal and dorsal axons invaded laterally. Each retinal quadrant terminated preferentially in its retinotopically appropriate sector of the colliculus. The arrangement of axons in the quokka visual pathway displays several novel features. Axon order is distinct throughout, involving a well-demarcated exchange of dorsal and ventral axons in the nerve and an axial rotation of nasal and temporal axons at the base of the tract; these relocations suggest decision regions for growing axons. The organisation presumably underlies the less extensive searching within the developing superior colliculus to generate retinotopic maps in the quokka and also in tammar wallaby [Marotte, J. Comp Neurol. 293:524-539, 1990] than in the rat [Simon and O'Leary, J. Neurosci. 12:1212-1232, 1992].

Rhythmic motor activity and interlimb co-ordination in the developing pouch young of a wallaby (Macropus eugenii)

1. The forelimb motor behaviour of developing wallaby was studied. A clock-like alternating movement was reactivated whenever the animal was removed from the pouch. 2. Forelimb stepping frequency increased during the first 3 weeks of development, while the phase relationship remained constant. Forelimb activity could be affected by altering the afferent feedback from the contralateral limb, or an increase in ambient temperature. 3. In vitro experiments were performed using an isolated brainstem-spinal cord preparation from animals up to 6 weeks postnatal. Fictive locomotor activity could be evoked by electrical stimulation or bath-applied NMDA (< 10 microM). 4. Bath-applied strychnine (10-25 microM) and bicuculline (10-50 microM) disrupted the phase relationship between motor pools, while rhythmic motor discharge remained in the absence of these inhibitory pathways. 5. The present findings indicate that the pattern generator that underlies the robust forelimb movement during the first journey to the pouch is retained for different motor functions during in-pouch development. The neural network that underlies such behaviour can be divided into two major components, a rhythm generator within each hemicord, and a pattern co-ordinating pathway which involve both glycinergic and GABAergic interneurones.

The initial stages of development of the retinocollicular projection in the wallaby (Macropus eugenii): distribution of ganglion cells in the retina and their axons in the superior colliculus

The time course of ingrowth of retinal projections to the superior colliculus in the marsupial mammal, the wallaby (Macropus eugenii), was determined by anterograde labelling of axons from the eye with horseradish peroxidase, from birth to 46 days, when axons cover the colliculus contralaterally and ipsilaterally. The position of retinal ganglion cells giving rise to these projections over this period was determined in fixed tissue by retrograde labelling from the colliculus with a carbocyanine dye. Axons first reach the rostrolateral contralateral colliculus 4 days after birth and extend caudally and medially, reaching the caudal pole at 18 days and the far caudomedial pole at 46 days. The first contralaterally projecting cells are in the central dorsal and temporal retina, followed by cells in the nasal and finally the ventral retina. They are distributed closer to the periphery with increasing age. The first sign of a visual streak appears by 18 days. Axons reach the ipsilateral colliculus a day later than contralateral axons and come from a similar region of the retina. The sparser ipsilateral projection reaches the caudal and medial collicular margins by 46 days but by 16-18 days, ganglion cells giving rise to this transient projection are already concentrated in the temporoventral retina. The orderly recruitment of ganglion cells from retinotopically appropriate regions of the retina as axons advance across the contralateral colliculus suggests that the projection is topographically ordered from the beginning. The ipsilateral projection is less ordered as cells are located in the temporoventral crescent at a time when their axons are still transiently covering the colliculus prior to becoming restricted to the rostral colliculus. Features of mature retinal topography such as the visual streak and the location of ipsilaterally projecting cells begin to be established very early in development, before the period of ganglion cell loss and long before eye opening at 140 days.

Topographic organization in the retinocollicular pathway of the fetal cat demonstrated by retrograde labeling of ganglion cells

The topographic organization of the developing retinocollicular pathway was assessed by making focal deposits of a retrograde tracer (usually rhodamine latex beads) into the superficial layers of the superior colliculus of fetal cats at known gestational ages. Subsequently, the distributions of labeled cells in the contralateral and ipsilateral retinas were examined. At all stages of development, a high density of labeled cells was found in a delimited area (core region) of both retinas. The locations of the retinal regions containing the high density of labeled cells varied with the locus of the tracer deposit in the superior colliculus in a manner consistent with the topographic organization of the mature cat's retinocollicular pathway. Additionally, some labeled ganglion cells, considered to be ectopic, were found to be scattered throughout the contralateral and ipsilateral fetal retinas. Such ectopic cells were few in number throughout prenatal development. For every 100 cells projecting to the appropriate region of the colliculus, we estimate that less than one ganglion cell makes a gross projection error. The incidence of ectopic cells did not differ between the contralateral and ipsilateral retina, even though the overall density of crossed labeled cells was always greater than that of uncrossed labeled cells. In the youngest fetal animals, tracer deposits into the caudal portion of the superior colliculus resulted in a core region of labeled cells in the contralateral nasal retina as well as in the nasal ipsilateral retina. Such uncrossed nasal cells, not seen in more mature animals, appear to innervate the appropriate topographic location of the superior colliculus, but on the wrong side of the brain. Most likely, these uncrossed nasal ganglion cells contribute to the widespread distribution of the ipsilateral retinocollicular pathway observed in fetal cats after intraocular injections of anterograde tracers (Williams and Chalupa, 1982). Collectively, our findings demonstrate that the developing retinocollicular pathway of the fetal cat is characterized by a remarkable degree of topographic precision.