Development of the chiasm of a marsupial, the quokka wallaby

Early development of the nervous system of the eutherian <i>Tupaia belangeri</i>

Abstract. The longstanding debate on the taxonomic status of Tupaia belangeri (Tupaiidae, Scandentia, Mammalia) has persisted in times of molecular biology and genetics. But way beyond that Tupaia belangeri has turned out to be a valuable and widely accepted animal model for studies in neurobiology, stress research, and virology, among other topics. It is thus a privilege to have the opportunity to provide an overview on selected aspects of neural development and neuroanatomy in Tupaia belangeri on the occasion of this special issue dedicated to Hans-Jürg Kuhn. Firstly, emphasis will be given to the optic system. We report rather "unconventional" findings on the morphogenesis of photoreceptor cells, and on the presence of capillary-contacting neurons in the tree shrew retina. Thereafter, network formation among directionally selective retinal neurons and optic chiasm development are discussed. We then address the main and accessory olfactory systems, the terminal nerve, the pituitary gland, and the cerebellum of Tupaia belangeri. Finally, we demonstrate how innovative 3-D reconstruction techniques helped to decipher and interpret so-far-undescribed, strictly spatiotemporally regulated waves of apoptosis and proliferation which pass through the early developing forebrain and eyes, midbrain and hindbrain, and through the panplacodal primordium which gives rise to all ectodermal placodes. Based on examples, this paper additionally wants to show how findings gained from the reported projects have influenced current neuroembryological and, at least partly, medical research.

Segregated hemispheric pathways through the optic chiasm distinguish primates from rodents

At the optic chiasm retinal fibers either cross the midline, or remain uncrossed. Here we trace hemispheric pathways through the marmoset chiasm and show that fibers from the lateral optic nerve pass directly toward the ipsilateral optic tract without any significant change in fiber order and without approaching the midline, while those from medial regions of the nerve decussate directly. Anterograde labeling from one eye shows that the two hemispheric pathways remain segregated through the proximal nerve and chiasm with the uncrossed confined laterally. Retrograde labeling from the optic tract confirms this. This clearly demonstrates that hemispheric pathways are segregated through the primate chiasm. Previous chiasmatic studies have been undertaken mainly on rodents and ferrets. In these species there is a major change in fiber order pre-chiasmatically, where crossed and uncrossed fibers mix, reflecting their embryological history when all fibers approach the midline prior to their commitment to innervate either hemisphere. This pattern was thought to be common to placental mammals. In marsupials there is no change in fiber order and uncrossed fibers remain confined laterally through nerve and chiasm, again, reflecting their developmental history when all uncrossed fibers avoid the midline. Recently it has been shown that this distinction is not a true dichotomy between placental mammals and marsupials, as fiber order in tree shrews and humans mirrors the marsupial pattern. Architectural differences in the mature chiasm probably reflect different developmental mechanisms regulating pathway choice. Our results therefore suggest that both the organization and development of the primate optic chiasm differ markedly from that revealed in rodents and carnivores.

Diversity in mammalian chiasmatic architecture: ipsilateral axons are deflected at glial arches in the prechiasmatic optic nerve of the eutherian Tupaia belangeri

Permanent ipsilaterally projecting axons approach the chiasmatic midline in rodents but are confined to lateral parts of the optic chiasm in marsupials. Hence, principally different mechanisms were thought to underlie axon pathway choice in eutherian (placental) and marsupial mammals. First evidence of diversity in eutherian chiasmatic architecture came from studies in the newborn and adult tree shrew Tupaia belangeri (Jeffery et al. [1998] J. Comp. Neurol. 390:183-193). Here, as in marsupials, ipsilaterally projecting axons do not approach the midline. The present study aims to clarify how the developing tree shrew chiasm is organized, how glial cells are arranged therein, and the extent to which the tree shrew chiasm is similar to that of marsupials or other eutherians. By using routinely stained serial sections as well as immunohistochemistry with antibodies against glial fibrillary acidic protein, vimentin, and medium-molecular-weight neurofilament protein, we investigated chiasm formation from embryonic day 18 (E18) to birth (E43). From E22 onward, ipsilaterally projecting axons diverged from contralaterally projecting axons in prechiasmatic parts of the optic nerve. They made sharp turns when arriving at glial arches found at the transition from the optic nerve to the chiasm. Thus, during the ingrowth period of axons, Tupaia belangeri and marsupials have specialized glial arrays in common, which probably help to deflect ipsilaterally projecting axons to lateral parts of the chiasm. Our observations provide new evidence of diversity in eutherian chiasmatic architecture and identify Tupaia belangeri as an appropriate animal model for studies on the mechanisms underlying axon guidance in the developing chiasm of higher primates.

Chiasm formation in man is fundamentally different from that in the mouse

At the optic chiasm axons make a key binary decision either to cross the chiasmal midline to innervate the contralateral optic tract or to remain uncrossed and innervate the ipsilateral optic tract. In rodents, midline interactions between axons from the two eyes are critical for normal chiasm development. When one eye is removed early in development the hemispheric projections from the remaining eye are disrupted, increasing the crossed projection at the expense of the uncrossed. This is similar to the abnormal decussation pattern seen in albinos. The decussation pattern in marsupials, however, is markedly different. Early eye removal in the marsupial has no impact on projections from the remaining eye. These differences are related to the location of the uncrossed projection through the chiasm. In rodents, axons that will form the uncrossed projection approach the chiasmal midline, while in marsupials they remain segregated laterally through the chiasm. Histological analysis of the optic chiasm in man provides anatomical evidence to suggest that, unlike in rodents, uncrossed axons are confined laterally from the optic nerve through to the optic tract and do not mix in each hemi-chiasm. This is a pattern similar to that found in marsupials. Electrophysiological evidence in human anophthalmics shows that the failure of one eye to develop in man has no impact on the hemispheric projections from the remaining eye. This strongly suggests that the mechanisms regulating chiasmal development in man differ from those in rodents, but may be similar to marsupials. This implies that optic chiasm formation in rodents and ferrets is not common to placental mammals in general.

Early midline interactions are important in mouse optic chiasm formation but are not critical in man: a significant distinction between man and mouse

The optic chiasm is one of the most popular models for studying axon guidance. Here axons make a key binary decision either to cross the midline to innervate the contralateral hemisphere or to remain uncrossed. In rodents, midline interactions between axons from the two eyes are critical for normal development, as early removal of one eye systematically disrupts hemispheric projections from the remaining eye, increasing the crossed projection at the expense of the uncrossed. This is similar to the abnormal decussation pattern seen in albinos. This pattern is markedly different in marsupials where early eye removal has no impact on projections from the remaining eye. These differences are related to the location of the uncrossed projection through the chiasm. In rodents these axons approach the midline whereas in marsupials they remain segregated laterally. We provide anatomical evidence in man suggesting that, unlike in rodents, uncrossed axons are confined laterally and do not mix in each hemi-chiasm, which is a pattern similar to that found in marsupials. Further, we demonstrate electrophysiologically, using visual cortical evoked potentials, that the failure of one eye to develop in man has no impact on the hemispheric projections from the remaining eye. These data demonstrate that the mechanisms regulating chiasmal development in man differ from those in rodents but may be similar to those in marsupials. We suggest that mouse models of the organization and development of the optic chiasm are not common to placental mammals in general.

Compensatory and transneuronal plasticity after early collicular ablation

Plasticity within the visual system was assessed in the quokka wallaby following unilateral superior collicular (SC) ablation at postnatal days (P) 8-10, prior to the arrival of retinal ganglion cell (RGC) axons. At maturity (P100), projections were traced from the eye opposite the ablation, and total RGC numbers were estimated for both eyes. Ablations were partial (28-89% of SC remaining) or complete (0-5% of SC remaining). Projections to the visual centers showed significant bilateral (P < 0.05) increases in absolute volume. Minor anomalous projections also formed within the deep, surviving non-retino-recipient layers of the ablated SC and via a small bundle of RGC axons recrossing the midline to innervate discrete patches in the SC contralateral to the lesion. Total absolute volume of projections did not differ between partial and complete ablations; moreover, values did not differ from normal (P > 0.05). Compared with normal, total RGC numbers were significantly (P < 0.05) reduced in the eye opposite the ablation but increased (P < 0.05) in the other eye. Consequently, the sum of the two RGC populations did not differ from normal (P > 0.05). As in rodents, the visual system in quokka compensates following injury by maintaining a set volume of arborization but does so by forming only minor anomalous projections. Furthermore, increased RGC numbers in the eye ipsilateral to the lesion indicate that compensation occurs transneuronally, thus maintaining total numbers of projecting neurons. The implication is that the visual system acts in concert following unilateral injury to maintain set values for RGC terminal arbors as well as their cell bodies.

Variations in the architecture and development of the vertebrate optic chiasm

At the vertebrate optic chiasm there is major change in fibre order and, in many animals, a separation of fibres destined for different hemispheres of the brain. However, the structure of this region is not uniform among all species but rather shows marked variations both in terms of its gross architecture and the pathways taken by different fibres. There also are striking differences in the developmental mechanisms sculpting this region even between closely related animals. In spite of this, recent studies have provided strong evidence for a remarkable degree of conservation in the molecular nature of the guidance signals and regulatory genes driving chiasmatic development. Here differences and similarities in chiasmatic organisation and development between separate groups of animals will be reviewed. While it may not be possible to ascribe a single set of factors that are universal components of the vertebrate chiasm, there are both strikingly similar elements as well as diverse features to the development, organisation and architecture of this region. This review aims to highlight key issues in the organisation and development of the vertebrate optic chiasm with a focus on comparing and contrasting the data that has been gleaned to date from different vertebrate groups.

Changes in expression of fibroblast growth factor receptors during development of the mouse retinofugal pathway

Retinal axons undergo several changes in organization as they pass through the region of the optic chiasm and optic tract. We used immunocytochemistry to examine the possible involvement of fibroblast growth factor receptors (FGFR) in these changes in retinal axon growth. In the retina, at all ages examined, prominent staining for FGFR was seen in the optic fiber layer and at the optic disk. At embryonic day 15 (E15), FGFR immunoreactivity was also detected in the ganglion cell layer, as defined by immunoreactivity for islet-1. At later developmental stages (E16 to postnatal day 0), FGFR were found in the optic fiber layer and the inner plexiform layer. In the ventral diencephalon, immunostaining for FGFR was first detected at E13 in a group of cells posterior to the chiasm. These cells appeared to match the neurons that are immunopositive for the stage-specific embryonic antigen-1 (SSEA-1). FGFR staining was also found on the retinal axons at E13. At E14-E16, when most axons are growing across the chiasm and the tract, a dynamic pattern of FGFR immunoreactivity was observed on the retinal axons. The staining was reduced when axons reached the midline but was increased when axons reached the threshold of the optic tract. These results suggest that axon growth and fiber patterning in distinct regions of the retinofugal pathway are in part controlled by a regulated expression of FGFR. Furthermore, the axons with elevated FGFR expression in the optic tract have a posterior border of rich FGFR expression in the lateral part of the diencephalon. This region overlaps with a lateral extension of the SSEA-1-positive cells, suggesting a possible relation of these cells to the elevated expression of FGFR.

Architecture of the optic chiasm and the mechanisms that sculpt its development

At the optic chiasm the two optic nerves fuse, and fibers from each eye cross the midline or turn back and remain uncrossed. Having adopted their pathways the fibers separate to form the two optic tracts. Research into the architecture and development of the chiasm has become an area of increasing interest. Many of its mature features are complex and vary between different animal types. It is probable that numerous factors sculpt its development. The separate ganglion cell classes cross the midline at different locations along the length of the chiasm, reflecting their distinct periods of production as the chiasm develops in a caudo-rostral direction. In some mammals, uncrossed axons are mixed with crossed axons in each hemi-chiasm, whereas in others they remain segregated. These configurations are the product of different developmental mechanisms. The morphology of the chiasm changes significantly during development. Neurons, glia, and the signals they produce play a role in pathway selection. In some animals fiber-fiber interactions are also critical, but only where crossed and uncrossed pathways are mixed in each hemi-chiasm. The importance of the temporal dimension in chiasm development is emphasized by the fact that in some animals uncrossed ganglion cells are generated abnormally early in relation to their retinal location. Furthermore, in albinos, where many cells do not exit the cell cycle at normal times, there are systematic chiasmatic abnormalities in ganglion cell projections.

The effects of early prenatal monocular enucleation on the routing of uncrossed retinofugal axons and the cellular environment at the chiasm of mouse embryos

Whereas it has been shown that early monocular enucleations produce a reduction in the uncrossed pathway from the surviving eye in rats and ferrets, similar evidence for binocular interactions in the development of the uncrossed component in mice is currently open to question. Using retrograde tracing, we have investigated the time course of changes in the uncrossed retinofugal pathway immediately after the early prenatal monocular enucleation in mouse embryos. Removal of one eye from C57 pigmented mice at embryonic day (E) 13 does not cause a reduction of the earliest uncrossed component from the central retina examined 1 day later at E14. However, a substantial reduction of the uncrossed pathway is seen at E15, the time when the major uncrossed projection first arises from the ventral temporal retina. This reduction is greater in E16 one-eyed embryos, indicating that most retinal axons from the ventral temporal retina rely on a binocular interaction for their turning at the chiasm. Further, early removal of one eye at E13 does not produce any obvious changes in the cytoarchitecture of RC-2-immunopositive radial glia at the chiasm, nor of the stage-specific antigen-1 (SSEA-1) -expressing neurons. This lack of changes in the cellular organization at the chiasm indicates that the reduction of the uncrossed pathway is probably produced by an elimination of binocular fibre interactions at the chiasm, rather than through a degenerative change of cellular elements at the chiasm as a consequence of the eye removal procedure.

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].

First evidence of diversity in eutherian chiasmatic architecture: tree shrews, like marsupials, have spatially segregated crossed and uncrossed chiasmatic pathways

In the optic chiasm of mammals, axons either cross the midline to the opposite side of the brain or remain uncrossed. In the eutherian species studied to date, uncrossed axons in the caudal nerve are found in all regions. In the chiasm, they are dispersed through the hemichiasm, with many axons approaching the midline and then turning back to enter the same side of the brain as the originating eye. In marsupials, by contrast, uncrossed axons never approach the midline; instead, they remain grouped in the lateral nerve and chiasm. The impression gained from these data is that there is a major difference in chiasmatic architecture between eutherian and marsupial mammals. Therefore, the mechanisms by which axons choose their route through the chiasm was also thought to differ between the two major groups of mammals. However, the present study shows that the chiasm of a highly visual eutherian mammal, the tree shrew, is similar to that found in marsupials, with uncrossed axons confined to lateral regions and not approaching the midline. However, unlike marsupials, in the tree shrew, optic fascicles in the chiasm are often separated by thick collagen bundles. It is probable that the chiasmatic structure described to date for eutherian mammals is not ubiquitous, as was previously thought, and theories explaining the mechanisms by which axons chose their route through the chiasm during development will have to be expanded.

Chiasmatic specificity in the regenerating mammalian optic nerve

The mammalian central nervous system is capable of regenerating; however, there is no evidence that the regenerating axons can navigate along their normal pathways and reestablish topographically organized projections: essential for functional return of vision. Here retinal ganglion cells in the opossum Monodelphis were birthdated with tritiated thymidine on the sixth postnatal day (P6), before being lesioned in the temporal retina at P8. Retrograde tracing with horseradish peroxidase injected into the ipsilateral optic tract at P24 showed that the temporal crescent had reformed behind the retinal lesion. By comparisons of cell and thymidine counts from lesioned and control regions of retina, it was estimated that about 40% of the normal number of ganglion cells are able to regenerate into the ipsilateral optic tract following a lesion in the temporal retina at P8. A clear line of decussation (separation of ipsilateral and contralateral projections) reformed in the lesioned temporal retina and regenerating ganglion cells labeled with DiI were turned at appropriate points on passing through the optic chiasm. This is evidence of chiasmatic specificity with regard to lesioned retinal ganglion cells regenerating into the ipsilateral optic tract.

Regeneration in the developing optic nerve: correlating observations in the opossum to other mammalian systems

Regeneration of severed axons within the central nervous system of adult mammals does not normally occur with any degree of success. During development, however, newly forming projections must send axons to distant sites and form appropriate connections with their targets: successful regeneration has been observed during this critical period. The opossum central nervous system develops during early postnatal life and has provided a useful experimental model to investigate this specialized mode of axonal regeneration in mammals. The presence of a clear decision point at the optic chiasm has also provided a useful site at which to investigate the navigational capacity of retinal ganglion cells regenerating along the optic nerve during this critical period. Regeneration failure occurs as the central nervous system progresses from this permissive, developing state to a mature, non-permissive adult state. Studies into the behaviour of glial and neuronal elements around this transition period can help elucidate some of the factors that need to be overcome if regeneration is ever to become successful in adult mammals. The regeneration characteristics of a lesioned projection are dependent upon its developmental stage and are also related to the proximity of axotomy along its pathway. A system of staging is proposed to correlate observations in the opossum optic nerve to other mammalian systems.

Changing topography of the RPE resulting from experimentally induced rapid eye growth

The retinal pigment epithelium (RPE) of the quokka wallaby. Setonix brachyurus, grows and changes throughout life. To investigate factors that determine changes in the quokka RPE, we have examined topography of this tissue in experimentally enlarged eyes. Unilateral eyelid suture was conducted at the time of normal eye opening, postnatal day (P) 110, and animals were examined at 1 or 1 1/2 years of age. The numbers and densities of RPE cells and the extent of multinucleation were compared with those in normal animals. Eyelid suture resulted in a 9.8% and 17.4% increase in retinal area at 1 and 1 1/2 years, respectively; a significant degree of myopia was associated with this enlargement. Cell density topography in experimental eyes was not the same as in controls. Cells from central retina were disproportionately larger in the experimental than control eyes. However, the RPE cell topography in sutured eyes was not the same as that of aged retinae of a similar size. Notably, in sutured eyes there was no development of the high or highest cell densities seen in equatorial and temporal central RPE in aged retinae, respectively. Furthermore, the degree of cell enlargement in peripheral regions was slight compared with that observed in similar-sized, aged retinae. There was no increase in RPE cell number; rather, average cell area increased accompanied by no change or a slight decrease in RPE thickness. Consequently, overall volume of cells did not change significantly. The large number of multinucleate cells normally seen in aged animals was not observed in experimentally enlarged eyes, implying that an increase in cell volume may be the trigger for multinucleation.

Cellular localisation of metabotropic glutamate receptors in the mammalian optic nerve: a mechanism for axon-glia communication

It has been proposed that neurotransmitter signalling can occur between axons and glia in the mammalian optic nerve in the absence of synaptic specialisations, and that this may be glutamate mediated. Here, the cellular distribution of five metabotropic glutamate receptors (mGluR's 1a, 1b, 1c, 2/3 and 5) have been assessed in the rat optic pathway using specific antibodies. Positive immunoreactivity is found for mGluR2/3 and 5. Both are found in axons, although only mGluR5 is present in the majority of these. Strong immunoreactivity for mGluR2/3 is found in cells in the optic pathway and thalamus. The cellular morphology and distribution is consistent with their being astrocytes. Examination of brain sections stained for mGluR2/3 is consistent with this notion, with many cells having end-feet processes terminating on blood vessels or the pial surface. The axonal immunoreactivity could represent the presence of these receptors on axons, but it is more probable that the receptor protein synthesised in the ganglion cell soma is being transported to the cell terminal in sufficient concentration to be revealed by immunohistochemistry. The reason for the axon-astrocyte signalling is unclear, and may be associated with metabolic coupling. In development, communication between axons and glia mediates a range of functions including pathway selection and myelination. It is probable that in the adult this form of signalling underpins a range of functions that have yet to be described.