Evidence for a driving role of ingrowing axons for the shifting of older retinal terminals in the tectum of fish

Diffusion and imaging properties of three new lipophilic tracers, NeuroVue Maroon, NeuroVue Red and NeuroVue Green and their use for double and triple labeling of neuronal profile

We describe here diffusion and imaging properties of three new lipophilic tracers, NeuroVue Maroon (near infrared), NeuroVue Red and NeuroVue Green. Using pair-wise comparisons between the new dyes and existing dyes (DiI, DiA, DiD, DiO, PKH2, PKH26) applied to the left and the right side of fixed spinal cord preparations, we show that NeuroVue Maroon (excitation maximum 647 nm) surpasses all other dyes in this study in signal to noise ratio. We also present data showing the utility of these new dyes for both double labeling and triple labeling in combination with each other or existing lipophilic tracers. Using mice bearing the PLP-eGFP transgene, we demonstrate that either NeuroVue Maroon or NeuroVue Red can readily be combined with eGFP labeling. Double labeling experiments using NeuroVue Red and eGFP allowed us to demonstrate that every fiber in the neonatal ear is surrounded by developing Schwann cells.

Metamorphic remodeling of the primary olfactory projection in Xenopus: developmental independence of projections from olfactory neuron subclasses

In adult Xenopus, the nasal cavity is divided into separate middle (MC) and principal (PC) cavities; the former is used to smell water-borne odorants, the latter air-borne odorants. Recent work has shown that olfactory neurons of each cavity express a distinct subclass of odorant receptors. Moreover, MC and PC axons project to distinct regions of the olfactory bulb. To examine the developmental basis for this specificity in the olfactory projection, we extirpated the developing MC from early metamorphic (stage 54-57) tadpoles and raised the animals through metamorphosis. In most lesioned animals, the MC partly regenerated. Compared with the unlesioned side, reduction of the region of the glomerular layer of the olfactory bulb receiving MC afferents ranged from 70% to 95%. PC afferents did not occupy regions of the olfactory bulb deprived of MC afferents. These results support a model in which intrinsic cues in the olfactory bulb control the projection pattern attained by ingrowing olfactory axons.

Position, guidance, and mapping in the developing visual system

Positional identity in the visual system affects the topographic projection of the retina onto its central targets. In this review we discuss gradients and positional information in the retina, when and how they arise, and their functional significance in development. When the axons of retinal ganglion cells leave the eye, they navigate through territory in the central nervous system that is rich in positional information. We review studies that explore the navigational cues that the growth cones of retinal axons use to orient towards their target and organize themselves as they make this journey. Finally, these axons arrive at their central targets and make a precise topographic map of visual space that is crucial for adaptive visual behavior. In the last section of this review, we examine the topographic cues in the tectum, what they are, when, and how they arise, and how retinal axons respond to them. We also touch on the role of neural activity in the refinement of this topography.

Development of the rabbit retina, III: Differential retinal growth, and density of projection neurons and interneurons

To provide a quantitative description of postnatal retinal expansion in rabbits, a new procedure was developed to map the retinae, which cover the inner surface of hemispheres or parts of rotation ellipsoids, in situ, onto a single plane. This method, as well as the known distribution of Müller cells per unit retinal surface area, were used to estimate the redistribution of specific subpopulations of Müller cells within different topographic regions of the retinae. Müller cells are known to exist as a stable population of cells 1 week after birth and can therefore be used as "markers" for determining tissue expansion. Our results show that differential retinal expansion occurs during development. Peripheral retinal regions expand at least twice as much as the central ones. Furthermore, there is a greater vertical than horizontal expansion. This differential retinal expansion leads to a corresponding redistribution of 5-hydroxytryptamine (5-HT) accumulating amacrine cells. Differential retinal expansion, however, does not account for all of the changes in the centro-peripheral density gradient of cells in the ganglion cell layer (GCL)--mostly retinal ganglion cells--during postnatal development. The changes in the ganglion cell layer were evaluated in Nissl-stained wholemount retinal preparations. Additionally, the difference between expansion-related redistribution of cells in the GCL and Müller cells was confirmed in wholemount preparations where Müller cells (identified as vimentin positive) and cells in the GCL (identified by fluorescent supravital dyes) were simultaneously labeled. It is assumed that many of the ganglion cells within the retinal center are not translocated during retinal expansion, possibly because their axons are fixed. In contrast, 5-HT accumulating amacrine cells--which are interneurons without a retinofugal axon--display a passive redistribution together with the surrounding retinal tissue.

Ipsilateral retinopetal projection of the nucleus olfactoretinalis (NOR) during development and regeneration: a DiI study in a cichlid fish

The development and regeneration of the ipsilateral retinopetal projection of the nucleus olfactoretinalis (NOR) in the cichlid fish Haplochromis burtoni was studied with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI) in fixed tissue. Throughout development most NOR cells projected to the contralateral retina. Only an insignificant, transient elevation of a projection to the ipsilateral retina was found in a few animals; however, after severing the contralateral processes of NOR cells by either enucleation or nerve crush, many animals had significantly more NOR cells with a regenerated process to the ipsilateral retina. Nevertheless, within a few weeks of surgery, the number of animals with ipsilaterally projecting cells were reduced to control values. The transiently enhanced ipsilateral projections to the retina imply changes in the guiding mechanism after these operations and the existence of control mechanisms against unusual connections to the retina in this bony fish.

Ipsilateral retinal projections into the tectum during regeneration of the optic nerve in the cichlid fish Haplochromis burtoni: a Dil study in fixed tissue

Retinal projections were experimentally manipulated in a bony fish to reveal conditions under which considerably enlarged ipsilateral projections developed and persisted. Three experimental groups were studied: animals after unilateral enucleation, after unilateral nerve crush, and after enucleation and crush of the remaining optic nerve. At 29 days after unilateral enucleation alone, no enhanced ipsilateral projection had developed. After nerve crush, however, large numbers of retinal fibers regenerated into the ipsilateral tectum. Retrogradely filled, ipsilaterally projecting ganglion cells were distributed throughout the entire retina. After 15 days regenerating retinal fibers covered the entire ipsilateral tectum. At later stages the ipsilateral projection showed progressive reduction in coverage of the tectum. Combining enucleation with nerve crush led to an ipsilateral projection that covered the tectum at 28 days and later. In this experimental situation the development of an ipsilateral projection appears to be a two-step process: (1) Fibers are rerouted to the ipsilateral side at the diencephalon, and (2) ipsilateral fibers persist in the tectum only in the absence of a contralateral projection while they appear to be eliminated in the other cases.