Ganglion cell death during retinal development in chick eyes explanted to the chorioallantoic membrane

Neuron death in the developing avian isthmo-optic nucleus, and its relation to the establishment of functional circuitry

The present review covers all the published data on neuron death in the developing avian isthmo-optic nucleus (ION), which provides a particularly convenient situation for studying the causes and consequences of neuron death in the development of the vertebrate central nervous system. The main conclusions are as follows: The naturally occurring neuron death in the ION is related both temporally and causally to the ION's formation of afferent and efferent connections. The ION neurons need to obtain both anterograde and retrograde survival signals in order to survive during a critical period in embryogenesis. They may compete, at least for the retrograde signals, but the nature of the competition is still unclear. The retrograde signals are modified by action potentials. Neurons dying from a lack of anterograde survival signals can be distinguished morphologically from ones dying from a lack of retrograde signals. The neuron death refines circuitry by selectively eliminating neurons with "aberrant" axons projecting to the "wrong" (i.e., ipsilateral) retina or to the "wrong" (topographically inappropriate) part of the contralateral retina.

The origin and development of retinal astrocytes in the mouse

Astrocytes, a class of glia which appear in the mammalian retina late in development, have been postulated either to originate in situ from Müller cells or extra-retinally from the optic stalk epithelium, only subsequently invading the eye. The site of origin and the developmental characteristics of retinal astrocytes were examined in the mouse, a species not previously studied for this purpose. Sections of normal eyes and stalks at different ages were examined. Cells positive for glial fibrillary acidic protein (GFAP) were first observed at post-conceptional day 17 at the optic disc end of the stalk. From this site, the GFAP-positive cells migrated into and across the retina at a rate of approximately 290 microns per day, reaching its edge by post-conceptional day 28. While migrating across the retina, the astrocytes progressively increased in size and morphological complexity, observations confirmed by measurement of their fractal dimension. Over the same period, a wave of differentiation swept along the stalk in the cranial direction. Further evidence that retinal astrocytes are born outside the retina emerged when foetal hemiretinae with or without optic stalks were explanted to the chorioallantoic membrane of the chick. When examined one to twelve days later, no explant cultured without the optic stalk contained GFAP-positive astrocytes, while explants with the stalk left attached contained relatively normal numbers of astrocytes. We observed, using fluorescence confocal microscopy, that retinal astrocytes in the mouse as in the rat, associate predominantly with blood vessels, not axonal bundles. It was of interest to determine whether this class of glia is essential to the normal cytoarchitectural development of the neural retina. Morphological analysis of the explants revealed no observable differences in cytoarchitecture or in the timing of developmental events between retinae maturing with or without astrocytes. It was therefore concluded that astrocytes may not be essential to the normal structural development of the murine retina.

Explantation of fetal murine retinae to the chorioallantoic membrane of the chicken embryo

A technique is here described for the culture of mammalian retinal explants on the chorioallantoic membrane of the developing chicken embryo. As an integral part of the central nervous system, the mammalian retina is characterised by its highly organised laminar structure and developmental timetable. Study of its prenatal development is, however, difficult to undertake in utero. In an attempt to render the organ of vision more accessible experimentally, fetal mouse retinae were explanted across major species barriers to the live chorioallantoic membrane of the chick. From 26 experiments, 128 explants (70% of the total) were recovered and 27 possessed a cytomorphology apparently identical to that of age-matched controls. The surviving retinae were analysed using a specifically devised set of criteria and they had developed a normal laminar structure (ganglion cell, inner plexiform, inner nuclear, outer plexiform and outer nuclear layers) but increased numbers of pyknotic profiles were present and somal sizes in the ganglion cell layer were significantly smaller. Such patterns have been obtained in other studies, both in vivo and in vitro, in which retinae had no access to their major targets in the brain, the superior colliculus and lateral geniculate nucleus. Explantation to the chorioallantoic membrane is thus a viable alternative for experiments requiring tissue isolation from natural surroundings since the explants are accessible for manipulation and observation while interacting with the host chick embryo. Furthermore, the technique allows examination of retinal differentiation, offering the opportunity to answer a number of important questions regarding development in the central nervous system.

The development of retinal ganglion cells deprived of their targets

The influence of central targets on the morphological differentiation of retinal ganglion cells was investigated in Xenopus laevis. Since the ganglion cells mature into distinct morphological subtypes after their axons have reached their central targets, it is possible that the target tissues may influence or specify this aspect of neuronal cell development. To test this idea, Xenopus eyebuds were target-deprived by transplantation to the flank region of host embryos where they developed ectopically. The grafted eyes grew at normal rates, but could not make any projections into the central nervous system. To examine the morphological differentiation of the retinal ganglion cells their structures were revealed using an in vitro retinal preparation and intracellular injections of the dye Lucifer yellow. The elaboration and maturation of ganglion cell dendrites were found to be indistinguishable between control and transplanted eyes throughout development. Thus, the development of retinal ganglion cells into distinct morphological classes can occur even when their axons do not interact with the appropriate central targets.

Prevention of optic nerve regeneration in the frog Hyla moorei transiently delays the death of some ganglion cells

We have previously reported that a proportion of ganglion cells die during optic nerve regeneration in the adult frog Hyla moorei (Humphrey and Beazley, '85). Here we assess the effect of preventing optic nerve regeneration on this cell loss. The optic nerve was crushed unilaterally and regeneration was allowed to progress unimpeded in one experimental series but was prevented by ligating or capping the nerve in another. We estimated total cell numbers in the ganglion cell layer from cresyl-stained wholemounts, comparing each experimental retina with its unoperated partner. At 70-78 days postcrush, mean cell numbers had fallen by 31.5% for frogs with unimpeded regeneration (N = 9), a significantly greater reduction than the 21.5% (N = 8) loss for the impeded regeneration series (p less than 0.001). Thereafter, cell numbers were stable for frogs with unimpeded regeneration. Cell death continued in the series with impeded regeneration, and losses exceeded those of frogs with unimpeded regeneration from 110 days postcrush. When regeneration was impeded, ganglion cell somas underwent an intense cell soma reaction and became arranged in rows radiating from the optic nerve head. Our findings indicate that some ganglion cells are transiently spared when regeneration of their axons is prevented. The abnormally extensive contacts formed between somas may delay ganglion cell loss. However, the eventual death of most ganglion cells shows them to be target-independent in the long term.

Loss of ganglion cells in fetal retina transplanted to rat cortex

Fetal retina survived and differentiated when placed into the cortex of newborn host rats. Two months after transplantation, anterograde tract tracing techniques failed to reveal any projections from these transplants into the host brain. Soma-size analysis of the cells in the ganglion cell layer of the transplants suggested that only displaced amacrine cells remained and that the ganglion cells had degenerated. The basis for these results is considered relative to trophic maintenance of ganglion cells, retinal axon guidance, and specificity of connections.

Organotypic cultures of neural retina: neurite outgrowth stimulated by brain extracts

A bioassay for the growth of retinal neurites was designed. Circular plugs 0.5-1.5 mm in diameter were excised from the chick neural retina at embryonic day 6 and cultured as organotypic explants on a collagen gel. After addition of medium or various tissue extracts cultures were incubated and the length of extending neurites measured. In control medium with 10% serum, neurites were sparse and reached less than 0.2 mm after 4 days. Extract of the optic lobe from 18-day-old chick embryos distinctly increased the density and length of neurites in a dose-dependent manner reaching 1.2 mm after 4 days. A unit of outgrowth activity corresponding to half the maximum length of fibers was defined. The activity of the optic lobe extracts was retained in fractions with nominal molecular weights over 100,000 daltons upon pressure dialysis. Extract from the forebrain hemispheres had nearly the same effect on retinal neurite growth. Mouse submandibular gland NGF failed to evoke retinal fiber growth. Our data imply that macromolecules of the developing brain can support the growth of optic axons.

Development of ganglion cells and their axons in organized cultures of fetal mouse retinal explants

Retinas from 13-15 day fetal mice were explanted alone, with adjacent eyeball tissue, or with nearby superior colliculus explants. The organotypic structure of the retina developed in situ, including photoreceptors, interneurons, plexiform layers, ganglion cells, and an optic fibre layer. Electrophysiologic recordings demonstrated that functional synaptic networks developed resembling bioelectric response patterns seen in situ. Within half-retinas, arrays of optic fibers converged to the optic nerve head; in co-cultures with tectum they could become myelinated. Large bundles of long, naked neurites--1 degree primary retinal fibers--emerged from the explant in the first few days in vitro; these could often be traced back to the optic nerve head and a detailed survey of their properties using horseradish peroxidase (HRP) tracing methods identified tham as ganglion cell axons. When growing upon collagen substrata, 1 degree fibers began to disintegrate during the second week in vitro; however, many 1 degree fibers that grew into superior colliculus explants were maintained for at least 5 weeks in vitro, where they formed elaborate, functional terminal arborizations. In a few cases, 1 degree fibers grew across skeletal muscle fibers and appeared to induce them to contract. A second type of neuritic outgrowth pattern appeared after the first week in vitro: 2 degrees retinal fibers. This was composed of a mixed population of interneuronal neurites; a small percentage was catecholaminergic. Our characterization of the morphologic properties of retinal ganglion cells and their axons in organotypic cultures provides the necessary background to interpret electrophysiologic mapping and neural-specificity analyses of retino-CNS co-cultures. This in vitro model system may have biological relevance to understanding the cues that control the development of the retinotectal projection in situ.

Antibodies to a neural cell adhesion molecule disrupt histogenesis in cultured chick retinae

Cell-surface proteins are believed to have important roles in cell-cell interactions during brain development, particularly in such processes as cellular adhesion, neurite outgrowth and synapse formation. The chick neural cell adhesion molecule, CAM, is a cell-surface protein specific to the nervous system and has been implicated in cell adhesion among cells and neurites of the developing retina and brain. Previous studies have shown that F(ab') fragments of antibodies directed against CAM inhibit the in vitro aggregation of cells obtained from 9-day embryonic chick retina. The specific antibody fragments also reduce the diameter of neurite fascicles that grow out from cultured dorsal root ganglia, apparently by blocking side-to-side adhesion between the neurites. In addition, anti-CAM antibodies alter the appearance of histotypic patterns in retinal cell aggregates maintained in culture for several days. We now demonstrate that the antibodies can disrupt histogenesis of the developing retina in organ culture, strengthening the notion that the cell-cell adhesion properties mediated by CAM are involved in the normal development of histological layers in the chick retina.