Formation of neuroblastic layers in chicken retinospheroids: the fibre layer of Chievitz secludes AChE-positive cells from mitotic cells

Neuronal differentiation in the early human retinogenesis

AIM

Our study investigates the differentiation of retinal stem cells towards different neuronal subtypes during the critical period of human eye development.

METHODS

Expression of the neuronal marker neurofilament 200 (NF200), tyrosine hydroxilase (TH) and choline acetyltransferase (ChAT) was seen by immunofluorescence in the 5th-12th - week stage of development in the human eye. Data was analysed by Mann-Whitney, Kruskal-Wallis and Dunn's post hoc tests.

RESULTS

NF200, TH and ChAT cells appeared in the 5th/6th week and gradually increased during further development. The proportion of TH positive areas were distributed similarly to NF200, with a higher proportion in the outer neuroblastic layer. The proportion of a ChAT positive surface was highest in the 5th/6th - week whilst from the 7th week onwards, its proportion became higher in the optic nerve and inner neuroblastic layers than in the outer layer, where a decrease of ChAT positive areas were seen.

CONCLUSIONS

Our study indicates a high differentiation potential of early retinal cells, which decreased with the advancement of development. The observed great variety of retinal phenotypic expressions results from a large scale of influences, taking place at different developmental stages.

PEDF counteracts DL-α-aminoadipate toxicity and rescues gliotoxic damages in RPE-free chicken retinal explants

Gliotoxic responses complicate human eye diseases, the causes of which often remain obscure. Here, we activated Müller cells (MCs) by the gliotoxin DL-α-aminoadipate (AAA) and assayed possible protective effects by pigment epithelium-derived factor (PEDF) in RPE-free retinal explants of the E6 chick embryo. These models are suited to analyze gliotoxic reactions in vitro, since the avian retina contains only Müller cells (MCs) as glial components, and the RPE-free explants are devoid of a major PEDF source. ChAT- and AChE-immunohistochemistry (IHC) revealed that AAA treatment disrupted the differentiation of cholinergic amacrine cells in the inner plexiform layer. At the applied concentration of 1 mM AAA, apoptosis of MCs was slightly increased, as shown by TUNEL and caspase-3 activity assays. Concomitantly, cell-free gaps emerged in the middle of the retina, where MCs were swollen and amassed glutamine synthetase (shown by GS and Vimentin IHC). AAA treatment strongly activated MCs, as shown by GFAP IHC, and by an increase of stress-related catalase activity. Remarkably, nearly all effects of AAA on MCs were effectively counter-balanced by 50 ng/ml PEDF co-treatment, as also shown by RT-PCR. These findings suggest that supplementation with PEDF can protect the retina against gliotoxic attacks. Further studies should establish whether PEDF similarly protects a gliotoxic human retina.

Morphogenesis of the different types of photoreceptors of the chicken (Gallus domesticus) retina and the effect of amblyopia in neonatal chicken

Despite the great variety in chicken photoreceptors, existing morphogenetic studies only deal with two types: rods and cones. We have therefore examined by scanning electron microscopy the first appearance and maturation of different retinal photoreceptors in 36 chicken embryos (Gallus domesticus), aged 5-19 days prehatching. On day 5 of incubation, chicken retinae were only composed of proliferating ventricular cells devoid of photoreceptors. On day 8, outer mitotic cells were separated from inner differentiating photoreceptors, by the transient layer of Chievitz. Ball-like protrusions appeared at the ventricular surface, representing the first signs of photoreceptor inner segment formation. From day 10 onward, double cones, single cones, and rods could be clearly distinguished, and occasional cilia were detected at their tip. On day 12, inner segments had increased in length and diameter, and frequently carried a cilium representing the beginning of outer segment formation. On day 14, most photoreceptors displayed a distinct outer segment. On day 19, photoreceptors had essentially assumed adult morphology. Based on the shape of their outer segments, two subtypes of cones and three subtypes of double cones could be distinguished. Throughout development, we observed microvilli close to maturing photoreceptors, either originating from their lateral sides, from their tip, or from Müller cells. Microvillus density peaked between day 12 and 14, indicating an important role in photoreceptor morphogenesis. Unilateral occlusion of the eyes of posthatching chicken reduced the proportion of double cones to single cones in the retina, indicating dependence of retinal morphogenesis upon functional activity of visual cells.

Exposure to Diazinon Alters In Vitro Retinogenesis: Retinospheroid Morphology, Development of Chicken Retinal Cell Types, and Gene Expression

Developing embryos are more vulnerable than adults to acute cholinergic intoxication by anticholinesterases, including organophosphorus pesticides. These agents affect the process of neural development itself, leading to permanent deficits in the architecture of the nervous system. Recent evidence on direct roles of acetylcholinesterase (AChE) on neuronal differentiation provides additional grounds for investigating the developmental toxicity of anticholinesterases. Therefore, the effect of the organophosphate diazinon on the development of chick retinal differentiation was studied by an in vitro reaggregate approach. Reaggregated spheres from dissociated retinal cells of the E6 chick embryo were produced in rotation culture. During the whole culture period of 10 days, experimental cultures were supplemented with different concentrations of the pesticide, from 20 to 120 microM diazinon. The pesticide-treated spheres were reduced in size, and their outer surface was irregular. More importantly, inner structural distortions could be easily traced because the structure of control spheroids can be well characterized by a histotypical arrangement of laminar parts homologous to the normal retina. Acetylcholinesterase activity in diazinon-treated spheres was reduced when compared with controls. As a dramatic effect of exposure to the pesticide, inner plexiform layer (IPL)-like areas in spheroids were not distinguishable anymore. Similarly, photoreceptor rosettes and Müller radial glia were strongly decreased, whereas apoptosis was stimulated. The expression of transcripts for choline-acetyltransferase and muscarinic receptors was affected, revealing an effect of diazinon on the cholinergic system. This further proves the significance of cholinesterases and the cholinergic system for proper nervous system development and shows that further studies of debilitating diazinon actions on development are necessary.

Characteristics of cellular proliferation in the developing human retina

The expression of proliferation-associated proteins Ki67 and PCNA was studied in the retinal rudiments of human embryos at 5-8 weeks of development; studies also addressed the numbers of nucleoli in the nuclei of neuroepithelial cells (with consideration of their distances to the apical surface) and DNA-synthesizing cells after transient (20 min) in vitro incubation in serum-free medium containing BrdU. The retinal rudiment of embryos at five weeks of development had neuroepithelium of the typical structure. BrdU-positive nuclei and nuclei with small numbers of nucleoli were located in the basal part of the ventricular zone. However, this organization was disrupted during the initial period of formation of the inner nuclear layer (six weeks). At this time, DNA-synthesizing cells were found even at the apical surface. Retinal rudiments of embryos at 6-7 weeks of development contained an additional area of cell proliferation in the Chievitz layer and the inner nuclear layer. In eight-week embryos, dividing cells were located in the outer nuclear layer, which again acquired the organization typical of neuroepithelium.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

Pigmented epithelium sustains cell proliferation and decreases expression of opsins and acetylcholinesterase in reaggregated chicken retinospheroids

We investigated the effect of the retinal pigmented epithelium on cell proliferation and differentiation in rosetted retinospheroids, which are retina-like spheres reaggregated in the complete absence of retinal pigmented epithelium from dissociated retinal cells of 6-day-old chick embryos in a rotation culture system. In spheroids raised in the absence of retinal pigmented epithelium (controls), acetylcholinesterase was expressed in cells of an inner nuclear-like layer and their neuropil matrices. Moreover, the ratio between rods and cones was found to be approximately normal throughout the spheroid. When spheroids were cultured in the presence of retinal pigmented epithelium monolayers, cell proliferation in spheroids as determined by BrdU labelling was significantly increased and extended for 1 week, while acetylcholinesterase protein levels and specific activities in homogenates were decreased to approximately 30%. At the same time, opsin immunoreactivity was completely suppressed within the spheroid and appeared slowly in cells around its periphery; i.e. the proportion of rhodopsin-positive cells decreased from 14 to 3%. This study reveals that the retinal pigmented epithelium in vitro sustains cell proliferation but inhibits the differentiation of acetylcholinesterase-positive cells and of photoreceptors.

Butyrylcholinesterase-positive cells of the developing chicken retina that are non-cholinergic and GABA-positive

Butyrylcholinesterase (BChE) is closely related to acetylcholinesterase (AChE), but its function in nervous system development or physiology is unclear. Here, the distribution of BChE was investigated by immunohistochemical methods in the developing chick retina. Using a specific anti-BChE antibody, we detected immunoreactivity associated with different cell types in two nuclear layers and in plexiform layers of the retina. At embryonic day 10 (E10), a transient BChE staining is detected in the inner plexiform layer (IPL) and in radial cells, the latter possibly representing Müller glia. At E12, a subpopulation of amacrine cells appeared, followed by cells in the middle and outer half of the inner nuclear layer. These cells at locations of amacrine, bipolar and horizontal cells represented the predominant three cell types persisting until hatching. The BChE+ amacrine cells were studied in more detail. Their distribution was not significantly different in the central and peripheral retina. Double labelling experiments revealed that BChE+ amacrine cells did not express choline acetyltransferase (ChAT), and, thus, are non-cholinergic. Only a minority of them coexpressed AChE. On the other hand, the majority of them colocalized with anti-GABA immunoreactivity. Taken together, these data support a hitherto unsuspected role of BChE in non-cholinergic cells, possibly in conjunction with GABA.

Influence of technique and transplantation site on rosette formation in rabbit retinal transplants

In order to determine mechanical and host-graft related interactions in the histogenesis of retinal transplants, a new technique for transplanting flat and comparatively large pieces of embryonic rabbit retina into adult rabbit eyes was elaborated. With the procedure, free-floating grafts in the epiretinal space survive, develop and differentiate largely without rosette formation, suggesting that the dissection and transplantation procedure is adequate for obtaining a normal development. On the other hand, subretinal transplants mature at an apparently faster pace than epiretinal transplants, but do not become regularly laminated. Outer segments do not develop well in the epiretinal transplants, whereas they do so in the subretinal ones, suggesting host-graft interactions by means of yet unknown diffusible factors.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Müller glia stabilizes cell columns during retinal development: lateral cell migration but not neuropil growth is inhibited in mixed chick-quail retinospheroids

Radial columnar organization of cell clones is a characteristic feature of vertebrate retinae that is structurally not understood. Here we provide in vitro evidence that Müller glia processes stabilize cells within columns. Dissociated embryonic chick retinal plus pigmented cells regenerate in vitro into fully laminated stratospheroids. After reaggregating chick and quail cells, quail-derived spheroid areas are detected as isolated sectors, as shown by a quail-specific antibody. Each sector contains one or multiple cell columns. The radial borders separating chick and quail sectors are fully congruent with the extension of 3A7-labelled Müller glia processes. While cell somata do not show any lateral interspecies mixing, quail-derived neuropil extends within the inner plexiform areas far into chick sectors. After selective damage of Müller cells by the gliotoxin DL-alpha-aminoadipic acid, the columnar organization is destabilized, as evidenced by a decrease in vimentin expression and by the migration of individual neurons out of their cell column. These data demonstrate that Müller cells actively stabilize cells within their columns, while neuritic growth is not hindered.

Contactin/F11 and tenascin-C co-expression in the chick retina correlates with formation of the synaptic plexiform layers

The neural immunoglobulin-like cell adhesion molecule contactin/F11 and the extracellular matrix glycoprotein tenascin-C are prominent molecules in the developing nervous system which interact in in vitro assays (Zisch et al., J. Cell Biol. 119, 203-213). To determine their potential role in neural development, the distribution of tenascin-C and contactin/F11 was examined in the developing chick retina. The onset of both tenascin-C and contactin/F11 expression coincides with the appearance of ganglion cell dendrides and neurites from bipolar and amacrine cells in the inner layer (IPL) at E8, and the extension of bipolar and horizontal cell processes in the outer plexiform layer (OPL) at E9. Contactin/F11 expression is co-ordinately upregulated with the TN190 and TN200 tenascin-C isoforms between embryonic day 8 (E8) and E17, while little, if any, of the TN220 isoform, which does not bind contactin/F11, is detected. In situ hybridization reveals that tenascin-C and contactin/F11 mRNAs are synthesized by different neuronal types. Tenascin-C mRNA probes hybridize to amacrine and displaced amacrine neurons, and horizontal neurons. In cultured retinal cells, tenascin-C is also present on process-bearing neurofilament-positive cells. Contactin/F11 mRNA is detected in bipolar cells or their precursors from E8-9, and later in horizontal and ganglion neurons. The highest levels and greatest overlap in the synaptic IPL and OPL are reached at E17, when the stratification of the retina is nearly complete. These results are consistent with a putative role for contactin/F11-tenascin-C interactions in the establishment of synaptic layers in the retina.

Overexpressed monomeric human acetylcholinesterase induces subtle ultrastructural modifications in developing neuromuscular junctions of Xenopus laevis embryos

Formation of a functional neuromuscular junction (NMJ) involves the biosynthesis and transport of numerous muscle-specific proteins, among them the acetylcholine-hydrolyzing enzyme acetylcholinesterase (AChE). To study the mechanisms underlying this process, we have expressed DNA encoding human AChE downstream of the cytomegalovirus promoter in oocytes and developing embryos of Xenopus laevis. Recombinant human AChE (rHAChE) produced in Xenopus was biochemically and immunochemically indistinguishable from native human AChE but clearly distinguished from the endogenous frog enzyme. In microinjected embryos, high levels of catalytically active rHAChE induced a transient state of over-expression that persisted for at least 4 days postfertilization. rHAChE appeared exclusively as nonassembled monomers in embryos at times when endogenous Xenopus AChE displayed complex oligomeric assembly. Nonetheless, cell-associated rHAChE accumulated in myotomes of 2- and 3-day-old embryos within the same subcellular compartments as native Xenopus AChE. NMJs from 3-day-old DNA-injected embryos displayed fourfold or greater overexpression of AChE, a 30% increase in postsynaptic membrane length, and increased folding of the postsynaptic membrane. These findings indicate that an evolutionarily conserved property directs the intracellular trafficking and synaptic targeting of AChE in muscle and support a role for AChE in vertebrate synaptogenesis.