Development of the rabbit retina: II. Müller cells

Transcriptomics of CD29+/CD44+ cells isolated from hPSC retinal organoids reveals a single cell population with retinal progenitor and Müller glia characteristics

Müller glia play very important and diverse roles in retinal homeostasis and disease. Although much is known of the physiological and morphological properties of mammalian Müller glia, there is still the need to further understand the profile of these cells during human retinal development. Using human embryonic stem cell-derived retinal organoids, we investigated the transcriptomic profiles of CD29+/CD44+ cells isolated from early and late stages of organoid development. Data showed that these cells express classic markers of retinal progenitors and Müller glia, including NFIX, RAX, PAX6, VSX2, HES1, WNT2B, SOX, NR2F1/2, ASCL1 and VIM, as early as days 10-20 after initiation of retinal differentiation. Expression of genes upregulated in CD29+/CD44+ cells isolated at later stages of organoid development (days 50-90), including NEUROG1, VSX2 and ASCL1 were gradually increased as retinal organoid maturation progressed. Based on the current observations that CD24+/CD44+ cells share the characteristics of early and late-stage retinal progenitors as well as of mature Müller glia, we propose that these cells constitute a single cell population that upon exposure to developmental cues regulates its gene expression to adapt to functions exerted by Müller glia in the postnatal and mature retina.

Age differential response to bevacizumab therapy in choroidal neovascularization in rabbits

Choroidal neovascularization (CNV) in young rabbits has been shown to have a rapid, robust response after treatment with bevacizumab, an anti-vascular endothelial growth factor (VEGF) medication. This investigation evaluates an age differential response to bevacizumab in older populations of rabbits using multimodal high resolution molecular imaging. Young (4 months old) and life span (14 months old) rabbits were given subretinal injections of Matrigel and VEGF to produce CNV. All CNV rabbit models were then treated with a bevacizumab intravitreal injection. Rabbits were then monitored longitudinally using photoacoustic microscopy (PAM), optical coherence tomography (OCT), color photography, and fluorescence imaging. Chain-like gold nanoparticle clusters (CGNP) conjugated with tripeptide arginylglycylaspartic acid (RGD) was injected intravenously for molecular imaging. Robust CNV developed in both young and old rabbits. After intravitreal bevacizumab injection, fluorescence signals were markedly decreased 90.13% in the young group. In contrast, old rabbit CNV area decreased by only 10.56% post-bevacizumab treatment. OCT images confirmed a rapid decrease of CNV in the young group. CGNPs demonstrated high PAM signal in old rabbits and minimal PAM signal in young rabbits after bevacizumab, indicating CNV regression. There is a significant difference in response to intravitreal bevacizumab treatment between young and old rabbits with CNV which can be monitored with multimodal molecular imaging. Old rabbits demonstrate significant persistent disease activity. This represents the first large eye model of persistent disease activity of CNV and could serve as the foundation for future investigations into the mechanism of persistent disease activity and the development of novel therapies.

International Harmonization of Nomenclature and Diagnostic Criteria (INHAND): Nonproliferative and Proliferative Lesions of the Rabbit

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions Project (www.toxpath.org/inhand.asp) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally accepted nomenclature for proliferative and non-proliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying microscopic lesions observed in most tissues and organs from the laboratory rabbit used in nonclinical safety studies. Some of the lesions are illustrated by color photomicrographs. The standardized nomenclature presented in this document is also available electronically on the internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous lesions as well as lesions induced by exposure to test materials. Relevant infectious and parasitic lesions are included as well. A widely accepted and utilized international harmonization of nomenclature for lesions in laboratory animals will provide a common language among regulatory and scientific research organizations in different countries and increase and enrich international exchanges of information among toxicologists and pathologists.

Gene Editing in Rabbits: Unique Opportunities for Translational Biomedical Research

The rabbit is a classic animal model for biomedical research, but the production of gene targeted transgenic rabbits had been extremely challenging until the recent advent of gene editing tools. More than fifty gene knockout or knock-in rabbit models have been reported in the past decade. Gene edited (GE) rabbit models, compared to their counterpart mouse models, may offer unique opportunities in translational biomedical research attributed primarily to their relatively large size and long lifespan. More importantly, GE rabbit models have been found to mimic several disease pathologies better than their mouse counterparts particularly in fields focused on genetically inherited diseases, cardiovascular diseases, ocular diseases, and others. In this review we present selected examples of research areas where GE rabbit models are expected to make immediate contributions to the understanding of the pathophysiology of human disease, and support the development of novel therapeutics.

Extrafoveal Müller cells detection in vivo in the human retina: A pilot study based on optical coherence tomography

Müller cells (MC) represent a key element for the metabolic and functional regulation of the vertebrate retina. The aim of the present study was to test the feasibility of a new method for the in-vivo detection and quantification of extrafoveal MC in human retina. We developed a new approach to isolate and analyse extrafoveal MC in vivo, starting from structural optical coherence tomography data. Our pilot investigation was based on the optical properties of MC, which are known to not interfere with the light reaching the outer retinal structures. We reconstructed MC in the macular region of 18 healthy subjects and the quantitative analyses revealed ~42,000/9 mm² cells detected. Furthermore, we included 2 patients affected by peripheral intraocular melanoma, with macular sparing, needing surgical enucleation. We used these two eyes to perform a qualitative comparison between our reconstructions and histological findings. Our study represents the first pilot investigation dedicated on the non-invasive isolation and quantification of MC, in-vivo, in human retina. Although we are aware that our study has several limitations, first of all related with the proper detection of foveal MC, because of the peculiar z-shape morphology, this approach may open new opportunities for the non-invasive in vivo analysis of MC, providing also potential useful perspectives in retinal diseases.

Postnatal mammalian retinal development: quantitative data and general rules

This article is aimed at providing comparative quantitative data about postnatal mammalian retina development, and at searching for some general rules at both the descriptive and the mechanistic level. In mammals the eye continues to grow, and the retina continues to expand, much after the end of retinal cytogenesis. Thus, although the total number of retinal cells remains constant after cessation of mitotic activity (and the end of 'physiological cell death'), the retinal surface area increases by a factor of two or more. In most mammals, ocular growth exceeds retinal expansion: the neural retina lines 70-80% of the inner ocular surface at the beginning but only about 40-60% in adults. Differential local expansion of the retina (the peripheral area increases more than the central one) can be explained by 'passive stretching' of the retinal tissue by the growing eyeball; it depends on the different biomechanical properties of the peripheral vs. central retinal tissue. The increasing retinal surface area allows for a re-distribution of cells such that the thickness of the (particularly, outer) nuclear layer(s) decreases proportional to the areal expansion. This causes a considerable developmental reduction of the number of cell nuclei 'stacked above each other' by a factor of more than two, and requires a translocation of the somata against their neighbors. We provide a physico-mathematical model of these oblique 'down-sliding' movements of the photoreceptor cell somata along the Müller cell process in the center of their columnar cell unit.

Morphometric analysis of the retina from horses infected with the Borna disease virus

Borna disease (BD) is a fatal disorder of horses, often characterized by blindness. Although degeneration of retinal neurons has been demonstrated in a rat model, there are controversial data concerning whether a similar degeneration occurs in the retina of infected horses. To investigate whether BD may cause degeneration of photoreceptors and possibly of other neuronal cells at least at later stages of the disease, we performed a detailed quantitative morphologic study of retinal tissue from Borna-diseased horses. BD was diagnosed by detection of pathognomonic Joest-Degen inclusion bodies in the postmortem brains. Paraffin sections of paraformaldehyde-fixed retinae were used for histologic and immunohistochemical stainings. Numbers of neurons and Müller glial cells were counted, and neuron-to-Müller cell ratios were calculated. Among tissues from 9 horses with BD, we found retinae with strongly altered histologic appearance as well as retinae with only minor changes. The neuron-to-Müller cell ratio for the whole retina was significantly smaller in diseased animals (8.5 +/- 0.4; P < .01) as compared with controls (17.6 +/- 0.8). It can be concluded that BD in horses causes alterations of the retinal histology of a variable degree. The study provides new data about the pathogenesis of BD concerning the retina and demonstrates that a loss of photoreceptors may explain the observed blindness in infected horses.

Switch of K+ buffering conditions in rabbit retinal Müller glial cells during postnatal development

Although spatial buffering of excess extracellular K+ by K+ channels is a main function of retinal glial (Müller) cells, there are severe limitations to long distance K+-spatial buffering that have been predicted for (immature) glial cells: (i) a lack of inwardly rectifying K+ (Kir) channels [Glia 21(1997) 46]; and (ii) high internal resistance of outgrowing (cable like) processes [W. Rall, Handbook of Physiology, Section 1, vol. 1, Part 1, American Physiological Society, Bethesda, 1977, pp. 39-97]. In order to determine if changes in developing Müller cells improve or worsen their capability of carrying K+ spatial buffering currents, we compared the whole-cell currents of acutely isolated Müller cells at 5, 11 and 28 postnatal days of rabbits. Both K+-spatial buffer limitations described above were found in early postnatal stage (5 days), however, the cells overcome these limitations shortly after 11 days. During the period of 11-28 days, rabbit Müller cells simultaneously increase stalk axial conductance and express Kir channels. Both processes take place during the critical stage of retinal maturation, and should dramatically improve "cable" K+-spatial buffering.

Selective staining by vital dyes of Müller glial cells in retinal wholemounts

Müller glial cells within the retina may respond to different signaling molecules with an elevation of their intracellular free calcium. To prove the localization of the recorded calcium responses in Müller cells within acutely isolated retinal wholemounts, retinal pieces from adult animals and humans were exposed to different vital dyes just after the calcium imaging records were finished. The dyes, Mitotracker Orange, Mitotracker Green, Celltracker Orange, Celltracker Green, and monochlorobimane, are all selectively taken up by Müller glial cells, while neuronal cells remain largely devoid of the dyes. By using this method, it can be demonstrated that the free calcium alterations within the wholemounts indeed occur within Müller cells. Moreover, the cross-sectional areas of (dye-filled) Müller glial cell bodies, as well as of (dye-free) neuronal cell bodies, can be measured in retinal wholemounts, and the spatial densities of both types of cells can be determined. The vital dye loading of Müller cells may facilitate investigations of stimulus-induced alterations of retinal glial cell physiology and morphology.

Upregulation of purinergic P2Y receptor-mediated calcium responses in glial cells during experimental detachment of the rabbit retina

To investigate injury-induced alterations of purinergic P2Y receptor-mediated calcium responses in glial (Müller) cells of the rabbit retina, neural retinae were experimentally detached from the pigment epithelium. The ATP-evoked calcium responses were recorded in the endfeet of glial cells at the vitread surface of retinal wholemounts. In control retinae, approximately 7% of the glial cells investigated showed ATP-evoked calcium responses. Within 24 h of detachment, significantly more retinal glial cells (42%) showed calcium responses, and glial ATP responsiveness increased further in retinae which were detached for 48 (44%) or for 72 h (64%). The results indicate that in the detached retina, glial cells upregulate their responsiveness to extracellular ATP within 24 h of injury. Thus, P2Y receptor-mediated signalling may be involved in the early steps of glial response to retinal injury.

ATP-evoked calcium responses of radial glial (Müller) cells in the postnatal rabbit retina

Here we show that rabbit Müller cell differentiation from radial glial progenitor cells is accompanied by a decreasing capability to respond to specific stimuli (depolarization and extracellular adenosine 5'-triphosphate [ATP]) with an elevation of intracellular calcium. Intracellular free calcium was recorded in retinal wholemounts from young (postnatal days [P] 2 to 31) and adult rabbits. Images were taken from the nerve fiber/ganglion cell layers where the endfeet of radial glial/ Müller cells can be identified after selective uptake of calcium-sensitive dyes. The area of responding endfeet was determined as the percentage of the total area occupied by Müller cell endfeet, as an estimate of the percentage of responding cells. In response to depolarization (50 mM potassium), an increase of intracellular free calcium occurred in 19% of cells from young postnatal retinae (P2-31) but only in 2% from adults. This depolarization-induced calcium rise was caused both by a calcium influx from extracellular space and by an intracellular calcium release. The latter response was inhibited by the P2 receptor blocker pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid (PPADS), indicating that extracellular calcium-independent ATP release into the extracellular space occurs during retinal depolarization. When extracellular ATP (200 microM) was applied, calcium responses were recorded in 83% of cells from young postnatal retinae (P2-6); in the course of further development, both the percentage of responding cells (7% in retinae from adult rabbits) and the amplitude of the calcium responses decreased. It is concluded that during the differentiation of immature radial glia into mature Müller cells, stimulus-evoked intracellular calcium signaling mechanisms change.

Müller cells in developing rats with inherited retinal dystrophy

Morphology and enzymes of Müller cells in the developing retina of RCS (Royal College of Surgeons) rats were investigated. RCS (rdy/rdy) rats with inherited retinal dystrophy were studied and RCS (-/+) rats served as normal controls. Rats underwent intracardiac perfusion with 4% paraformaldehyde and the eyes were enucleated on postnatal days P1, 4,10, 21, 35, and 100. Eyes were then fixed with 4% paraformaldehyde, and silver enhancing technique was applied to show glutamine synthetase (GS) and glial fibrillary acidic protein (GFAP). For solubilized retinas, Western blot analysis and enzyme-linked immunosorbent assay (ELISA) were performed to detect GS and GFAP in the extracts. Immunohistochemistry showed GS expression first on P10. It increased later in both normal and dystrophic retinas. GFAP was not expressed in normal retinas, but Müller cells of dystrophic retinas were stained on P35 and P100. GS immunoblots were recognized on P21 and later in both normal and dystrophic retinas with similar densities, while GFAP immunoblots were observed only on P35 and P100, and only in dystrophic retinas. ELISA demonstrated increased GS concentrations with the development in both normal and dystrophic retinas, but no significant difference was observed between them. GFAP concentrations had no significant difference on P21 between both groups, those of normal ones remained unchanged later, while those of dystrophic rats were remarkably increased on P35 and P100. Müller cells might be affected following the progressive degeneration of photoreceptor cells and react to the glio-neuronal relationship.

A role for the fibroblast growth factor receptor in cell fate decisions in the developing vertebrate retina

The mature vertebrate retina contains seven major cell types that develop from an apparently homogenous population of precursor cells. Clonal analyses have suggested that environmental influences play a major role in specifying retinal cell identity. Fibroblast growth factor-2 is present in the developing retina and regulates the survival, proliferation and differentiation of developing retinal cells in culture. Here we have tested whether fibroblast growth factor receptor signaling biases retinal cell fate decisions in vivo. Fibroblast growth factor receptors were inhibited in retinal precursors in Xenopus embryos by expressing a dominant negative form of the receptor, XFD. Dorsal animal blastomeres that give rise to the retina were injected with cDNA expression constructs for XFD and a control non-functional mutant receptor, D48, and the cell fates of transgene-expressing cells in the mature retina determined. Fibroblast growth factor receptor blockade results in almost a 50% loss of photoreceptors and amacrine cells, and a concurrent 3.5-fold increase in Muller glia, suggesting a shift towards a Muller cell fate in the absence of a fibroblast growth factor receptor signal. Inhibition of non-fibroblast-growth-factor-mediated receptor signaling with a third mutant receptor, HAVO, alters cell fate in an opposite manner. These results suggest that it is the balance of fibroblast growth factor and non-fibroblast growth factor ligand signals that influences retinal cell genesis.

Mechanisms of hyperoxia-induced reductions in retinal blood flow in newborn pig

Although reductions in retinal blood flow (RBF) in response to acute hyperoxia are well described, the mechanistic basis of this response has yet to be clarified. The present study was undertaken in order to determine the possible involvement of two arachidonic acid-derived vasoconstrictors, the cyclooxygenase metabolite thromboxane and the cytochrome P450 metabolite 20-HETE, as well as the involvement of the peptide endothelin and superoxide free radical. Fluorescein videoangiography was performed on the intact eyes of isoflurane-anesthetized newborn piglets. RBF responses to 20 min of hyperoxia were calculated from the angiograms off-line, using changes in mean arteriovenous transit times and arteriolar and venular diameters. The effect of hyperoxia (PaO2=351+/-9 mmHg; n=39) on RBF was examined in each animal under control conditions and again after intravitreal perivascular administration of drugs that block the synthesis or receptors of known vasoconstrictors. Estimated RBF decreased by a maximum of 42+/-3% in the 7 animal groups in response to 20 min of hyperoxia. The magnitude and time course of the change in RBF resulting from two successive hyperoxic challenges did not differ, and were unaffected by intravitreal administration of vehicle. The response to hyperoxia was attenuated 46+/-6 (n=6; P=0.001) after intravitreal CGS 22652 (2 nmol), a combined thromboxane synthesis inhibitor and receptor antagonist. DDMS (12.5 nmol), a competitive inhibitor of the P450 enzyme omega-hydroxylase that forms 20-HETE, blocked hyperoxic constriction by 23+/-7% (n=6; P=0.01). Intravitreal pretreatment with TBC 1241z (2 nmol), a receptor antagonist of the peptide endothelin, blocked the hyperoxic response by 26+/-5% (n=6; P=0.01). A combination of CGS 22652 (2 nmol), DDMS (12.5 nmol), and TBC 1241z (2 nmol), blocked the hyperoxic flow response by 51+/-3% (n=5; P=0.003). Administration of a combination of superoxide dismutase (10 U intravitreally, 10000 U kg-1 of the polyethylene glycol-conjugate intravenously) and catalase (10 U intravitreally, 10000 U kg-1 intravenously) was without effect on hyperoxia-induced reductions in RBF (n=5). The present results indicate that the arachidonic acid metabolites thromboxane and 20-HETE, and the peptide endothelin, participate in mediating the acute reduction in RBF in response to hyperoxia.

Glia cells of the monkey retina--II. Müller cells

In this paper, for the first time a quantitative description of the morphology and distribution of Müller cells in the macaque monkey retina using immunohistochemistry and high resolution confocal laser scanning microscopy is given. By their morphological features Müller cells are ideally adapted to their neuronal environment in the various retinal layers, with a dense network of horizontal processes, especially in the inner plexiform layer, and close contacts to neuronal somata especially in the outer nuclear layer and ganglion cell layer. Morphology varies with retinal eccentricity. The thickness of the inner trunk increases significantly with increasing retinal eccentricity. According to the overall thickness of the retina, Müller cells in central retina are longer than in peripheral regions. In the parafoveal region, the outer trunks of Müller cells in the outer plexiform layer are immensely elongated. These Müller fibres can reach lengths of several hundred micrometers as they travel through the outer plexiform layer from the foveal centre towards the foveal border where they enter the inner nuclear layer. Müller cell density varies between 6000 cells/mm2 in far peripheral and peak densities of > 30,000 cells/mm2 in the parafoveal retina. There is a close spatial relationship between Müller cells and blood vessels in the monkey retina, suggesting a role of Müller cells in the formation of the blood-retinal barrier, in the uptake of nutrients and the disposal of metabolites.

QRI, a retina-specific gene, encodes an extracellular matrix protein exclusively expressed during neural retina differentiation

Neural retina development results from growth arrest of neuroectodermal precursors and differentiation of postmitotic cells. The QRI gene is specifically expressed in Müller retinal glial cells. Its expression coincides with the stage of withdrawal from the cell cycle and establishment of differentiation and is repressed upon induction of retinal cell proliferation by the v-src gene product. In this report, we show that the QR1 gene encodes several glycosylated proteins that are secreted and can either associate with the extracellular matrix or remain diffusible in the medium. By using pulse-chase experiments, the 100-103 kDa forms seem to appear first and are specifically incorporated into the extracellular matrix, whereas the 108 and 60 kDa polypeptides appear later and are detected as soluble forms in the culture medium. We also report that expression of the QR1 gene is developmentally regulated in the chicken. Its mRNA is first detectable at embryonic day 10, reaches a maximal level at embryonic day 15 and is no longer detected at embryonic day 18. Immunolocalization of the QR1 protein in chicken retina sections during development shows that expression of the protein parallels the differentiation pattern of post-miotic cells (in particular Müller cells and rods), corresponding to the two differentiation gradients in the retina: from the ganglion cell layer to the inner nuclear layer and outer nuclear layer, and from the optic nerve to the iris. At embryonic day 10, expression of the QR1 protein(s) is restricted to the optic nerve region and the inner nuclear layer, colocalizing with Müller cell bodies. As development proceeds, QR1 protein localization spreads towards the iris and towards the outer nuclear layer, following Müller cell elongations towards the photoreceptors. Between embryonic days 16 and 18, the QR1 protein is no longer detectable in the optic nerve region and is concentrated around the basal segment of the photoreceptors in the peripheral retina. Our results suggest a role for the QR1 gene product in the process of growth arrest and establishment of photoreceptor differentiation.

Dystroglycan expression in the wild type and mdx mouse neural retina: synaptic colocalization with dystrophin, dystrophin-related protein but not laminin

Alpha- and beta-dystroglycan (alpha- and beta-DG) are members of a dystrophin-associated glycoprotein complex (DGC) in skeletal muscle which binds to agrin and laminin, and has been postulated to be involved in myoneural snyapse formation. The absence of functional dystrophin in Duchenne muscular dystrophy (DMD) and in one of its animal models, the mdx mouse, leads to a reduction of alpha- and beta-DG in muscle, and is often associated with mental retardation and abnormal retinal synaptic transmission in DMD. Using immunohistochemistry, we find that alpha- and beta-DG are expressed in the outer plexiform layer of both wild type and mdx retina, where both dystrophin and dystrophin-related protein (DRP), but not laminin are present. In situ hybridization identifies two neuronal populations, photoreceptors and retinal ganglion cells, that express DG mRNA. Alpha- and beta-DG are also expressed in the inner limiting membrane and around blood vessels where they colocalize with laminin and DRP. Western blot analysis revealed the expression of several dystrophin isoforms in wild type and mdx retina, possibly explaining the unaltered expression of alpha- and beta-dystroglycan in the mdx central nervous system (CNS). Our results support the hypothesis that alpha- and beta-DG can interact with dystrophin and DRP in the CNS and perform functions analogous to those of the DGC in muscle.

Müller glial cells of the tree shrew retina

The tree shrew is one of the few mammalian species whose retinae are strongly cone dominated, which is usually the case in reptilian and avian retinae. Müller cells of the tree shrew (Tupaia belangeri) retina were studied by transmission electron microscopy of tissue sections and freeze-fracture replicas, by immunolabeling of the intermediate filament protein vimentin in radial paraffin sections and in whole retinae, as well as by intracellular dye injection in slices of retinae. In addition, enzymatically isolated cells were stained by Pappenheim's panoptic staining method. The cells showed an ultrastructure that is similar to other mammalian Müller cells with two exceptions: Due to the extensive lateral fins of cone inner segments, the apical microvilli of Müller cells are arranged in peculiar palisades, and the basket-like Müller cell sheaths around neuronal somata in both nuclear layers consist of unusual multilayered membrane lamellae. Unlike Müller cells in other mammalian species studied thus far, but similar to reptilian and avian Müller cells, those of tree shrews commonly have two or more vitread processes rather than one main trunk. Müller cell densities range between some 13,000 mm-2 in the periphery and about 20,000 mm-2 in the retinal center. Neuron:(Müller)glial cell ratios were estimated to be 7.9:1 in the center and 6.2:1 in the periphery. For each Müller cell, about 1.5 (cone) photoreceptor cells, four or five interneurons of the inner nuclear layer, and about one cell of the ganglion cell layer were counted. This is a much lower number of neurons per Müller cell than in most other mammals studied.

Development of A-type (axonless) horizontal cells in the rabbit retina

The development of A-type horizontal cells (HC) was studied in the rabbit retina between embryonic day (E)24 and adulthood [the day of birth was called postnatal day (P)1 and corresponds to E31-32]. The cells were visualized by several methods 1) by immunolabeling with antibodies to neurofilament 70,000 (NF-70kD), 2) by immunolabeling with antibodies to a calcium binding protein (CaBP-28kD), 3) by two different methods of silver impregnation, and 4) by histochemical demonstration of NADH-diaphorase activity. Most methods labeled A-type HC only in the dorsal retina; thus, our study is restricted to HC of this region. HC densities were determined at each developmental stage. The cells were drawn at scale, and size, quotient of symmetry, and topographical orientation of dendritic trees were studied by image analysis. The growth of HC dendritic fields was correlated with data on the postnatal local retinal expansion, which is known to be driven by the intraocular pressure (after cessation of retinal cell proliferation at P9). This expansion was evaluated in an earlier paper (Reichenbach et al. [1993] Vis. Neurosci. 10:479-498) by using local subpopulations of Müller cells as "markers" of distinct topographic regions of the retinae. After E24, when the final number of HC is established, we can discriminate three distinct developmental stages of A-type HC. During the first stage, between E24 and E27, the young cells are often vertically oriented and may extend their first short dendrites within (the primordia of) both plexiform layers. The irregular HC mosaic at E24 shows a significant difference to all other stages. The second stage begins after birth when the dendritic trees of the cells are already restricted to the outer plexiform layer. Between P3 and P9, their dendritic trees enlarge more than the surrounding retinal tissue expands, and the coverage factor almost doubles from 2.5 to 4.4. The third stage occurs after P9 when the growth rate of dendritic tree areas corresponds to that of the local retinal tissue expansion caused by "passive stretching" of the postmitotic tissue, and the coverage factor remains constant. This is compatible with the view that mature synaptic connections of A-type HC are mostly established after the first week of life and are then maintained.

Vitread proliferation of filamentous processes in avian Müller cells and its putative functional correlates

In order to examine to what extent the neuronal and metabolic activities of avascular vertebrate retinae are reflected in the morphology of their Müller cells we have studied (by using several monoclonal antibodies) the morphology of Müller cells in two species of diurnal birds (chicken, Gallus domesticus, and pigeon, Columba livia) and one species of nocturnal saltwater crocodiles (Crocodylus porosi). In all three species, the outer nuclear layer is relatively thin and the Müller cell trunks divide into rootlets that wrap around the photoreceptors. In both diurnal birds studied, the trunks of Müller cells in the inner plexiform layers invariably divide into numerous fine filamentous processes that terminate in small expansions covering most of the vitreal surface of the retina. Furthermore, the networks of filamentous processes of birds' Müller cells exhibit conspicuous horizontal lamination in the inner plexiform layer. In contrast, the filamentous processes arising from the individual Müller cell trunks of the crocodile, if present, are much less numerous and less widely spread than those of diurnal birds. It is proposed that the splitting of the Müller cell trunks into numerous filamentous processes terminating in small vitreal expansions represents a morphological adaptation for: 1) effective spatial buffering of K+ ions in thick and presumably metabolically highly active, yet avascular, avian retinae, and 2) effective absorption and distribution of nutrients leaking from the vitreally located supplemental nutritive organ, the pecten.

Early appearance and transient expression of putative amino acid neurotransmitters and related molecules in the developing rabbit retina: an immunocytochemical study

We have studied, by immunocytochemistry, the ontogeny of GABA, glycine, glutamate, glutamine, and taurine-containing cells in the rabbit retina. Amacrine cells show GABA immunoreactivity by embryonic day 25 (E25) and throughout postnatal life. By contrast, ganglion cells and horizontal cells are only transiently GABA-immunoreactive (-IR); few appear GABA-IR by the third postnatal week. At maturity, glycine is present in amacrine cells and in some bipolar cells. During development, putative ganglion cells transiently contained glycine between E25 and postnatal day 3 (P3), whereas immunolabelling in presumed amacrine cells and bipolar cells persists after birth. Ganglion cells, bipolar cells, photoreceptors, and some amacrine cells are glutamate-IR in the adult retina. Glutamate immunoreactivity first appears in the somata and processes of cytoblastic cells by E20 and is prominent by E25. Surprisingly, ganglion cells are not strongly glutamate-IR until just before eye-opening, at postnatal day 10 (P10), coincident with the appearance of glutamine in their somata and in Müller glial cells. Bipolar cells are glutamate-IR before they or Müller cells contain high levels of glutamine (at P10). Glutamate immunoreactivity in photoreceptors is progressively restricted to the inner segments by eye-opening. At no stage are presumed horizontal cells glutamate-IR or glutamine-IR, but some amacrine cells show glutamate- and glutamine-IR by P10. Taurine is localized to photoreceptors and Müller glial in the adult retina. Some cytoblasts are taurine-IR at E20; with ensuing development, taurine labelling becomes restricted primarily to Müller cells and photoreceptors; some putative bipolar cells may also be labelled. However, for a few days around birth, cells resembling horizontal cells, also show taurine immunoreactivity. The early appearance and often transient expression of these amino acids in retinal cells suggests that these neuroactive molecules may be involved in the structural and functional development of the retina.

Development of dendritic trees of rabbit retinal alpha ganglion cells: relation to differential retinal growth

To provide a quantitative description of the postnatal development of dendritic trees in alpha ganglion cells of the rabbit retina, these cells were stained either by intracellular injection of Lucifer yellow or by application of the lipophilic dye DiI. This was done at three developmental stages: postnatal day (P) 8/9, P 16/17, and in adults. For different retinal locations we quantified the alpha cell dendritic field area, the number of dendritic branch points, and the average dendritic length between branch points. According to the alpha cell location, the data were collected in three groups representing the retinal center, midperiphery, and far periphery, respectively. The data were then correlated with the postnatal retinal expansion which is known to differ among the above topographic regions of the retinae (Reichenbach et al., 1993). Our results show that the growth of alpha ganglion cell dendrites is not proportional to, but significantly exceeds, that of the local retinal tissue. Between P 8/9 and adulthood, the area of central alpha cells increases almost six-fold from 26,000 to 144,000 microns 2 (retinal expansion: 2.2-fold), and that of peripheral cells more than 15-fold from 35,000 to 556,000 microns 2 (retinal expansion: four-fold). During this period, the coverage factor of alpha cell dendritic fields increases about three-fold, and reaches adult levels of about 3 (retinal center) and 2.2 (periphery), respectively. The number of dendritic branch points remains nearly constant, and the distance between them increases by a factor close to the square root of the factor by which the dendritic field area grows. Thus, it appears that, from the second postnatal week on, dendritic trees of rabbit alpha ganglion cells increase by intense "interstitial growth," rather than by outgrowth of (new) dendritic branches. This growth pattern is different from that of some other rabbit retinal ganglion cell types, and of alpha ganglion cells of the cat retina, whose dendritic trees expand at a rate equal to or less than that of the surrounding retinal tissue. The consequences for synaptic contacts with bipolar and amacrine cells are discussed; they suggest a high degree of synaptic plasticity during normal postnatal retinal growth.

Growth factor effects on the proliferation of different retinal glial cells in vitro

Vascularized mammalian retinae contain two distinct neuroglial cells types, radially oriented Müller cells and astrocytes, which are located in the nerve fiber layer. These cell types derive from different precursor cells and proliferate during ontogenesis at distinct schedules. The aim of the present study was to disclose whether growth factors, which are known to interfere with the development of neuroglial cells in the central nervous system, like basic and acidic fibroblast growth factor (aFGF and bFGF), epidermal growth factor (EGF) and platelet-derived growth factor, have similar or distinct effects on the proliferative capacity of retinal astrocytes and Müller cells. These questions were tested by applying growth factors to cultured astrocytes and Müller cells from early postnatal rabbit retina. Proliferating cells were identified by double labeling experiments combining cell type specific markers with bromodeoxyuridine immunocytochemistry and [3H]thymidine incorporation experiments, respectively. In addition, we used the anatomical advantage of the rabbit retina. Its peripheral part is astroglial cell-free. Cultures prepared from this part of the retina (P-cultures) contain Müller cells, microglial cells and neurons, while cultures from the 'central part', the medullary rays (MR) region contain, in addition, astrocytes and oligodendrocytes. Our studies show that Müller cell proliferation is stimulated by EGF in a dose dependent manner, while astrocyte proliferation is stimulated by aFGF and bFGF. The proliferation of O4-positive glial precursor cells is stimulated by aFGF, bFGF and platelet-derived growth factor, but not by EGF. Microglial cells, which are a minor population in these cultures, do not respond to either of these factors.

Development of the rabbit retina. V. The question of 'columnar units'

A qualitative and quantitative description of the columnar units in the mammalian retina, and a discussion of their ontogeny and putative functions is given. Columnar arrangements of cells exist in the developing retina which can be observed by means of scanning electron microscopy. In the adult retina, each Müller cell ensheaths a columnar group of neuronal cells. Counting the number of cells in radial H/E stained sections at various developmental stages reveals a constant ratio of neuronal cells per Müller cell, independent of the developmental stage (after postnatal day 9), and independent of the retinal topography. Such groups of cells always consist of one Müller cell, 11 rod photoreceptor cells, about 2 bipolar cells, and 1 to 2 amacrine cells. Retinal ganglion cells, cone photoreceptor cells, and horizontal cells are more sparsely distributed in the retina than these units; since they are known to arise earlier in the ontogenesis than other cell types they are considered to exist independently of the columnar units. It is suggested that the units arise by migration of groups of preneurons along a common Müller (precursor) cell; these preneurons and the corresponding Müller cell may be clonally related. In the adult retina, such columns might constitute metabolic and functional units.

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.

Morphology and distribution of Müller cells in the retina of the toad Bufo marinus

We have previously shown that an antibody against neuron-specific enolase (NSE) selectively labels Müller cells (MCs) in the anuran retina (Wilhelm et al. 1992). In the present study the light- and electron-microscopic morphology of MCs and their distribution were described in the retina of the toad, Bufo marinus, using the above antibody. The somata of MCs were located in the proximal part of the inner nuclear layer and were interconnected with each other by their processes. The MCs were uniformly distributed across the retina with an average density of 1500 cells/mm2. Processes of MCs encircled the somata of photoreceptor cells isolating them from each other by glial sheath, except for those of the double cones. Some of the photoreceptor pedicles remained free of glial sheath. Electron-microscopic observations confirmed that MC processes provide an extensive scaffolding across the neural retina. At the outer border of the ganglion cell layer these processes formed a non-continuous sheath. The MC processes traversed through the ganglion cell layer and spread beneath it between the neuronal somata and the underlying optic axons. These processes formed a continuous inner limiting membrane separating the optic fibre layer from the vitreous tissue. Neither astrocytic nor oligodendrocytic elements were found in the optic fibre layer. The significance of the uniform MC distribution and the functional implications of the observed pattern of MC scaffolding are discussed.

Organotypic slice culture of the mammalian retina

Vertical slices of 6-day postnatal (P6) rat retina were cut at a thickness of 100 microns and cultured using the roller-tube technique. After 14-21 days in vitro there was significant distortion of normal retinal architecture, but localized areas of the slices showed the typical pattern of layering of mature retina. The following immunocytochemical markers were used to characterize the different retinal cell types: antibodies against protein kinase C (PKC), calcium binding protein (CabP 28kD), neurofilaments (NF), glia-specific antibodies (GFAP, vimentin), and transmitter-specific antibodies (GABA, TH). The expression of these markers was compared in P6 retina, adult retina, and slice culture. To further characterize the cultured cells, patch-clamp recordings were performed in combination with intracellular injection of Lucifer Yellow (LY). Transmitter- and voltage-gated membrane currents were recorded from morphologically identified neurons. The experiments show that a mammalian slice culture can be used to study differentiation and function of retinal cell types.

Development of the rabbit retina. IV. Tissue tensility and elasticity in dependence on topographic specializations

A method is introduced for the quantification of specific compliance and the elasticity of small pieces of living retinal tissue. These pieces are fixed at their margins by means of tissue glue, and loaded with a small iron spherule the bending force of which can be gradually enhanced by the action of an electromagnet. Retinal bending caused by such calibrated forces is measured by a horizontal light microscope, and used for estimations of specific compliance and elasticity of the tissue. Three different particular regions of the rabbit retina--periphery, visual streak, and (prospective) medullary rays--were tested at several post-natal developmental stages. From very early stages on (day 2 p.p.) up to adulthood the peripheral retina was found to be significantly more tensile than the two other central regions. This can be shown to depend greatly on the thickness of the tissue which is lower in the retinal periphery. During early post-natal development, all retinal regions except the (prospective) medullary rays become thinner. The tensility of the tissue increases, with the exception of the medullary rays which reduce their compliance strongly. In the adult retina, however, the tensility of all retinal regions is reduced as compared with the neonatal tissue. This seems to be caused by a constant gradual increase of the elasticity of the retina during development which, in turn, may be caused by several developmental parameters, e.g. the formation of synapses, the outgrowth of glial side branches ensheathing neighbouring neuronal cells, or a reduction in extracellular clefts. It is proposed that these differences in tensility between different retinal regions, may be the cause for differential retinal expansion driven by the intraocular pressure. Thus, simple mechanical features of the tissue may contribute to the formation of important topographic specializations of the retina, e.g. the visual streak as the site of highest visual acuity.