The rabbit retina Müller cell. A fine structural and cytochemical study

Glycogen in retinal horizontal cells of the African mud catfish Clarias gariepinus (Burchell, 1822) and its physiological significance

This paper reports on glycogen store in the retinal horizontal cells (HC) of the African mud catfish Clarias gariepinus, as seen by histochemical reaction with periodic acid Schiff (PAS) and transmission electron microscopy in light- as well as dark-adapted state. Glycogen is abundant in the large somata and less in their axons, characterised ultrastructurally by many microtubules and extensive gap junctions interconnecting them. There was no apparent difference in glycogen content in HC somata between light- and dark adaptation, but the axons clearly showed absence of glycogen in dark condition. The HC somata (presynaptic) make synapses with dendrites in the outer plexiform layer. Müller cell inner processes, which contain more densely packed glycogen, invest the HC. Other cells of the inner nuclear layer do not show any appreciable content of glycogen. Rods, but not cones, contain abundant glycogen in their inner segments and synaptic terminals. It is likely that glycogen is used as energy substrate in hypoxia for this species that dwell muddy aquatic environment with low oxygen content. They appear to have high energy demand, and a high glycogen content in HC could act as a ready source to fulfil physiological processes, like microtubule-based transport of cargo from the large somata to axons and the maintenance of electrical activities across the gap junctions between the axonal processes. It is also likely that they can supplement glucose to the neighbouring inner nuclear layer neurons, which are clearly devoid of glycogen.

Cholesterol homeostasis in the vertebrate retina: biology and pathobiology

Cholesterol is a quantitatively and biologically significant constituent of all mammalian cell membrane, including those that comprise the retina. Retinal cholesterol homeostasis entails the interplay between de novo synthesis, uptake, intraretinal sterol transport, metabolism, and efflux. Defects in these complex processes are associated with several congenital and age-related disorders of the visual system. Herein, we provide an overview of the following topics: (a) cholesterol synthesis in the neural retina; (b) lipoprotein uptake and intraretinal sterol transport in the neural retina and the retinal pigment epithelium (RPE); (c) cholesterol efflux from the neural retina and the RPE; and (d) biology and pathobiology of defects in sterol synthesis and sterol oxidation in the neural retina and the RPE. We focus, in particular, on studies involving animal models of monogenic disorders pertinent to the above topics, as well as in vitro models using biochemical, metabolic, and omic approaches. We also identify current knowledge gaps and opportunities in the field that beg further research in this topic area.

Change in phospholipid species of retinal layer in traumatic optic neuropathy model

Injured optic nerves induce death in almost all retinal ganglion cells (RGC) and cause a loss of axons. To date, we have studied injured RGC axon regeneration by using a traumatic optic nerve injury (TONI) rodent model, and we revealed that axonal regeneration is induced by the graft of an autologous peripheral nerve. The efficient approach to the regeneration of axons thus needs an environmental adjustment of RGC. However, the RGC environment induced by TONI remains unknown. Here, we analyzed female and male C57BL/6 mouse retinal tissue alterations in detail after TONI and focused on the major phospholipid species that are enriched in the whole retina. Reactive astrocyte accumulation, glia scar formation, and demyelination were observed in the injured optic nerve area, while RGC cell death, astrocyte accumulation, and Glial fibrillary acidic protein (GFAP) positive Müller cell increases were detected in the retinal layer. Furthermore, phosphatidylinositol (PI) 18:0/20:4 was localized to three nuclear layer structures: the ganglion cell layer (GCL), the inner nuclear layer (INL), and the outer nuclear layer (ONL) in control retina; however, the localization of 18:0/20:4 PI in TONI was disturbed. Meanwhile, phosphatidylserine (PS) 18:0/22:6 showed that the expression was specifically in the inner plexiform layer (IPL) with similar signal intensity in both cases. Other PS species and phosphatidylethanolamine (PE) were differentially localized in the retinal layer; however, the expressions of PE including docosahexaenoic acid (DHA) were affected by TONI. These results suggest that not only GCL but also other retinal layers were influenced by TONI.

Multiple intrinsic factors act in concert with Lhx2 to direct retinal gliogenesis

Müller glia (MG) are the principal glial cell type in the vertebrate retina. Recent work has identified the LIM homeodomain factor encoding gene Lhx2 as necessary for both Notch signaling and MG differentiation in late-stage retinal progenitor cells (RPCs). However, the extent to which Lhx2 interacts with other intrinsic regulators of MG differentiation is unclear. We investigated this question by investigating the effects of overexpression of multiple transcriptional regulators that are either known or hypothesized to control MG formation, in both wildtype and Lhx2-deficient RPCs. We observe that constitutively elevated Notch signaling, induced by N1ICD electroporation, inhibited gliogenesis in wildtype animals, but rescued MG development in Lhx2-deficient retinas. Electroporation of Nfia promoted the formation of cells with MG-like radial morphology, but did not drive expression of MG molecular markers. Plagl1 and Sox9 did not induce gliogenesis in wildtype animals, but nonetheless activated expression of the Müller marker P27^Kip1 in Lhx2-deficient cells. Finally, Sox2, Sox8, and Sox9 promoted amacrine cell formation in Lhx2-deficient cells, but not in wildtype retinas. These findings demonstrate that overexpression of individual gliogenic factors typically regulates only a subset of characteristic MG markers, and that these effects are differentially modulated by Lhx2.

Müller glial cell reprogramming and retina regeneration

Müller glia are the major glial component of the retina. They are one of the last retinal cell types to be born during development, and they function to maintain retinal homeostasis and integrity. In mammals, Müller glia respond to retinal injury in various ways that can be either protective or detrimental to retinal function. Although these cells can be coaxed to proliferate and generate neurons under special circumstances, these responses are meagre and insufficient for repairing a damaged retina. By contrast, in teleost fish (such as zebrafish), the response of Müller glia to retinal injury involves a reprogramming event that imparts retinal stem cell characteristics and enables them to produce a proliferating population of progenitors that can regenerate all major retinal cell types and restore vision. Recent studies have revealed several important mechanisms underlying Müller glial cell reprogramming and retina regeneration in fish that may lead to new strategies for stimulating retina regeneration in mammals.

Cellular signaling and factors involved in Müller cell gliosis: neuroprotective and detrimental effects

Müller cells are active players in normal retinal function and in virtually all forms of retinal injury and disease. Reactive Müller cells protect the tissue from further damage and preserve tissue function by the release of antioxidants and neurotrophic factors, and may contribute to retinal regeneration by the generation of neural progenitor/stem cells. However, Müller cell gliosis can also contribute to neurodegeneration and impedes regenerative processes in the retinal tissue by the formation of glial scars. This article provides an overview of the neuroprotective and detrimental effects of Müller cell gliosis, with accounts on the cellular signal transduction mechanisms and factors which are implicated in Müller cell-mediated neuroprotection, immunomodulation, regulation of Müller cell proliferation, upregulation of intermediate filaments, glial scar formation, and the generation of neural progenitor/stem cells. A proper understanding of the signaling mechanisms implicated in gliotic alterations of Müller cells is essential for the development of efficient therapeutic strategies that increase the supportive/protective and decrease the destructive roles of gliosis.

Immunocytochemical analysis of glycogen phosphorylase isozymes in the developing and adult retina of the domestic chicken (Gallus domesticus)

Glycogen is the major energy reserve in neural tissues including the retina. A key-enzyme in glycogen metabolism is glycogen phosphorylase (GP) which exists in three differentially regulated isoforms. By applying isozyme-specific antibodies it could be demonstrated that the GP BB (brain), but not the GP MM (muscle) isoform is expressed in the chicken retina in neuronal and glial (Müller) cells. In the embryonic chicken retina, GP showed a development-dependent expression pattern. Double-labeling experiments with cell type-specific antibodies revealed that GP is expressed in various layers of the retina some of which, e.g., the photoreceptor inner segments, are known to be sites of high energy consumption. This suggests important roles of GP BB, and therefore glycogen, in early differentiation, spontaneous wave generation and in formation and stabilization of synapses.

Glycogen phosphorylase isozyme pattern in mammalian retinal Müller (glial) cells and in astrocytes of retina and optic nerve

Müller cells, the radially oriented dominant macroglial cells of the retina, are known to contain abundant glycogen as well as the key enzyme for its degradation, glycogen phosphorylase (GP), but the expressed isozyme pattern is unknown. To elucidate the isoform expression pattern, specific antisera directed against the brain (BB) and muscle (MM) isoforms of GP were applied to retinal sections, isolated Müller cells, and sections of the optic nerve. We show that Müller cells of rat, rabbit, guinea pig, and mouse retina exclusively express the BB isoform. Astrocytes of rat and rabbit optic nerve, as well as retina express only the BB isoform. In contrast, astrocytes in the brain and spinal cord as well as the epithelial cells of the pars caeca and of the ciliary body express both the BB and MM isoform. This result may indicate some differences in the role of glycogen in retinal macroglia and brain astrocytes, reflecting a local specialization of macroglia in the retina proper.

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.

Localization of cathepsin D in rat ocular tissues. An immunohistochemical study

Numerous studies have demonstrated that cathepsin D as a major lysosomal acid protease plays an important role in the degradation of protein in several tissues. An important function of the retinal pigment epithelium is to interact with the photoreceptor cells in the renewal process. During the renewal process, the RPE cell phagocytosis discarded photoreceptor discs which are then degraded in the RPE phagolysosomes. It is believed that cathepsin D plays a main role in the degradation of rod outer segments and rhodopsin into glycopeptides. The cellular localization of cathepsin D immunoreactivity was examined at the light microscopic level in the ocular tissues of non-affected RCS-rdy+ rats strain by use of the alkaline phosphatase-antialkaline phosphatase (APAAP) technique. The presence of cathepsin D immunoreactivity was found in the cell cytoplasm of the following ocular tissues: retinal pigment epithelium; Müller cells; ganglion cells; pigmented and non-pigmented ciliary body; iris tissue; epithelium and endothelium of the cornea; endothelium of various vessels, including the tunica vasculosa lentis. High activity of cathepsin D was found in the RPE cells, as well as in the cytoplasm of Müller cells, especially expressed in their foot plates lying close to the inner limiting membrane.

Immunocytochemical demonstration of glycogen phosphorylase in Müller (glial) cells of the mammalian retina

Glycogen phosphorylase (GP) was immunocytochemically detected in Müller cells of the rabbit and rat retina using a monoclonal antibody raised against bovine brain GP. Immunofluorescence and immunoenzymatic procedure were applied on isolated, Müller cells and sections of paraformaldehyde-fixed, paraffin-embedded retinas. All methods used revealed positive immunostaining. GP immunoreactivity was most intense in the Müller cell endfeet and the pericarya, corresponding to the nerve fibre layer and the inner nuclear layer in the retina. The presence of GP in Müller cells stresses the important role of these glial cells in the energy metabolism of the mammalian retina.

Ultrastructure and permeability of the Schwann cell layer surrounding the giant axon of the squid

The ultrastructure of the Schwann cell layer surrounding the giant axon of the squid Alloteuthis subulata is described, and the permeability of extracellular compartments assessed by exposure to electron-dense tracers. Morphometric analysis is used to deduce the number, size and shape of the Schwann cells, and the routes for ion flux across the Schwann cell layer. Axons (mean diameter 233 microns) were surrounded by a 1-2 microns thick layer of Schwann cells which were approximately 1 micron thick, approximately 70 microns long and approximately 23 microns wide. There were around 62,000 Schwann cells per cm2 axon surface. The outer (abaxonal) surface of the Schwann cells was invaginated, with evidence for a covering of fine Schwann cells processes; the inner (adaxonal) surface of the Schwann cells was less folded. The percentage area occupied by mesaxonal cleft openings to the axon and to the basal lamina was 0.02% and 1.09% respectively. A system of tubules, the glial tubular system, occupied 3.9% of the Schwann cell volume, and opened to both axonal and basal lamina surfaces, with more elaborate lattice-like clusters towards the basal side of the cell. Tubule openings accounted for 0.26% of the surface area facing the axon and 0.37% of the area facing the basal lamina (where there was greater clustering of openings). The electron dense tracers horseradish peroxidase, ionic lanthanum and tannic acid filled mesaxon clefts, glial tubular system and periaxonal space. If ion flux occurred via the mesaxonal clefts, a theoretical series resistance (Rsth) of > 20 omega cm2, would be predicted, whereas if it occurred via the tubular system, the figure would be < 2 omega cm2, closer to physiological estimates. The results presented show that the glial tubular system is likely to be the major route for ion flux into and across the Schwann cell layer, and for clearance of K+ from the periaxonal space during periods of axonal stimulation. The implications for K+ homeostasis in the axonal microenvironment are discussed.

Müller cells in vascular and avascular retinae: a survey of seven mammals

Eight monoclonal antibodies were used to label Müller cells in four mammals that have vascular retinae (cats, dogs, humans, and rats) and in three with avascular retinae (echidnas, guinea pigs, and rabbits). Müller cells were found to have a fairly uniform retinal distribution in seven species, with a mean density of 8,000-13,000 cells mm-2. Müller cells in avascular retinae differ from their vascular counterparts in four respects. First, they are shorter than those in vascular retinae. This difference is mainly due to a reduction in the thickness of the outer nuclear layer. Second, the trunks of Müller cells in avascular retinae tend to be thicker, although those in echidnas are an exception to this trend. Third, Müller cell rootlets in avascular retinae follow a more tortuous course than those in vascular retinae, reflecting the fact that photoreceptor nuclei in the two types of retina have different shapes and stacking patterns. Fourth, due to a reduction in the density of photoreceptors in avascular retinae, there are fewer neurones per Müller cell. Although these four features may enable Müller cells to assist the nutrition of neurones in the inner layers of avascular retinae, they are unlikely to be morphological specializations that have evolved for that purpose. Rather, these features appear to be a direct consequence of the fact that avascular retinae are thinner and have a differently organised outer nuclear layer. These features aside, Müller cells in avascular retinae closely resemble their counterparts in vascular retinae.

In vitro retinal and erythrocyte polyol pathway regulation by hormones and an aldose reductase inhibitor

The effects of a high-glucose medium, insulin, and an aldose reductase inhibitor (ONO-2235) on sorbitol accumulation were compared in the human erythrocyte and the rabbit retina, while the effects of epinephrine on in vitro sorbitol accumulation were investigated in the human and rabbit retina. In both erythrocytes and the retina, linear increments of sorbitol accumulation were observed in a dose-dependent manner with 5 to 50 mM glucose. These increments were markedly inhibited by 100 microM ONO-2235 but not by insulin (400 microU/ml). In the presence of 5 mM glucose, a dose-dependent increase of the sorbitol content of the rabbit retina was seen following epinephrine stimulation (0.4-4.0 microM and this was markedly reduced by 100 microM ONO-2235. Moreover, both 50 mM glucose and 4.0 microM epinephrine increased the sorbitol content of the retina from a diabetic patient, and the glucose-induced increment in sorbitol was significantly reduced by 100 microM ONO-2235. Our data suggested that aldose reductase inhibitors might be useful for the treatment of diabetic retinopathy, since the polyol pathway appears to be an important factor in its pathogenesis, and that catecholamines might have some role in the activation of the retinal polyol pathway.

Müller cells in adult rabbit retinae: morphology, distribution and implications for function and development

We describe the morphology and distribution of Müller cells in wholemounts of rabbit retinae labelled with either monoclonal antibodies (anti-Vimentin, 3H3, 4D6, and 4H11), or intracellular horseradish peroxidase. Several new features of Müller cell organization are noted. First, Müller cells appear to compose a single morphological class and their morphology varies systematically with retinal thickness. Second, in contrast to other retinal glia, Müller cells have a neuronlike distribution, with a peak density of 10,700-15,000 cells per mm2 at the visual streak and a minimum density of 4,400-6,000 per mm2 at both the superior and inferior retinal edges. There are 4.2 +/- 0.5 x 10(6) Müller cells per retina. Third, unlike in other species, rabbit Müller cells do not contact blood vessels, suggesting that they do not participate in the transfer of metabolites or in the blood:retinal barrier. Fourth, each Müller cell has a vitread endfoot about 20-40 microns in diameter composed of numerous fimbriae. The fimbriae from a single Müller cell generally contact several axon fascicles in the nerve fibre layer, and at each point along its length each fascicle is enclosed by the overlapping fimbriae from several Müller cells. Fifth, in the inner and outer plexiform layers, numerous filamentous branchlets extend 20 microns or more from the radial trunk, interweaving with branchlets from nearby Müller cells to form dense and continuous strata. In the ganglion cell layer and outer nuclear layer, Müller cell processes completely wrap neuronal somata, whereas in the inner nuclear layer they partially wrap somata. We discuss the functional and developmental implications of these observations.

Morphological differentiation of the Müller cell: Golgi and electron microscopy study in the chick retina

The sequence of morphological differentiation of Müller cells in the chick retina was investigated in relation to the differentiation of the retinal neurons using the Golgi method. From the beginning of differentiation, the Müller cell develops spurs and lateral processes. Some of these glial processes become transformed into accessory prolongations of the Müller cell. From the 17th or 18th day of incubation, the morphology of the Müller cells is similar to that of the adult retina. On the basis of their inner prolongation, two types of Müller cells were identified. The first type, with diffuse and abundant descending processes, is identical to that described classically. The second type is a cell characterized by sparse and scanty inner ramifications. This report also describes electron microscopic observations of Müller cells and their enwrapping relationship with the axons of the optic nerve fiber layer.

Agonist-induced glycogenolysis in rabbit retinal slices and cultures

1. The effects of different putative retinal transmitters and/or modulators on glycogenolysis in rabbit retinal slices and in retinal Müller cell cultures were examined. 2. Incubation of rabbit retinal slices or primary retinal cultures (either 3-5 day-old or 25-30 day-old) in a buffer solution containing [3H]-glucose resulted in the accumulation of newly synthesized [3H]-glycogen. 3. Noradrenaline (NA), isoprenaline, vasoactive intestinal peptide (VIP), 5-hydroxytryptamine (5-HT) and 8-hydroxy-dipropylaminetetralin (8-OH-DPAT) stimulated the hydrolysis of this newly formed 3H-polymer. The potency order of maximal stimulations was: VIP greater than NA greater than isoprenaline greater than 5-HT greater than 8-OH-DPAT. 4. The putative retinal transmitters, dopamine, gamma-aminobutyric acid (GABA), glycine and taurine and the muscarinic agonist carbachol (CCh) had no effect on [3H]-glycogen content. 5. The glycogenolytic effects of NA/isoprenaline and 5-HT/8-OH-DPAT appear to be mediated by beta-adrenoceptors and 5-HT1 receptors (possibly 5-HT1A), respectively while the VIP-induced response involved another receptor subtype. 6. Agonists which mediated [3H]-glycogen hydrolysis also stimulated an increase in adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation. Both responses are blocked to a similar extent by the same antagonists and so are probably mediated via the same receptor subtypes. Moreover, dibutyryl cyclic AMP (db cyclic AMP) promoted tritiated glycogen breakdown in the three retinal preparations. 7. Not all receptors linked to cyclic AMP production however promote glycogenolysis. Dopamine and apomorphine stimulated cyclic AMP formation via D1-receptors without influencing glycogenolysis. These receptors are exclusively associated with neurones.

Cell type-specific distribution of cathepsin B and D immunoreactivity within the rabbit retina

The cellular localization of cathepsin B and D immunoreactivity was demonstrated at the light microscopic level in the retina of adult rabbits by use of the peroxidase-antiperoxidase technique. Antisera were raised against rat liver enzymes. Whereas cathepsin D immunoreactivity was confined to Müller (glial) cells, cathepsin B was demonstrated in some, but not all, neuronal cell types. It is proposed that the two enzymes might carry different functions within the neuronal versus glial compartment.

Attempt to classify glial cells by means of their process specialization using the rabbit retinal Müller cell as an example of cytotopographic specialization of glial cells

The rabbit retinal Müller cell is one of the most widely studied glial cell types, and it has all forms of contacts that a glial cell can express, viz. 1) to a (ventricular) fluid space, 2) to a mesenchymal borderline (basal lamina), and 3) to neuronal compartments. This cell demonstrates the local adaptation of cell processes to the microenvironment with which they are in contact. Summarizing available data on Müller cells and other glial cell types, it is concluded that the structure with which the process is in contact determines the type of glial cell process that develops. The type I process has microvilli, desmosome-like junctions, and high Na+,K+-ATPase activity; this type of process is in direct contact with a fluid such as cerebrospinal fluid. The type II endfoot-bearing process contains gliofilaments and has a high K+ conductivity; this type of process is covered by a basal lamina and is in contact with mesenchyme. The type III sheath-bearing process insulates neuronal compartments and expresses suitable membrane properties for glia-neuronal communication. Since structurally similar processes have been shown to have similar physiological properties, a new systematic classification of glial cells is proposed, based on the presence or absence of defined types of cell processes. This approach is believed to provide new insights into the function of neuroglia in both the central and peripheral nervous systems, in vertebrates and invertebrates, and even during ontogenetic development.

Comparative ultrastructural study of rabbit Müller cells in vitro and in situ

The ultrastructural appearance of cultured rabbit glial cells (putative Müller cells) has been compared with that of rabbit Müller cells in situ. Electron microscopy disclosed that the cultured cells had a basement membrane comparable to the inner limiting membrane of the retina and that villous processes adjacent to zonulae adherentes were reminiscent of fibre baskets and the outer limiting membrane. Cultured cells showed apico-basal polarisation in specimens from early passages. It was noted however that cultured cells showed a number of features which were different from normal Müller cells in situ. These included prominent rough endoplasmic reticulum, sparse smooth endoplasmic reticulum and relatively electron lucent cytoplasmic matrices. These findings resembled the appearances found in proliferating Müller cells in the human eye. The ultrastructural differences noted in the cultured Müller cells suggest that the cultured cells are in an active form.

Distribution of Müller cells in the turtle retina: an immunocytochemical study

Müller cells are the major type of glial cell in the vertebrate retina, and appear to participate in important structural and metabolic functions. Although the morphological features of Müller cells have been extensively studied, their topographic distribution across the retina has not been previously reported. We have used a Müller cell-specific monoclonal antibody, 19-33, to study the distribution of Müller cells in turtle retina. The antibody was obtained during a search for cell type-specific monoclonal antibodies in the rat retina. Immunoblotting studies show that 19-33 reacts with a 58 KDa protein that is present in Müller cells. Immunocytochemical studies with en face sections of turtle retina show that the density of Müller cells is fairly uniform across the retina although there are small regional differences. We estimate that the mean Müller cell density is about 1600 cells mm-2 of turtle retina and that each turtle retina contains about 54,000 Müller cells.

Evidence for gluconeogenesis in the amphibian retina

Evidence for the existence of a gluconeogenic pathway was provided in the amphibian retina. It was found that [3H]glutamate was converted to [3H]glucose derived from [3H]glutamate was incorporated into glycogen. The rate for this incorporation was found to be essentially the same in both light- and dark-adapted retinas: 0.147 vs. 0.142 nmol (mg protein X 2 hr)-1, respectively. However, the rate of incorporation was found to decline progressively with time. The rate for the incorporation of label derived from glutamate into glycogen was found to be considerably less than that for [3H]glucose: 10.2 nmol (mg protein X 2 hr)-1. The activity of a key gluconeogenic enzyme, fructose-1,6-bisphosphatase, also was demonstrated in retinal supernatants, approximately 1 nmol (mg X min)-1, and the activity of this enzyme was found to be inhibited both by adenosine monophosphate and by fructose-2,6-bisphosphate.

Postnatal development of radial glial (Müller) cells of the rabbit retina

Radial glial (Müller) cells were isolated from postnatal rabbit retinae by enzymatic dissociation in papain-containing solution, air-dried, and submitted to Pappenheim's panoptic stain. Morphometric data of these cells were evaluated by light microscopy. During postnatal development, the cells become substantially thicker and shorter, their nuclei lose the rod shape and move more toward scleral layers, and the nucleus-cytoplasm volume relation decreases. Whereas the cell volume increases from birth on, substantial outgrowth of fine side branches within the plexiform layers fails to occur before electrical activity is established there, i.e. after postnatal day 9. A model is proposed relating the growth of sheath-bearing glial processes to local protein synthesis stimulated by external K+ accumulation due to neuronal activity. Early myelinated nerve fibers are suggested to bear mechanical resistance to growing radial glial processes thus causing a splitting of these processes when they enter developing nerve fiber layers.

Microtubule-associated protein 4 antibody: a new marker for astroglia and oligodendroglia

An antibody to a 240,000 dalton microtubule-associated protein, microtubule-associated protein 4, was used to illustrate the distribution of this protein in semi-thin sections of the central nervous system. Immunofluorescence microscopy indicated that microtubule-associated protein 4 was restricted to non-neuronal elements of the brain and spinal cord. Astrocytes, oligodendrocytes and "specialized" glia, including tanycytes, Bergmann glia and Muller cells, contained microtubule-associated protein 4. This distribution of microtubule-associated protein 4 in neural tissue in distinct from that described for the other major brain microtubule-associated proteins, microtubule-associated protein 1 and microtubule-associated protein 2. The reactivity of MAP 4 antibody with these glia demonstrates the antigenic relatedness of these cells and further distinguishes glia from other elements of the nervous system.

Morphological and structural study of Landolt's club in the chick retina

Light and electron microscopy were used to study Landolt's club of the bipolar cells in the newborn chick retina as well as in early embryonic stages. In the embryo, the bipolar cells were connected to the outer limiting membrane by Landolt's club. Some of the bipolar cells disconnect from this membrane, by complete retraction of Landolt's club, giving rise to bipolar cells without this process. The newly hatched chick, was used for analysis of the ultrastructure of Landolt's club. Zones of apposition between Muller cells and Landolt's club are associated with cytoplasmic vesicles in both cells. Muller cells appear to transmit vesicular material, possibly nutrients, to bipolar cells through Landolt's club. Thus, Landolt's club provides substrates to bipolar cells in the poorly vascularized region of the chick retina.

Scanning and transmission electron microscopy of Müller cells isolated from rabbit retina

Proteolytic enzymes were applied in the dissociation of rabbit retina to obtain isolated Müller cells. Both trypsin and papain were used, and in some cases the trypsinization was shortly interrupted by tissue fixation before it was continued. Scanning electron microscopy mostly confirmed the previous descriptions of these cells, which were based on different techniques. In addition, some new cell extensions were observed that derived from the nuclear region. The papain procedure ensured the best cell preservation with a free surface and limited contaminant material attached. Roughly 90% of the cells excluded trypan blue. It is concluded that these cells should be preferred for biochemical studies, because their cytology also turned out to be intact as judged from electron microscopic sections.

Experimental epiretinal membranes induced by intravitreal carbon particles

We injected 20-nm carbon particles into the vitreous of 14 young rabbits that were killed eight to ten weeks later. Histologic examination showed partial posterior vitreous detachments, epiretinal cellular proliferation, and membranes in all eyes and retinal detachments in five eyes. Electron microscopy disclosed that the epiretinal membranes were formed mainly by Müller cell expansions, astrocytes, and macrophages. Müller cells penetrated the internal limiting membrane and removed carbon particles from the vitreous by endocytosis. The experiments indicated that gaps are produced in the internal limiting membrane by glial cells and macrophages that invade the vitreous in an attempt to remove foreign material. The experimental epiretinal membranes resembled idiopathic preretinal gliosis or macular pucker.

Cochlear transduction: an integrative model and review

A model for cochlear transduction is presented that is based on considerations of the cell biology of its receptor cells, particularly the mechanisms of transmitter release at recepto-neural synapses. Two new interrelated hypotheses on the functional organization of the organ of Corti result from these considerations, one dealing with the possibility of electrotonic interaction between inner and outer hair cells and the other with a possible contributing source to acoustic emissions of cochlear origin that results from vesicular membrane turnover.

The ultrastructure of monkey foveal photoreceptors, with special reference to the structure, shape, size, and spacing of the foveal cones

A systematic electron microscopic study was made of the structure of foveal cones of Macaca spp. Transverse sections of inner (IS) and outer segments (OS) were made in sequence, from the pigment epithelial zone (PEZ) to the outer limiting membrane (OLM). The smallest diameters of hundreds of cone sections were measured from electron micrographs with a Zeiss particle-size analyzer, and analyzed statistically. Some details are also included about Cebus photoreceptors. It is claimed in the literature that foveal cones are rod-like (cylindrical) and untapered. Our study shows the foveolar cone to be a tapered structure. There has been some confusion between the foveola, which is rod-free, and the fovea, which has a high concentration of cones, but is not rod-free. Within the fovea, as the ratio of cones to rods falls from infinity to 1, with distance from the central bouquet of cones, the cone center-to-center distances increase, the inner segment diameters increase, and the number of cones/sq mm decreases. The tapered calycal processes are more massive in M. irus than M. mulatta, and the lateral fins are better developed. Lateral fins are not present in the foveola. The cones are arranged in straight lines.

A quantitative comparison between the ganglion cell populations and axonal outflows of the visual streak and periphery of the rabbit retina

A vertical density profile of the ganglion cells 2 mm temporal of the optic nerve head in the rabbit retina has been produced by counting somata in the cresyl-violet-stained, ganglion cell layer of a flat-mounted retina. Somata classified as ganglion cells were characterized by obvious Nissl staining in an extensive cytoplasm and typically had diameters greater than 9 micrometer. The accuracy of the profile, and thus of the classification criteria, has been substantiated by electron micrographic determination of the numbers of ganglion cell axons arising within local regions of known area on the same retina. This study indicates that Vaney and Hughes' estimate ('76) of 547,100 presumed ganglion cells in the rabbit retina should be changed to 373,500 ganglion cells. The latter value is within the statistical error of their optic nerve count of 394,000 fibers. The mean diameter of ganglion cells 6 mm from the visual streak in the inferior periphery (density: 550 cells/mm2) was 28% greater than that of cells on the peak of the streak (density: 5,400 cells/mm2), although the form of the ganglion cell diameter distribution did not change markedly with eccentricity. The increase in the mean size of ganglion cells in the periphery appeared to be approximately matched by an increase in the size of their axons. Larger axons became myelinated farther from the edge of the myelinated band than did smaller axons. Within the ganglion cell layer there was another population of cells which were quite distinct from the obvious neuroglia: Their nuclei were similar to those of the larger ganglion cells and many appeared to have Nissl granules within their limited cytoplasm. About half of this heterogeneous population was classified as "coronate cells," which were characterized by the partial nuclear encapsulation of their eccentric cytoplasm.

Retinal glycogen content during ischaemia

In the normal rabbit retina glycogen particles were only observed in the Müller cell. The glycogen was evenly distribution throughout the cytoplasm. The inner retina contained large amounts of Müller cell cytoplasm and consequently possessed substantial quantities of glycogen. The outer retina contained little Müller cell cytoplasm and only small amounts of Glycogen. Following total acute ischaemia induced by high intraocular pressure, the amount of glycogen in the retina fell rapidly so that by 45-60 min of ischaemia it was absent from the Müller cell. In the rabbit, ischaemia preferentially damaged the outer retina and especially the visual cells. This pattern of damage was thought to be a reflection of the amount of glycogen present in the different regions of the retina.

Vitreoretinal juncture; healing of experimental wounds

Healing of mechanically-induced, minimal wounds of vitreoretinal juncture in non-vascularized retina of rabbits was studied with electron microscopy at 1 and 3 days, and 1, 2, 6 and 10 weeks. Neuroectodermal scar was formed by two processes, each having a specific anatomical relationship to wound. Accessory gliocytosis, in which accessory glia adjacent to wound become phagocytic and proliferate; following this their progeny migrate to wound and progressively differentiate into fibrous astrocytes to fill in the wound proper. Plexiform gliosis, in which the Müller cell side branches proliferate to form a layer about the perimeter of wound. The retinal inner limiting lamina did not regenerate. The significance of these findings in relation to epiretinal membrane formation in man is discussed.

Electron microscope autoradiographic detection of sites of protein synthesis in the rabbit retina Müller cells

Rabbit retinas were incubated in medium containing 500 microCi of [(3)H]leucine for 3 min, and transferred to medium without isotope for another 7, 17, 37, 57, and 117 min. Retinal pieces were fixed in paraformaldehyde and osmium tetroxide and embedded in Epon. Thin sections were autoradiographed with Ilford L4 emulsion, and a quantitative study of silver grain distribution per Müller cell portion, and per Müller cell organelle, was carried out. Grain density per unit area was high over the middle cell portion at each incubation interval. Silver grains were numerous over background cytoplasm (which comprised free ribosomes) but their percentage was constant at all times and their relative concentration low. Silver grains were numerous and highly concentrated, at pulse incubation, over the rough endoplasmic reticulum (RER) and then decreased sharply, but this decline coincided with an increase over the Golgi complex, peaking at 20 min. Another peak appeared over the cell periphery at 60 min. These findings suggest the simultaneous synthesis of two types of proteins in Müller cells; structural proteins in background cytoplasm and proteins of secretory type in the RER.