The structure of rabbit retinal Müller (glial) cells is adapted to the surrounding retinal layers

Summary Radial glial (Müller) cells of the rabbit retina were studied by various techniques including Golgi impregnation, scanning electron microscopy, horseradish peroxidase application, and staining of enzymatically isolated cells. This combination of methods produced detailed information on the specialized morphology of the Müller cells within the different topographical regions of the retina, and of the Müller cell processes within the various retinal layers. As a general rule, the retinal periphery contains short thick Müller cells with big endfeet, whereas the thick central retina is occupied by long slender cells with small endfeet. Independent of their location within the retina, Müller cell processes were found to be adapted to the structure of the surrounding retinal layers. Within the outer and inner nuclear layers, Müller cell processes (and somata) extend thin cytoplasmic bubbles ensheathing the neuronal somata, as do the velate astrocytes in the brain. In the plexiform layers, Müller cells extend many fine side branches between the neuropil, comparable to the protoplasmic astrocytes of the brain. In the thick myelinated nerve fibre layer of the central retina the Müller cell processes are rather smooth, similar to those of fibrous astrocytes. It is concluded that the neuronal microenvironment determines the morphology of a given glial process, or even of a part of a glial process running through a specialized neuronal compartment.     

The characterization of a zebrafish mid‐hindbrain mutant,mid‐hindbrain gone (mgo)

The vertebrate mid‐hindbrain boundary (MHB) is a crucial morphological structure required for patterning and neural differentiation of the midbrain and anterior hindbrain. We isolated a novel zebrafish mutant, MHB gone (mgo), that exhibited a defective MHB. Expression of engrailed3 in the prospective MHB was absent at the 1‐somite stage, suggesting that initiation of the isthmic organizer was disrupted in mgo mutants. Complementation test with mgo and noi, in which the pax2a gene is mutated, infer that the mgo mutant may represent a novel noi allele. However, pronephric, otic vesicle, and commissural axonal defects described in noi mutants were not associated with mgo mutants. Genetic mapping revealed that the mgo mutation is linked to the Pax2a locus, but no mutation was detected in pax2a exons or within intron‐exon boundaries. Based on these findings, we propose that the mgo mutation genetically interacts with pax2a required for the initiation of MHB formation. Developmental Dynamics 238:899–907, 2009. © 2009 Wiley‐Liss, Inc.     

An ultramicroscopic study on the distribution of Müller cell processes in the outer retinal layers of the zebrafish

The zebrafish, Brachydanio rerio, is a good model for studying the development of various organs. We have assayed the distribution pattern of Müller cell processes in zebrafish retinas by electron microscopy. In the outer nuclear (ON) layer, multiple layers of Müller cell processes were present along both sides of the large pyramidal endings of the synaptic terminals. We found that the inner segments (ISs) of the zebrafish photoreceptors (PRs), including the cones, double cones and rods, were arranged in different planes, and that the Müller cell processes formed multilayered sheaths around virtually all PR compartments except their outer segments. Thus, Müller cell processes beyond the outer limiting membrane (OLM) are more easily observed in zebrafish retina than in the retinas of other species. To our knowledge, this is the first study to show the exact ultrastructural distribution of Müller cell processes around the OLM and the PR layer in zebrafish retina.     

Expression and cell localization of brain-derived neurotrophic factor and TrkB during zebrafish retinal development

Brain‐derived neurotrophic factor (BDNF) signaling through TrkB regulates different aspects of neuronal development, including survival, axonal and dendritic growth, and synapse formation. Despite recent advances in our understanding of the functional significance of BDNF and TrkB in the retina, the cell types in the retina that express BDNF and TrkB, and the variations in their levels of expression during development, remain poorly defined. The goal of the present study is to determine the age‐dependent changes in the levels of expression and localization of BDNF and TrkB in the zebrafish retina. Zebrafish retinas from 10 days post‐fertilization (dpf) to 180 dpf were used to perform PCR, Western blot and immunohistochemistry. Both BDNF and TrkB mRNAs, and BDNF and full‐length TrkB proteins were detected at all ages sampled. The localization of these proteins in the retina was very similar at all time points studied. BDNF immunoreactivity was found in the outer nuclear layer, the outer plexiform layer and the inner plexiform layer, whereas TrkB immunoreactivity was observed in the inner plexiform layer and, to a lesser extent, in the ganglion cell layer. These results demonstrate that the pattern of expression of BDNF and TrkB in the retina of zebrafish remains unchanged during postembryonic development and adult life. Because TrkB expression in retina did not change with age, cells expressing TrkB may potentially be able to respond during the entire lifespan of zebrafish to BDNF either exogenously administered or endogenously produced, acting through paracrine mechanisms.     

Müller glia and phagocytosis of cell debris in retinal tissue

Müller cells are the predominant glial cell type in the retina of vertebrates. They play a wide variety of roles in both the developing and the mature retina that have been widely reported in the literature. However, less attention has been paid to their role in phagocytosis of cell debris under physiological, pathological or experimental conditions. Müller glia have been shown to phagocytose apoptotic cell bodies originated during development of the visual system. They also engulf foreign molecules that are injected into the eye, cone outer segments and injured photoreceptors. Phagocytosis of photoreceptor cell debris in the light‐damaged teleost retina is primarily carried out by Müller cells. Once the microglial cells become activated and migrate to the photoreceptor cell layer, the phagocytic activity of Müller cells progressively decreases, suggesting a possible mechanism of communication between Müller cells and neighbouring microglia and photoreceptors. Additionally, it has been shown that phagocytic Müller cells acquire proliferating activity in the damaged teleost retina, suggesting that engulfment of apoptotic photoreceptor debris might stimulate the Müller glia to proliferate during the regenerative response. These findings highlight Müller glia phagocytosis as an underlying mechanism contributing to degeneration and regeneration under pathological conditions.     

Frizzled 8a function is required for oligodendrocyte development in the zebrafish spinal cord

Oligodendrocytes are the myelinating cells in the central nervous system. The development of oligodendrocytes is mediated by complex signaling networks, including Wnt signaling. Although Wnt signaling has been studied in various aspects of neurogenesis, the distinct roles of various Frizzled receptors that mediate the Wnt signaling in the CNS remain virtually unknown. In order to understand the specific function of Wnt signaling in oligodendrocyte development, we focused on the Frizzled 8a (Fz8a) receptor. Here we show that Fz8a plays a critical role in the specification and maturation of oligodendrocyte progenitor cells (OPCs) in the ventral spinal cord. Loss of Fz8a function perturbed the proliferation and organization of radial glial cells that give rise to OPCs in the ventral precursor region of spinal cord. In addition, we demonstrate that Wnt signaling activation after the specification of OPCs blocks the formation of mature oligodendrocytes and results in the elimination of OPCs.     

Loss of adenomatous polyposis coli (apc) results in an expanded ciliary marginal zone in the zebrafish eye

The distal region of neural retina (ciliary marginal zone [CMZ]) contains stem cells that produce non‐neural and neuronal progenitors. We provide a detailed gene expression analysis of the eyes of apc mutant zebrafish where the Wnt/β‐catenin pathway is constitutively active. Wnt/β‐catenin signaling leads to an expansion of the CMZ accompanied by a central shift of the retinal identity gene sox2 and the proneural gene atoh7. This suggests an important role for peripheral Wnt/β‐catenin signaling in regulating the expression and localization of neurogenic genes in the central retina. Retinal identity genes rx1 and vsx2, as well as meis1 and pax6a act upstream of Wnt/β‐catenin pathway activation. Peripheral cells that likely contain stem cells can be identified by the expression of follistatin, otx1, and axin2 and the lack of expression of myca and cyclinD1. Our results introduce the zebrafish apc mutation as a new model to study signaling pathways regulating the CMZ. Developmental Dynamics 239:2066–2077, 2010. © 2010 Wiley‐Liss, Inc.     

Zebrafish scarb2a insertional mutant reveals a novel function for the Scarb2/Limp2 receptor in notochord development

Background: Scarb2 or Limp2 belong to a subfamily of Scavenger receptors described as lysosomal transmembrane glycosylated receptors, that are mutated in the human syndrome AMRF (action myoclonus‐renal failure). The zebrafish insertional mutant scarb2a^hi1463Tg has notochord defects, the notochord is a defining feature of chordates running along the center of the longitudinal axis and it is essential for forming the spinal column in all vertebrates. Results: There are three paralogous scarb2 genes in zebrafish; scarb2a, scarb2b, and scarb2c. Both Scarb2a and Scarb2b proteins lack the classical di‐leucine motif. We found that scarb2a^hi1463Tg homozygous zebrafish embryos have a null mutation impairing vacuole formation in the notochord and simultaneously disrupting proper formation of the basement membrane resulting in its thickening at the ventral side of the notochord, which may be the cause for the anomalous upward bending observed in the trunk. Through whole‐mount in situ hybridization, we detected scarb2a mRNA expression in the notochord and in the brain early in development. However, it is puzzling that scarb2a notochord mRNA expression is short‐lived in the presumptive notochord and precedes the complete differentiation of the notochord. Conclusions: This work describes a novel function for the Scarb2 receptor as an essential glycoprotein for notochord development. Developmental Dynamics 245:508–519, 2016. © 2015 Wiley Periodicals, Inc.     

Cyclin D1 inactivation extends proliferation and alters histogenesis in the postnatal mouse retina

Background: The cell‐cycle regulator Cyclin D1 is expressed in embryonic retinal progenitor cells (RPCs) and regulates their cell‐cycle rate and neurogenic output. We report here that Cyclin D1 also has important functions in postnatal retinal histogenesis. Results: The initial production of Müller glia and bipolar cells was enhanced in Cyclin D1 knockout (Ccnd1^?/?) retinas. Despite a steeper than normal rate of depletion of the RPC population at embryonic ages, postnatal Ccnd1^?/? retinas exhibited an extended window of proliferation, neurogenesis, and gliogenesis. Cyclin D3, normally confined to Müller glia, was prematurely expressed in Ccnd1^?/? RPCs. However, Cyclin D3 did not compensate for Cyclin D1 in regulating cell‐cycle kinetics or neurogenic output. Conclusions: The data presented in this study along with our previous finding that Cyclin D2 was unable to completely compensate for the absence of Cyclin D1 indicate that Cyclin D1 regulates retinal histogenesis in ways not shared by the other D‐cyclins. Developmental Dynamics 241:941–952, 2012. © 2012 Wiley Periodicals, Inc.