Structural changes in the developing retina maintained in vitro

The Transient Intermediate Plexiform Layer, a Plexiform Layer-like Structure Temporarily Existing in the Inner Nuclear Layer in Developing Rat Retina

The retina is a highly specialised part of the brain responsible for visual processing. It is well-laminated; three layers containing five different types of neurons are compartmentalised by two synaptic layers. Among the retinal layers, the inner nuclear layer (INL) is composed of horizontal, bipolar, and amacrine cell types. Bipolar cells form one sublayer in the distal half of the IPL, while amacrine cells form another sublayer in the proximal half, without any border-like structure. Here, we report that a plexiform layer-like structure exists temporarily in the border between the bipolar and amacrine sublayers in the INL in the rat retina during retinal development. This transient intermediate plexiform layer (TIPL) appeared at postnatal day (PD) 7 and then disappeared around PD 12. Most apoptotic cells in the INL were found near the TIPL. These results suggest that the TIPL may contribute to the formation of sublayers and the cell number limit in the INL.

Primary retinal cultures as a tool for modeling diabetic retinopathy: an overview

Experimental models of diabetic retinopathy (DR) have had a crucial role in the comprehension of the pathophysiology of the disease and the identification of new therapeutic strategies. Most of these studies have been conducted in vivo, in animal models. However, a significant contribution has also been provided by studies on retinal cultures, especially regarding the effects of the potentially toxic components of the diabetic milieu on retinal cell homeostasis, the characterization of the mechanisms on the basis of retinal damage, and the identification of potentially protective molecules. In this review, we highlight the contribution given by primary retinal cultures to the study of DR, focusing on early neuroglial impairment. We also speculate on possible themes into which studies based on retinal cell cultures could provide deeper insight.

Microglial cells in organotypic cultures of developing and adult mouse retina and their relationship with cell death

Organotypic cultures of retinal explants allow the detailed analysis of microglial cells in a cellular microenvironment similar to that in the in situ retina, with the advantage of easy experimental manipulation. However, the in vitro culture causes changes in the retinal cytoarchitecture and induces a microglial response that may influence the results of these manipulations. The purpose of this study was to analyze the influence of the retinal age on changes in retinal cytoarchitecture, cell viability and death, and microglial phenotype and distribution throughout the in vitro culture of developing and adult retina explants. Explants from developing (3 and 10 postnatal days, P3 and P10) and adult (P60) mouse retinas were cultured for up to 10 days in vitro (div). Dead or dying cells were recognized by TUNEL staining, cell viability was determined by flow cytometry, and the numbers and distribution patterns of microglial cells were studied by flow cytometry and immunocytochemistry, respectively. The retinal cytoarchitecture was better preserved at prolonged culture times (10 div) in P10 retina explants than in P3 or adult explants. Particular patterns of cell viability and death were observed at each age: in general, explants from developing retinas showed higher cell viability and lower density of TUNEL-positive profiles versus adult retinas. The proportion of microglial cells relative to the whole population of retinal cells was higher in explants fixed immediately after their dissection (i.e., non-cultured) from adult retinas than in those from developing retinas. This proportion was always higher in non-cultured explants than in explants at 10 div, suggesting the death of some microglial cells during the culture. Activation of microglial cells, as revealed by their phenotypical appearance, was observed in both developing and adult retina explants from the beginning of the culture. Immunofluorescence with the anti-CD68 antibody showed that some activated microglial cells were CD68-positive but others were CD68-negative. Flow cytometry using CD68-labeling revealed that the percentage of CD68-positive microglial cells was much higher in developing than in adult retina explants, despite the activation of microglia in both types of explants, indicating that CD68-labeling was more closely related to the maturity degree of microglia than to their activation. Some swollen activated microglial cells entered the outer nuclear layer in developing and adult cultured retinal explants, whereas this layer was devoid of microglia in non-cultured explants. There was no apparent correlation between the distribution of microglia and that of TUNEL-labeled profiles. However, some swollen activated microglial cells in the outer and inner nuclear layers engulfed clusters of cell nuclei that were negative or weakly positive for TUNEL. This engulfment activity of microglia mimicked that observed in degenerative pathologies of the retina. We conclude that organotypic cultures from developing retinas show a higher rate of cell viability and better preservation of the normal cytoarchitecture in comparison to those obtained from adult retinas. In addition, the features of microglial response in cultured retinal explants show them to be a useful model for studying interactions between microglial cells and degenerating neurons in retinal diseases.

Tailoring substrates for long-term organotypic culture of adult neuronal tissue

Organotypic tissue cultures are highly promising for performing in vivo type studies in vitro. Currently, however, very limited survival times of only a few days for adult tissue often severely limit their application. Here, superhydrophilic nanostructured substrates with ideal material properties ensure tissue adhesion, essential for organotypic culture, while migration of single cells out of the tissue is hampered. Tuning substrate properties, for the first time, adult neuronal tissue could be cultured for 14 days with no indications of degeneration.

Progenitor cell-derived factors enhance photoreceptor survival in rat retinal explants

Explantation of postnatal rat retinas is associated with degenerative events that show morphological similarities to human retinal degenerative disorders. The most evident morphological features are photoreceptor apoptosis involving caspase-3 and Müller cell activation. The purpose of the present study was to determine the content of protective factors in rat retinal progenitor cells and analyze the influence of the identified factors on the survival of photoreceptor cells and retinal gliosis. Tissue inhibitors of matrix metalloproteinase-1 (TIMP-1) and vascular endothelial growth factor (VEGF) were identified as putative beneficial factors, and their combined effect was examined in rat retinal explant cultures. Photoreceptor apoptosis was estimated by cell counts of cleaved caspase-3 and caspase-12 immunolabeled as well as TUNEL labeled cells. TIMP-1 and VEGF in combination significantly suppressed photoreceptor apoptosis involving caspase-3 activation. Cell counts of caspase-12 and TUNEL labeled photoreceptors showed no significant difference between the experiment and control retinas. TIMP-1 and VEGF appeared to have no effect on Müller cell activation as measured by GFAP and Ki-67 immunohistochemistry. Our data suggest that TIMP-1 and VEGF in combination promote the survival of photoreceptor cells in rat retinal explants, possibly by affecting a caspase-3 signaling pathway.

Studies of host–graft interactionsin vitro

Progenitor and stem cell transplantation represent therapeutic strategies for retinal disorders that are accompanied by photoreceptor degeneration. The transplanted cells may either replace degenerating photoreceptors or secrete beneficial factors that halt the processes of photoreceptor degeneration. The present study analyzes whether rat retinal progenitor cells differentiated into photoreceptor phenotypic cells in neurospheres have a potential to interact with rat retinal explants. Immunocytochemistry for rhodopsin and synaptophysin indicated photoreceptor cell-like differentiation in neurospheres that were stimulated by basic fibroblast growth factor and epidermal growth factor. Differentiation into neural phenotypes including photoreceptor cells was effectively blocked by an addition of leukemia inhibitory factor. Grafting of neurospheres onto retinal explants demonstrated a consistent penetration of glial cell processes into the explanted tissue. On the other hand, the incorporation of donor cells into explants was very low. A general finding was that neurospheres grafting was associated with local decrease in Müller cell activation in the explants. Further characterization of these effect(s) could provide further insight into progenitor cell-based therapies of retinal degenerative disorders.