Surface morphology of explants from the human retinal pigment epithelium in culture. A scanning electron microscopic study

Inner limiting membrane as membranous support in RPE sheet-transplantation

PURPOSE

To investigate the inner limiting membrane as a scaffold for retinal pigment epithelium cells (RPE).

METHODS

Human donor eyes (n = 10) from the Eye Bank Tübingen were used to collect the inner limiting membrane (ILM). These human donor eyes and additional porcine eyes (n = 11) served to isolate RPE cells. A human RPE cell line (ARPE-19) was used as control. RPE cells were cultured on ILM for 3 and 7 days. Phase-contrast photographs of the cells in culture were obtained. Morphology and ultrastructural changes were evaluated by light and transmission electron microscopy.

RESULTS

Porcine RPE cells adhere and proliferate when seeded on human ILM. The cells maintained their cuboidal morphology, were polarized, disclosed microvilli on the apical surface, formed intercellular junctions and did not dedifferentiate. Human RPE cells obtained from cadaver eyes barely adhered to the ILM and did not form an intact monolayer. ARPE-19 cells formed a dense colony and maintained epithelial features.

CONCLUSION

The ILM is an ideal matrix to establish an intact RPE monolayer and has the potential to be used as sheet for subretinal transplantation.

In vitro transplantation of fetal human retinal pigment epithelial cells onto human cadaver Bruch's membrane

Retinal pigment epithelium transplantation has been proposed as adjunctive treatment for age-related macular degeneration following surgical excision of choroidal neovascular membranes. The goal of this study was to develop a model to evaluate retinal pigment epithelium transplantation onto human Bruch's membrane in vitro. We investigated the ability of cultured fetal human retinal pigment epithelium to colonize human cadaver Bruch's membrane, determined the incubation time needed to form a monolayer and to exhibit apical microvilli and tight junctions, and assessed the production of basement membrane. Freshly enucleated (less than 48 hours old) human eyes were cut through the pars plana, and the anterior segment, vitreous, and retina were removed. The native retinal pigment epithelium was debrided with a surgical sponge. Bruch's membrane and choroid at the macula were trephined with a 7.0 mm diameter trephine and then incubated with 1/2 ml of Dulbecco's modified Eagle's medium +15% fetal calf serum+basic fibroblast growth factor (1 ng ml-1), and fetal human retinal pigment epithelium at a concentration of 242,000 cells ml-1. Specimens were incubated for 1, 4, 6, 8, 12, or 24 hours. The specimens were fixed in half strength Karnovsky's fixative, processed, and analysed with scanning and transmission electron microscopy. The retinal pigment epithelium covered the debrided macular specimens to different degrees at different incubation times. After 1 hour, the cells started to attach and flatten (median percent coverage: 78%). The extent of Bruch's membrane coverage by fetal retinal pigment epithelium varied greatly between specimens. After 4-6 hours, the cells covered the entire debrided surface in a monolayer (median percent coverage: 97.2% at 4 hours, 99.8% at 6 hours). Tight junctions were observed, and the cells had few apical microvilli. The lateral cell borders were obliquely oriented with respect to Bruch's membrane, and the nuclei were elongated, exhibited prominent nucleoli, and were oriented parallel to Bruch's membrane. After 6-8 hours, cells started to become hexagonal (median percent coverage at 8 hours: 99.97%). Cells attached to the inner collagenous layer tended to be flatter than cells attached to residual native basement membrane. At 12 and 24 hours, expression of hexagonal shape, tight junctions, and apical microvilli were observed more frequently (median percent coverage: 99.87% at 12 and 100% at 24 hours). No newly formed basement membrane was observed at these time points. In separate experiments comparing attachment in the presence and absence of native RPE basement membrane, the presence of native retinal pigment epithelial basement membrane promoted the early attachment of the cells and more rapid expression of normal morphology. This in vitro system provides a reproducible way to study the adherence of retinal pigment epithelium to normal and diseased human Bruch's membrane.

DNA synthesis in human retinal pigment epithelium in organ culture

Retinal pigment epithelium from human eyes was maintained in organ culture, and DNA synthesis was visualized by autoradiography of cultures exposed to labelled thymidine. In Ham's F10 medium, labelling of cells was observed after 7 as well as after 14 days in vitro. At both stages the labelled cells were characteristically located adjacent to areas of epithelial damage. Cells remote from such areas did not incorporate thymidine, This pattern was observed in epithelia removed from the posterior pole as well as from the periphery of the fundus. Several types of routine media and media with human as well as foetal bovine serum (FBS) were found to support DNA synthesis in the cultured epithelium, labelled cells were not observed in epithelium maintained in Ham's F10 without serum, or with low serum concentration. Our study demonstrates, that DNA synthesis does take place in organ cultured human retinal pigment epithelium when the appropriate culture conditions are used. Our findings indicate that induction of DNA synthesis in the cells is not promoted by an indiscriminate effect of the medium but related to the presence of epithelial damage.

Argon laser lesions in the human RPE in vitro

Argon laser lesions were produced in human retinas in eyes to be enucleated because of choroidal melanomas. Lesions were subsequently isolated and maintained in vitro for one month and evaluated by light, transmission and scanning electron microscopy. In the cultured lesions ingrowing cells with a well recognizable epithelial morphology were observed to cover Bruch's membrane in the photocoagulated area, and to grow as sheets on sensory retinal tissue and on detached necrotic pigment epithelium. This growth was predominantly monolayered. In areas, however, overlapping, detachment of cells from the epithelium into the photocoagulated debris, and formation of tubulovescicular structures were observed. The present study demonstrates that lesions with a known potential for reactive changes in the retinal pigment epithelium in vivo can be isolated and cultured with a maintained potential for some of the typical reactive patterns. This opens for investigations on their genesis and on agents modifying these patterns under controlled in vitro conditions.

Human retinal pigment epithelium in long term explant culture

Explants of human retinal pigment epithelium were maintained in culture in various types of media, and examined by light and transmission electron microscopy. After one month in vitro, the central areas showed a monolayered configuration with distinct polarity and presence of ruthenium red stainable material on the apical surface. On the peripheral areas of Bruch's membrane, multilayered lesions were observed to develop and to extend from the monolayered epithelium and past the cut edge in Bruch's membrane. Cells in these lesions contained little melanin and generally lacked an apico-basal polarity. Ruthenium red staining revealed the presence of electron dense material on the apical surface of the lesions as well as in the extracellular space between cells in the various layers. Development of multilayered lesions with deposition of extracellular material are seen in various chorio-retinal disorders, including senile macular degenerations and also subsequent to laser and cryo-therapy. The findings in the present study point to the explant culture system as a valuable tool in the study of important aspects of chorio-retinal pathology.

Culture of retinal pigment epithelium from chorio-retinal biopsies

The recently devised methods for surgical sampling of chorio-retinal tissue permits morphological evaluation of retina in various diseases. In the present report it is demonstrated, that the retinal pigment epithelium in such samples can be isolated as a viable cell population, and that the cell mass can be increased sufficiently in culture to permit biochemical as well as morphological studies. This procedure thereby expands the diagnostic potential of the sample tissue.

Connections between the sensory retina and the retinal pigment epithelium

The study reports on the presence of connections between the visual receptor outer segments and the apical villi on the retinal pigment epithelium in the human eye. The mechanisms underlying sensory retinal adhesion to the pigment epithelium, recognition and attachment of visual receptor outer segments to the pigment epithelium during phagocytosis, and communication and transport of substances between the two layers of retina have been difficult to define, and molecular connections between the two layers of retina could play an important role in these processes.

Reactive lesions at the chorio-retinal junction. An experimental study

Addition of increased amounts of serum and a protein (peroxidase from horseradish) to the culture medium caused structural alterations in previously confluent cultures composed of cells from the human retinal pigment epithelium and choroid. The alterations consisted in the formation of bi- and multi-layered strands, plaques and nodular structures and in the detachment of cells with formation of nude areas. These alterations resemble the frequently observed reactive alterations at the chorio-retinal junction in eyes with inflammation, trauma, retinal detachment and certain degenerative disease processes as the senile disciform macula degeneration.