Effects of RPE-cell factors secreted from permselective fibers on retinal cells in vitro.

This study was undertaken to determine if retinal pigment epithelial (RPE) cells encased in permselective hollow fibers survive in a tissue culture environment and secrete a diffusible trophic factor(s) that may affect retinal cell survival in vitro. In this study, RPE cells were isolated from 6- to 8-day-old Long-Evans rats, then loaded into hollow fibers. The RPE-cell fibers were then cultured for at least one week in serum-containing medium. These RPE-cell fibers were subsequently co-cultured with cells isolated from retinas of day 2 Long-Evans rats in a defined medium. For at least 6 days in culture, opsin-positive cells were observed on the surface of larger flat cells. Over 80% of the small, round cells immunostained for opsin. However, opsin-immunostained cells were seldom seen in cultures with control fibers, that lacked RPE cells. In addition, conditioned medium collected from either the RPE-cell fibers or cultured RPE cells affected survival of opsin-positive retinal cells in culture in a manner similar to that of the RPE-cell fibers. Furthermore, selected growth factors such as epidermal, nerve and fibroblast growth factors, were unable to sustain retinal cell survival and affect morphological development as seen in RPE-CM supplemented cultures. In vivo companion developmental studies demonstrated that few opsin-positive cell bodies were observed in retinas of day 2 Long-Evans rats, the age corresponding to the stage of retinal cell isolation. In retinas of day 5 Long-Evans rats, the age corresponding to the end point of the in vitro assay, a dramatic increase in the number of opsin-immunostained cell bodies was noted, which corresponds to the developmental sequence also seen in culture. Light and electron microscopic examination revealed that the RPE cells cultured in the hollow tubes maintained an RPE-like structure for several months, in that these cells contained melanosomes and extended microvilli from their apical border and formed junctional complexes with adjacent cells. Results of this study confirm our earlier findings that RPE cells secrete an apparent novel factor(s) that affects retinal cell survival in vitro and, most significantly, the described encapsulation/secretion mechanism may provide a convenient method to deliver such factors for further in vivo testing of this phenomenon.