Isolation and characterization of flat cells, a subpopulation of the embryonic chick retina

When the embryonic neutral retina is dissociated into single cells which are maintained in stationary culture, the neuronal cells associate on the surfaces of a second population which we refer to as flat cells. The flat cells appear in the culture in significant numbers after 2 days and are required for neuronal cell attachment. We have been able to isolate pure flat cells from early cultures of mixed retina cells and have identified several antigens which support the concept that these cells are related to the glia. The cells have been tested by immunofluorescence for glial fibrillary acidic protein and have been found positive. Cell surfaces were labeled by transfer of tritiated galactose from UDP-galactose to endogenous acceptors in the presence of exogenous galactosyl transferase. After SDS-PAGE and fluorography, the surface glycoproteins of flat cells were seen to be significantly different from those of the original retina, and from chick fibroblasts. Immunoelectron microscope studies of detergent-extracted flat cells have demonstrated a complex network of intermediate filaments and actin fibers. We conclude that the flat cells are derived from the glia subpopulation of the retina and have adapted to the tissue culture environment by assuming this configuration. The unique surface properties of flat cells may be related to their role as an intermediate substrate between the neuronal cells and the tissue culture dish.