Response of Müller cells to growth factors alters with time in culture.

We have developed an explant culture technique, using the retinae of newborn guinea pigs, that reliably yields cultures of Müller cells showing uniform morphology and phenotype. Since the guinea pig retina is avascular and lacks astrocytes, Müller cells are the only glial cell-type and the only vimentin-positive population present. Virtually all passaged cells (greater than 98%) contain vimentin-positive intermediate filaments and no glial fibrillary acidic protein (GFAP) has been detected using a range of GFAP antibodies known to label astrocytes in the guinea pig optic nerve. Most vimentin-positive cells were also labeled with an antibody to carbonic anhydrase II, an enzyme which in the retina is specific for Müller cells. Proliferating Müller cells were identified within the inner nuclear layer of retinal fragments as early as 2 days in culture using bromodeoxyuridine (BrdU) and vimentin double labeling. Cultured Müller cells change their growth characteristics with successive passaging. The length of the cell cycle increases from 25.4 h for cells at first passage, to 66.7 h for cells at fourth passage. Altered responses to mitogens were also observed with passaging. First-passage cultures responded to basic fibroblast growth factor (bFGF) but not to several other factors tested including interleukin-2 (IL-2). In contrast, older cultures were highly responsive to IL-2 but showed a minimal response to bFGF. The altered responsiveness to mitogens observed in vitro may be relevant to changes in growth control of Müller cells in the developing and mature retina. The guinea pig retina provides an ideal mammalian tissue for generating Müller cell cultures that are free of astrocytes, endothelial cells, and pericytes, the most frequent contaminants of retinal glial cultures. The monolayers obtained show a high degree of homogeneity and are well suited for studies of Müller cell function.