The distribution of Na+,K(+)-ATPase in the retinal pigmented epithelium from chicken embryo is polarized in vivo but not in primary cell culture.

The polarity of retinal pigmented epithelia (RPE) from chicken embryos was studied in primary cell culture. Since cultured RPE approximates the morphological polarity of RPE in vivo, we investigated whether this polarity extends to the distribution of plasma membrane proteins that are peculiar to RPE. In contrast to other epithelia, the Na+,K(+)-ATPase of RPE is located in the apical rather than basolateral plasma membrane. To examine this property, we cultured RPE on extracellular matrix-coated filters. Primary cultures were compared to embryonic RPE in situ using electron microscopy and indirect immunofluorescence of frozen sections. The viability and morphology of RPE was improved by using a serum-free medium containing a bovine pituitary extract in conjunction with an extracellular matrix coating derived from Engelbreth-Holm-Swarm tumors. Cultured RPE mimicked the morphology of RPE in vivo with microvilli and junctional complexes on the apical pole and infoldings along the basolateral plasma membrane. Functional tight junctions formed as demonstrated by an EDTA-sensitive, transepithelial electrical resistance, and by the retention of [3H]inulin added to the apical chamber. In 2 hr, only 4-6% of the [3H]inulin crossed the monolayer, compared to 24% in control filters. Despite these features of polarity, the Na+,K(+)-ATPase was detected in both apical and basolateral membranes by immunofluorescence. In embryonic eyes in which the neural retina was removed, the Na+,K(+)-ATPase was confined to the apical membrane. In addition, the polarity of cultured RPE was probed with vesicular stomatitis virus. In contrast to other epithelia, budding virus particles were observed emerging from the apical, as well as basolateral, domain further suggesting the cultured cells were only partially polarized. These data indicate that structural criteria are inadequate to determine if cultured RPE have become polarized in the same manner as the epithelium in vivo.