Apical orientation of the microtubule organizing center and associated gamma-tubulin during the polarization of the retinal pigment epithelium in vivo

Simple epithelial cells express a morphological and functional polarity along their apical-to-basal axis. During the development of epithelia, a unique reorganization of microtubule arrays is thought to play a fundamental role in the establishment of cell polarity. To begin to understand this process in vivo, we have determined the distribution of gamma-tubulin within developing chicken retinal pigment epithelium (RPE). gamma-Tubulin is a recently discovered centrosomal protein that plays a role in nucleating microtubule growth from the centrosome. Although the RPE monolayer becomes established during embryonic Day 3, cell polarity gradually develops and matures over the next 10-13 days. Our studies reveal that gamma-tubulin is located in a distinct focus subjacent to the apical membrane by embryonic Day 3, the beginning of the polarization process. Using primary cell cultures, we examined the relationship between the establishment of junctional complexes and the reorganization of microtubule arrays. Despite the recovery of junctional complexes and a transepithelial electrical resistance, cultured cells failed to relocate gamma-tubulin foci to a position subjacent to the apical membrane. Rather, these foci remained in the juxtanuclear region. These data indicate that the rearrangement of unique, epithelial microtubule arrays requires more than cell-cell and cell-basement membrane interactions.