Cell polarity changes and migration during early development of the avian peripheral auditory system.

The intracellular location of organelles was studied in avian embryos immediately before and during the initial detachment of cells from the dorsoanterolateral wall of the otocyst, using light and electron microscopy. The Golgi apparatus was silver-impregnated, and its location within the otic epithelium was determined quantitatively. The present study demonstrates that a sub-population of cells of the dorsoanterolateral otic epithelium changes the intracellular location of its organelles, particularly the Golgi apparatus and the centrosome, from an apical to a basal position. Concomitantly, cells lose specializations characteristically present at apical (tight junctions, microvilli) and basal (basal lamina) surfaces. At basolateral cell surfaces, filopodia form ahead of the Golgi apparatus and centrosome and penetrate the previously continuous underlying basal lamina. Thereafter, cells detach from the otocyst and migrate medially toward the hind-brain. Thus, concomitant with changes in surface polarity, the cells that comprise the dorsoanterolateral wall of the otocyst undergo profound changes in the intracellular location of their organelles, especially the Golgi apparatus and the centrosome, so that by the time cells detach from the otic epithelium a reversal in their "normal" internal polarity has occurred. We suggest that the change in cell polarity may be related to the mechanisms that allow cells to leave the otocyst.