Fine structural aspects of the shift of zonula occludens and cytoorganelles during the inversion of cell polarity in cultured porcine thyroid follicles.

Porcine thyroid follicles, when isolated by enzymatic digestion and suspended in Eagle's MEM containing 10% fetal calf serum, undergo inversion of cellular polarity. After isolation, the strands for the tight junctions (zonulae occludentes) between follicle cells begin to move towards the side of the medium and gather at this side of the lateral plasma membrane during 24 h of incubation. Around this time, microvilli of many follicular cells protrude into the culture-medium. The elements of the Golgi apparatus are located at the luminal as well as the culture-medium side of the cytoplasm, and also at the lateral side of the nucleus after 24 h of suspension culture, and by 94 h of incubation almost all elements of this organelle, as well as lysosomes and the central cilium have migrated to the side of the medium. The migration of the zonulae occludentes is considered to be the initial change in the reversal of the polarity of this cell.