Polarity reversal of inside-out thyroid follicles cultured within collagen gel: structure of the junctions assessed by freeze-fracture and lanthanum permeability.

The organization of tight junctional complexes (TJs) was studied in cultured porcine thyroid cells during the inversion of polarity induced by collagen-embedding of inside-out follicles, using freeze-fracture replicas and lanthanum penetration. During the early steps of polarity reversal, freeze-fractures showed that TJs generally persisted. They increased in width and progressively branched out into the basolateral surfaces, towards the basal pole. Later, the number of TJ strands decreased and gap junctions inserted within TJ networks were found between cells in reversed follicles, in the same manner as in typically polarized follicles, embedded in collagen or in suspension. The de novo formation of TJ complexes was rarely found in the reversing structures. Despite the heterogeneity of TJs assessed by freeze-fracture, impermeability to lanthanum tracer was noted in inside-out structures. During the reversal process, some TJs remained unstained, whereas others displayed permeability to lanthanum. This heterogeneity might be due to the "opening" of a small number of junctions (perhaps only one by aggregate). When the process was achieved after 48 hr in collagen, the tightness of the junctions was complete, confirmed by the absence of lanthanum in luminal cavities of newly formed follicles.