THYROID EPITHELIAL MORPHOGENESIS IN-VITRO - A ROLE FOR BUMETANIDE-SENSITIVE CL- SECRETION DURING FOLLICULAR LUMEN DEVELOPMENT

The structural and functional unit of the thyroid gland is the follicl e, consisting of a closed lumen surrounded by a single layer of polari zed epithelial cells. In this paper we have attempted to characterize the process of lumenal development when primary cultures of porcine th yroid cells reorganized to form follicles. Cells incubated with the lo op diuretic, bumetanide, an inhibitor of NaK2Cl cotransport, aggregate d but failed to form normal follicles. Laser scanning confocal microsc opy combined with immunohistochemical markers of thyroid cell-surface proteins demonstrated that in the presence of bumetanide cells polariz ed and assembled ZO-1-containing tight junctions separating their apic al and basolateral membrane domains. Cultures formed small lumena but their subsequent growth was inhibited by bumetanide. Electrophysiologi cal studies confirmed that bumetanide-sensitive Cl- transport was the major contributor to the transepithelial electrical potential differen ce across the follicular wall after 48 h incubation. Other potential m echanisms did not contribute significantly to follicular lumenal growt h. In particular, bumetanide did not affect cell proliferation and, in contrast to tissue follicles, thyroglobulin could not be detected wit hin the lumena of cultured follicles. We conclude that thyroid follicu lar reorganization involves two distinct and separate phases of lumena l development: initial lumen formation which probably reflects the ass embly of a specialized apical membrane domain; and subsequent lumenal growth which is mediated by the inward transport of Cl- by polarized e pithelial cells. (C) 1984 Academic Press, Inc.