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 follicle, consisting of a closed lumen surrounded by a single layer of polarized epithelial cells. In this paper we have attempted to characterize the process of lumenal development when primary cultures of porcine thyroid cells reorganized to form follicles. Cells incubated with the loop diuretic, bumetanide, an inhibitor of NaK2Cl cotransport, aggregated but failed to form normal follicles. Laser scanning confocal microscopy combined with immunohistochemical markers of thyroid cell-surface proteins demonstrated that in the presence of bumetanide cells polarized and assembled ZO-1-containing tight junctions separating their apical and basolateral membrane domains. Cultures formed small lumena but their subsequent growth was inhibited by bumetanide. Electrophysiological studies confirmed that bumetanide-sensitive Cl- transport was the major contributor to the transepithelial electrical potential difference across the follicular wall after 48 h incubation. Other potential mechanisms did not contribute significantly to follicular lumenal growth. In particular, bumetanide did not affect cell proliferation and, in contrast to tissue follicles, thyroglobulin could not be detected within the lumena of cultured follicles. We conclude that thyroid follicular reorganization involves two distinct and separate phases of lumenal development: initial lumen formation which probably reflects the assembly of a specialized apical membrane domain; and subsequent lumenal growth which is mediated by the inward transport of Cl- by polarized epithelial cells.